3. PREAMBLE TO THE DESCRIPTION: The following specification particularly describes the invention and the manner in which it is to be performed: 2 FIELD OF THE INVENTION The present invention relates to audio signal coding and, in particular, to an apparatus 5 and method for stereo filling in multichannel coding. BACKGROUND OF THE INVENTION Audio coding is the domain of compression that deals with exploiting redundancy and 10 irrelevancy in audio signals. In MPEG USAC (see, e.g., [3]), joint stereo coding of two channels is performed using complex prediction, MPS 2-1-2 or unified stereo with band-limited or full-band residual signals. MPEG surround (see, e.g., [4]) hierarchically combines One-To-Two (OTT) and 15 Two-To-Three (TTT) boxes for joint coding of multichannel audio with or without transmission of residual signals. In MPEG-H, Quad Channel Elements hierarchically apply MPS 2-1-2 stereo boxes followed by complex prediction/MS stereo boxes building a fixed 4x4 remixing tree, (see, 20 e.g., [1]). AC4 (see, e.g., [6]) introduces new 3-, 4- and 5- channel elements that allow for remixing transmitted channels via a transmitted mix matrix and subsequent joint stereo coding information. Further, prior publications suggest to use orthogonal transforms like 25 Karhunen-Loeve Transform (KLT) for enhanced multichannel audio coding (see, e.g., [7]). For example, in the 3D audio context, loudspeaker channels are distributed in several height layers, resulting in horizontal and vertical channel pairs. Joint coding of only two channels as defined in USAC is not sufficient to consider the spatial and perceptual 30 relations between channels. MPEG Surround is applied in an additional pre-/ postprocessing step, residual signals are transmitted individually without the possibility of joint stereo coding, e.g. to exploit dependencies between left and right vertical residual signals. In AC-4 dedicated N- channel elements are introduced that allow for efficient encoding of joint coding parameters, but fail for generic speaker setups with more 35 channels as proposed for new immersive playback scenarios (7.1+4, 22.2). MPEG-H Quad Channel element is also restricted to only 4 channels and cannot be dynamically applied to arbitrary channels but only a pre-configured and fixed number of channels. 3 The MPEG-H Multichannel Coding Tool allows the creation of an arbitrary tree of discretely coded stereo boxes, i.e. jointly coded channel pairs, see [2]. 5 A problem that often arises in audio signal coding is caused by quantization, e.g., spectral quantization. Quantization may possibly result in spectral holes. For example, all spectral values in a particular frequency band may be set to zero on the encoder side as a result of quantization. For example, the exact value of such spectral lines before quantization may be relatively low and quantization then may lead to a situation, where the spectral values 10 of all spectral lines, for example, within a particular frequency band have been set to zero. On the decoder side, when decoding, this may lead to undesired spectral holes. Modern frequency-domain speech/audio coding systems such as the Opus/Celt codec of the IETF [9], MPEG-4 (HE-)AAC [10] or, in particular, MPEG-D xHE-AAC (USAC) [11], 15 offer means to code audio frames using either one long transform – a long block – or eight sequential short transforms – short blocks – depending on the temporal stationarity of the signal. In addition, for low-bitrate coding these schemes provide tools to reconstruct frequency coefficients of a channel using pseudorandom noise or lower-frequency coefficients of the same channel. In xHE-AAC, these tools are known as noise filling and 20 spectral band replication, respectively. However, for very tonal or transient stereophonic input, noise filling and/or spectral band replication alone limit the achievable coding quality at very low bitrates, mostly since too many spectral coefficients of both channels need to be transmitted explicitly. 25 MPEG-H Stereo Filling is a parametric tool which relies on the use of a previous frame’s downmix to improve the filling of spectral holes caused by quantization in the frequency domain. Like noise filling, Stereo Filling operates directly in the MDCT domain of the MPEG-H core coder, see [1], [5], [8]. 30 However, using of MPEG Surround and Stereo Filling in MPEG-H is restricted to fixed channel pair elements and therefore cannot exploit time-variant inter-channel dependencies. 35 The Multichannel Coding Tool (MCT) in MPEG-H allows adapting to varying inter-channel dependencies but, due to usage of single channel elements in typical operating configurations, does not allow Stereo Filling. The prior art does not disclose perceptually 4 optimal ways to generate previous frame’s downmixes in case of time-variant, arbitrary jointly coded channel pairs. Using noise filling as a substitute for stereo filling in combination with the MCT to fill spectral holes would lead to noise artifacts, especially for tonal signals. 5 SUMMARY OF THE INVENTION The object of the present invention is to provide improved audio coding concepts. The object of the present invention is solved by an apparatus for decoding according to claim 10 1, by an apparatus for encoding according to claim 15, by a method for decoding according to claim 18, by a method for encoding according to claim 19, by a computer program according to claim 20 and by an encoded multichannel signal according to claim 21. 15 An apparatus for decoding an encoded multichannel signal of a current frame to obtain three or more current audio output channels is provided. A multichannel processor is adapted to select two decoded channels from three or more decoded channels depending on first multichannel parameters. Moreover, the multichannel processor is adapted to generate a first group of two or more processed channels based on said selected 20 channels. A noise filling module is adapted to identify for at least one of the selected channels, one or more frequency bands, within which all spectral lines are quantized to zero, and to generate a mixing channel using, depending on side information, a proper subset of three or more previous audio output channels that have been decoded, and to fill the spectral lines of frequency bands, within which all spectral lines are quantized to 25 zero, with noise generated using spectral lines of the mixing channel. According to embodiments, an apparatus for decoding a previous encoded multichannel signal of a previous frame to obtain three or more previous audio output channels, and for decoding a current encoded multichannel signal of a current frame to obtain three or more 30 current audio output channels is provided. The apparatus comprises an interface, a channel decoder, a multichannel processor for generating the three or more current audio output channels, and a noise filling module. 35 The interface is adapted to receive the current encoded multichannel signal, and to receive side information comprising first multichannel parameters. 5 The channel decoder is adapted to decode the current encoded multichannel signal of the current frame to obtain a set of three or more decoded channels of the current frame. The multichannel processor is adapted to select a first selected pair of two decoded 5 channels from the set of three or more decoded channels depending on the first multichannel parameters. Moreover, the multichannel processor is adapted to generate a first group of two or more processed channels based on said first selected pair of two decoded channels to obtain 10 an updated set of three or more decoded channels. Before the multichannel processor generates the first pair of two or more processed channels based on said first selected pair of two decoded channels, the noise filling module is adapted to identify for at least one of the two channels of said first selected pair 15 of two decoded channels, one or more frequency bands, within which all spectral lines are quantized to zero, and to generate a mixing channel using two or more, but not all of the three or more previous audio output channels, and to fill the spectral lines of the one or more frequency bands, within which all spectral lines are quantized to zero, with noise generated using spectral lines of the mixing channel, wherein the noise filling module is 20 adapted to select the two or more previous audio output channels that are used for generating the mixing channel from the three or more previous audio output channels depending on the side information. A particular concept of embodiments that may be employed by the noise filling module 25 that specifies how to generate and fill noise is referred to as Stereo Filling. Moreover, an apparatus for encoding a multichannel signal having at least three channels is provided. 30 The apparatus comprises an iteration processor being adapted to calculate, in a first iteration step, inter-channel correlation values between each pair of the at least three channels, for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and for processing the selected pair using a multichannel processing operation to derive initial multichannel parameters for the selected pair and to 35 derive first processed channels. 6 The iteration processor is adapted to perform the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive further multichannel parameters and second processed channels. 5 Moreover, the apparatus comprises a channel encoder being adapted to encode channels resulting from an iteration processing performed by the iteration processor to obtain encoded channels. Furthermore, the apparatus comprises an output interface being adapted to generate an 10 encoded multichannel signal having the encoded channels, the initial multichannel parameters and the further multichannel parameters and having an information indicating whether or not an apparatus for decoding shall fill spectral lines of one or more frequency bands, within which all spectral lines are quantized to zero, with noise generated based on previously decoded audio output channels that have been previously decoded by the 15 apparatus for decoding. Moreover, a method for decoding a previous encoded multichannel signal of a previous frame to obtain three or more previous audio output channels, and for decoding a current encoded multichannel signal of a current frame to obtain three or more current audio 20 output channels is provided. The method comprises: - Receiving the current encoded multichannel signal, and receiving side information comprising first multichannel parameters. 25 - Decoding the current encoded multichannel signal of the current frame to obtain a set of three or more decoded channels of the current frame. - Selecting a first selected pair of two decoded channels from the set of three or more decoded channels depending on the first multichannel parameters. 30 - Generating a first group of two or more processed channels based on said first selected pair of two decoded channels to obtain an updated set of three or more decoded channels. 35 Before the first pair of two or more processed channels is generated based on said first selected pair of two decoded channels, the following steps are conducted: 7 - Identifying for at least one of the two channels of said first selected pair of two decoded channels, one or more frequency bands, within which all spectral lines are quantized to zero, and generating a mixing channel using two or more, but not 5 all of the three or more previous audio output channels, and filling the spectral lines of the one or more frequency bands, within which all spectral lines are quantized to zero, with noise generated using spectral lines of the mixing channel, wherein selecting the two or more previous audio output channels that are used for generating the mixing channel from the three or more previous audio output 10 channels is conducted depending on the side information. Furthermore, a method for encoding a multichannel signal having at least three channels is provided. The method comprises: 15 - Calculating, in a first iteration step, inter-channel correlation values between each pair of the at least three channels, for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and processing the selected pair using a multichannel processing operation to derive initial multichannel parameters for the selected pair and to derive first processed 20 channels. - Performing the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels to derive further multichannel parameters and second processed channels. 25 - Encoding channels resulting from an iteration processing performed by the iteration processor to obtain encoded channels. And: - Generating an encoded multichannel signal having the encoded channels, the 30 initial multichannel parameters and the further multichannel parameters and having an information indicating whether or not an apparatus for decoding shall fill spectral lines of one or more frequency bands, within which all spectral lines are quantized to zero, with noise generated based on previously decoded audio output channels that have been previously decoded by the apparatus for decoding. 35 8 Moreover, computer programs are provided, wherein each of the computer programs is configured to implement one of the above-described methods when being executed on a computer or signal processor, so that each of the above-described methods is implemented by one of the computer programs. 5 Furthermore, an encoded multichannel signal is provided. The encoded multichannel signal comprises encoded channels and multichannel parameters and information indicating whether or not an apparatus for decoding shall fill spectral lines of one or more frequency bands, within which all spectral lines are quantized to zero, with spectral data generated based on previously decoded audio output channels that have been previously 10 decoded by the apparatus for decoding. BRIEF DESCRIPTION OF DRAWINGS In the following, embodiments of the present invention are described in more detail with 15 reference to the figures, in which: Fig. 1a shows an apparatus for decoding according to an embodiment; Fig. 1b shows an apparatus for decoding according to another embodiment; 20 Fig. 2 shows a block diagram of a parametric frequency-domain decoder according to an embodiment of the present application; Fig. 3 shows a schematic diagram illustrating the sequence of spectra forming the 25 spectrograms of channels of a multichannel audio signal in order to ease the understanding of the description of the decoder of Fig. 2; Fig. 4 shows a schematic diagram illustrating current spectra out of the spectrograms shown in Fig. 3 for the sake of alleviating the understanding of the description of 30 Fig. 2; Fig. 5a and 5b show a block diagram of a parametric frequency-domain audio decoder in accordance with an alternative embodiment according to which the downmix of the previous frame is used as a basis for inter-channel noise filling; 35 Fig. 6 shows a block diagram of a parametric frequency-domain audio encoder in accordance with an embodiment; 9 Fig. 7 shows a schematic block diagram of an apparatus for encoding a multichannel signal having at least three channels, according to an embodiment; 5 Fig. 8 shows a schematic block diagram of an apparatus for encoding a multichannel signal having at least three channels, according to an embodiment; Fig. 9 shows a schematic block diagram of a stereo box, according to an embodiment; 10 Fig. 10 shows a schematic block diagram of an apparatus for decoding an encoded multichannel signal having encoded channels and at least two multichannel parameters, according to an embodiment; Fig. 11 shows a flowchart of a method for encoding a multichannel signal having at least 15 three channels, according to an embodiment; Fig. 12 shows a flowchart of a method for decoding an encoded multichannel signal having encoded channels and at least two multichannel parameters, according to an embodiment; 20 Fig. 13 shows a system according to an embodiment; Fig. 14 shows in scenario (a) a generation of combination channels for a first frame in scenario, and in scenario (b) a generation of combination channels for a second 25 frame succeeding the first frame according to an embodiment; and Fig. 15 shows an indexing scheme for the multichannel parameters according to embodiments. 30 DETAILED DESCRIPTION OF THE INVENTION Equal or equivalent elements or elements with equal or equivalent functionality are denoted in the following description by equal or equivalent reference numerals. 35 In the following description, a plurality of details are set forth to provide a more thorough explanation of embodiments of the present invention. However, it will be apparent to those 10 skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form rather than in detail in order to avoid obscuring embodiments of the present invention. In addition, features of the different embodiments described hereinafter may be 5 combined with each other, unless specifically noted otherwise. Before describing the apparatus 201 for decoding of Fig. 1a, at first, noise filling for multichannel audio coding is described. In embodiments, the noise filing module 220 of Fig. 1a may, e.g., be configured to conduct on or more of the technologies below that are 10 described regarding noise filling for multichannel audio coding. Fig. 2 shows a frequency-domain audio decoder in accordance with an embodiment of the present application. The decoder is generally indicated using reference sign 10 and comprises a scale factor band identifier 12, a dequantizer 14, a noise filler 16 and an 15 inverse transformer 18 as well as a spectral line extractor 20 and a scale factor extractor 22. Optional further elements which might be comprised by decoder 10 encompass a complex stereo predictor 24, an MS (mid-side) decoder 26 and an inverse TNS (Temporal Noise Shaping) filter tool of which two instantiations 28a and 28b are shown in Fig. 2. In addition, a downmix provider is shown and outlined in more detail below using reference 20 sign 30. The frequency-domain audio decoder 10 of Fig. 2 is a parametric decoder supporting noise filling according to which a certain zero-quantized scale factor band is filled with noise using the scale factor of that scale factor band as a means to control the level of the 25 noise filled into that scale factor band. Beyond this, the decoder 10 of Fig. 2 represents a multichannel audio decoder configured to reconstruct a multichannel audio signal from an inbound data stream 30. Fig. 2, however, concentrates on decoder’s 10 elements involved in reconstructing one of the multichannel audio signals coded into data stream 30 and outputs this (output) channel at an output 32. A reference sign 34 indicates that decoder 30 10 may comprise further elements or may comprise some pipeline operation control responsible for reconstructing the other channels of the multichannel audio signal wherein the description brought forward below indicates how the decoder’s 10 reconstruction of the channel of interest at output 32 interacts with the decoding of the other channels. 35 The multichannel audio signal represented by data stream 30 may comprise two or more channels. In the following, the description of the embodiments of the present application 11 concentrate on the stereo case where the multichannel audio signal merely comprises two channels, but in principle the embodiments brought forward in the following may be readily transferred onto alternative embodiments concerning multichannel audio signals and their coding comprising more than two channels. 5 As will further become clear from the description of Fig. 2 below, the decoder 10 of Fig. 2 is a transform decoder. That is, according to the coding technique underlying decoder 10, the channels are coded in a transform domain such as using a lapped transform of the channels. Moreover, depending on the creator of the audio signal, there are time phases 10 during which the channels of the audio signal largely represent the same audio content, deviating from each other merely by minor or deterministic changes therebetween, such as different amplitudes and/or phase in order to represent an audio scene where the differences between the channels enable the virtual positioning of an audio source of the audio scene with respect to virtual speaker positions associated with the output channels 15 of the multichannel audio signal. At some other temporal phases, however, the different channels of the audio signal may be more or less uncorrelated to each other and may even represent, for example, completely different audio sources. In order to account for the possibly time-varying relationship between the channels of the 20 audio signal, the audio codec underlying decoder 10 of Fig. 2 allows for a time-varying use of different measures to exploit inter-channel redundancies. For example, MS coding allows for switching between representing the left and right channels of a stereo audio signal as they are or as a pair of M (mid) and S (side) channels representing the left and right channels’ downmix and the halved difference thereof, respectively. That is, there are 25 continuously – in a spectrotemporal sense – spectrograms of two channels transmitted by data stream 30, but the meaning of these (transmitted) channels may change in time and relative to the output channels, respectively. Complex stereo prediction – another inter-channel redundancy exploitation tool – enables, 30 in the spectral domain, predicting one channel’s frequency-domain coefficients or spectral lines using spectrally co-located lines of another channel. More details concerning this are described below. In order to facilitate the understanding of the subsequent description of Fig. 2 and its 35 components shown therein, Fig. 3 shows, for the exemplary case of a stereo audio signal represented by data stream 30, a possible way how sample values for the spectral lines of 12 the two channels might be coded into data stream 30 so as to be processed by decoder 10 of Fig. 2. In particular, while at the upper half of Fig. 3 the spectrogram 40 of a first channel of the stereo audio signal is depicted, the lower half of Fig. 3 illustrates the spectrogram 42 of the other channel of the stereo audio signal. Again, it is worthwhile to 5 note that the “meaning” of spectrograms 40 and 42 may change over time due to, for example, a time-varying switching between an MS coded domain and a non-MS-coded domain. In the first instance, spectrograms 40 and 42 relate to an M and S channel, respectively, whereas in the latter case spectrograms 40 and 42 relate to left and right channels. The switching between MS coded domain and non-coded MS coded domain 10 may be signaled in the data stream 30. Fig. 3 shows that the spectrograms 40 and 42 may be coded into data stream 30 at a time-varying spectrotemporal resolution. For example, both (transmitted) channels may be, in a time-aligned manner, subdivided into a sequence of frames indicated using curly 15 brackets 44 which may be equally long and abut each other without overlap. As just mentioned, the spectral resolution at which spectrograms 40 and 42 are represented in data stream 30 may change over time. Preliminarily, it is assumed that the spectrotemporal resolution changes in time equally for spectrograms 40 and 42, but an extension of this simplification is also feasible as will become apparent from the following 20 description. The change of the spectrotemporal resolution is, for example, signaled in data stream 30 in units of the frames 44. That is, the spectrotemporal resolution changes in units of frames 44. The change in the spectrotemporal resolution of the spectrograms 40 and 42 is achieved by switching the transform length and the number of transforms used to describe the spectrograms 40 and 42 within each frame 44. In the example of Fig. 3, 25 frames 44a and 44b exemplify frames where one long transform has been used in order to sample the audio signal’s channels therein, thereby resulting in highest spectral resolution with one spectral line sample value per spectral line for each of such frames per channel. In Fig. 3, the sample values of the spectral lines are indicated using small crosses within the boxes, wherein the boxes, in turn, are arranged in rows and columns 30 and shall represent a spectral temporal grid with each row corresponding to one spectral line and each column corresponding to sub-intervals of frames 44 corresponding to the shortest transforms involved in forming spectrograms 40 and 42. In particular, Fig. 3 illustrates, for example, for frame 44d, that a frame may alternatively be subject to consecutive transforms of shorter length, thereby resulting, for such frames such as frame 35 44d, in several temporally succeeding spectra of reduced spectral resolution. Eight short transforms are exemplarily used for frame 44d, resulting in a spectrotemporal sampling of 13 the spectrograms 40 and 42 within that frame 42d, at spectral lines spaced apart from each other so that merely every eighth spectral line is populated, but with a sample value for each of the eight transform windows or transforms of shorter length used to transform frame 44d. For illustration purposes, it is shown in Fig. 3 that other numbers of transforms 5 for a frame would be feasible as well, such as the usage of two transforms of a transform length which is, for example, half the transform length of the long transforms for frames 44a and 44b, thereby resulting in a sampling of the spectrotemporal grid or spectrograms 40 and 42 where two spectral line sample values are obtained for every second spectral line, one of which relates to the leading transform, the other to the trailing transform. 10 The transform windows for the transforms into which the frames are subdivided are illustrated in Fig. 3 below each spectrogram using overlapping window-like lines. The temporal overlap serves, for example, for TDAC (Time-Domain Aliasing Cancellation) purposes. 15 Although the embodiments described further below could also be implemented in another fashion, Fig. 3 illustrates the case where the switching between different spectrotemporal resolutions for the individual frames 44 is performed in a manner such that for each frame 44, the same number of spectral line values indicated by the small crosses in Fig. 3 result for spectrogram 40 and spectrogram 42, the difference merely residing in the way the 20 lines spectrotemporally sample the respective spectrotemporal tile corresponding to the respective frame 44, spanned temporally over the time of the respective frame 44 and spanned spectrally from zero frequency to the maximum frequency fmax. Using arrows in Fig. 3, Fig. 3 illustrates with respect to frame 44d that similar spectra may 25 be obtained for all of the frames 44 by suitably distributing the spectral line sample values belonging to the same spectral line but short transform windows within one frame of one channel, onto the un-occupied (empty) spectral lines within that frame up to the next occupied spectral line of that same frame. Such resulting spectra are called “interleaved spectra” in the following. In interleaving n transforms of one frame of one channel, for 30 example, spectrally co-located spectral line values of the n short transforms follow each other before the set of n spectrally co-located spectral line values of the n short transforms of the spectrally succeeding spectral line follows. An intermediate form of interleaving would be feasible as well: instead of interleaving all spectral line coefficients of one frame, it would be feasible to interleave merely the spectral line coefficients of a proper subset of 35 the short transforms of a frame 44d. In any case, whenever spectra of frames of the two 14 channels corresponding to spectrograms 40 and 42 are discussed, these spectra may refer to interleaved ones or non-interleaved ones. In order to efficiently code the spectral line coefficients representing the spectrograms 40 5 and 42 via data stream 30 passed to decoder 10, same are quantized. In order to control the quantization noise spectrotemporally, the quantization step size is controlled via scale factors which are set in a certain spectrotemporal grid. In particular, within each of the sequence of spectra of each spectrogram, the spectral lines are grouped into spectrally consecutive non-overlapping scale factor groups. Fig. 4 shows a spectrum 46 of the 10 spectrogram 40 at the upper half thereof, and a co-temporal spectrum 48 out of spectrogram 42. As shown therein, the spectra 46 and 48 are subdivided into scale factor bands along the spectral axis f so as to group the spectral lines into non-overlapping groups. The scale factor bands are illustrated in Fig. 4 using curly brackets 50. For the sake of simplicity, it is assumed that the boundaries between the scale factor bands 15 coincide between spectrum 46 and 48, but this does not need to necessarily be the case. That is, by way of the coding in data stream 30, the spectrograms 40 and 42 are each subdivided into a temporal sequence of spectra and each of these spectra is spectrally subdivided into scale factor bands, and for each scale factor band the data stream 30 20 codes or conveys information about a scale factor corresponding to the respective scale factor band. The spectral line coefficients falling into a respective scale factor band 50 are quantized using the respective scale factor or, as far as decoder 10 is concerned, may be dequantized using the scale factor of the corresponding scale factor band. 25 Before changing back again to Fig. 2 and the description thereof, it shall be assumed in the following that the specifically treated channel, i.e. the one the decoding of which the specific elements of the decoder of Fig. 2 except 34 are involved with, is the transmitted channel of spectrogram 40 which, as already stated above, may represent one of left and right channels, an M channel or an S channel with the assumption that the multichannel 30 audio signal coded into data stream 30 is a stereo audio signal. While the spectral line extractor 20 is configured to extract the spectral line data, i.e. the spectral line coefficients for frames 44 from data stream 30, the scale factor extractor 22 is configured to extract for each frame 44 the corresponding scale factors. To this end, 35 extractors 20 and 22 may use entropy decoding. In accordance with an embodiment, the scale factor extractor 22 is configured to sequentially extract the scale factors of, for 15 example, spectrum 46 in Fig. 4, i.e. the scale factors of scale factor bands 50, from the data stream 30 using context-adaptive entropy decoding. The order of the sequential decoding may follow the spectral order defined among the scale factor bands leading, for example, from low frequency to high frequency. The scale factor extractor 22 may use 5 context-adaptive entropy decoding and may determine the context for each scale factor depending on already extracted scale factors in a spectral neighborhood of a currently extracted scale factor, such as depending on the scale factor of the immediately preceding scale factor band. Alternatively, the scale factor extractor 22 may predictively decode the scale factors from the data stream 30 such as, for example, using differential 10 decoding while predicting a currently decoded scale factor based on any of the previously decoded scale factors such as the immediately preceding one. Notably, this process of scale factor extraction is agnostic with respect to a scale factor belonging to a scale factor band populated by zero-quantized spectral lines exclusively, or populated by spectral lines among which at least one is quantized to a non-zero value. A scale factor belonging to a 15 scale factor band populated by zero-quantized spectral lines only may both serve as a prediction basis for a subsequent decoded scale factor which possibly belongs to a scale factor band populated by spectral lines among which one is non-zero, and be predicted based on a previously decoded scale factor which possibly belongs to a scale factor band populated by spectral lines among which one is non-zero. 20 For the sake of completeness only, it is noted that the spectral line extractor 20 extracts the spectral line coefficients with which the scale factor bands 50 are populated likewise using, for example, entropy coding and/or predictive coding. The entropy coding may use context-adaptivity based on spectral line coefficients in a spectrotemporal neighborhood of 25 a currently decoded spectral line coefficient, and likewise, the prediction may be a spectral prediction, a temporal prediction or a spectrotemporal prediction predicting a currently decoded spectral line coefficient based on previously decoded spectral line coefficients in a spectrotemporal neighborhood thereof. For the sake of an increased coding efficiency, spectral line extractor 20 may be configured to perform the decoding of the spectral lines 30 or line coefficients in tuples, which collect or group spectral lines along the frequency axis. Thus, at the output of spectral line extractor 20 the spectral line coefficients are provided such as, for example, in units of spectra such as spectrum 46 collecting, for example, all of the spectral line coefficients of a corresponding frame, or alternatively collecting all of 35 the spectral line coefficients of certain short transforms of a corresponding frame. At the 16 output of scale factor extractor 22, in turn, corresponding scale factors of the respective spectra are output. Scale factor band identifier 12 as well as dequantizer 14 have spectral line inputs coupled 5 to the output of spectral line extractor 20, and dequantizer 14 and noise filler 16 have scale factor inputs coupled to the output of scale factor extractor 22. The scale factor band identifier 12 is configured to identify so-called zero-quantized scale factor bands within a current spectrum 46, i.e. scale factor bands within which all spectral lines are quantized to zero, such as scale factor band 50c in Fig. 4, and the remaining scale factor bands of the 10 spectrum within which at least one spectral line is quantized to non-zero. In particular, in Fig. 4 the spectral line coefficients are indicated using hatched areas in Fig. 4. It is visible therefrom that in spectrum 46, all scale factor bands but scale factor band 50b have at least one spectral line, the spectral line coefficient of which is quantized to a non-zero value. Later on it will become clear that the zero-quantized scale factor bands such as 50d 15 form the subject of the inter-channel noise filling described further below. Before proceeding with the description, it is noted that scale factor band identifier 12 may restrict its identification onto merely a proper subset of the scale factor bands 50 such as onto scale factor bands above a certain start frequency 52. In Fig. 4, this would restrict the identification procedure onto scale factor bands 50d, 50e and 50f. 20 The scale factor band identifier 12 informs the noise filler 16 on those scale factor bands which are zero-quantized scale factor bands. The dequantizer 14 uses the scale factors associated with an inbound spectrum 46 so as to dequantize, or scale, the spectral line coefficients of the spectral lines of spectrum 46 according to the associated scale factors, 25 i.e. the scale factors associated with the scale factor bands 50. In particular, dequantizer 14 dequantizes and scales spectral line coefficients falling into a respective scale factor band with the scale factor associated with the respective scale factor band. Fig. 4 shall be interpreted as showing the result of the dequantization of the spectral lines. 30 The noise filler 16 obtains the information on the zero-quantized scale factor bands which form the subject of the following noise filling, the dequantized spectrum as well as the scale factors of at least those scale factor bands identified as zero-quantized scale factor bands and a signalization obtained from data stream 30 for the current frame revealing whether inter-channel noise filling is to be performed for the current frame. 35 17 The inter-channel noise filling process described in the following example actually involves two types of noise filling, namely the insertion of a noise floor 54 pertaining to all spectral lines having been quantized to zero irrespective of their potential membership to any zeroquantized scale factor band, and the actual inter-channel noise filling procedure. Although 5 this combination is described hereinafter, it is to be emphasized that the noise floor insertion may be omitted in accordance with an alternative embodiment. Moreover, the signalization concerning the noise filling switch-on and switch-off relating to the current frame and obtained from data stream 30 could relate to the inter-channel noise filling only, or could control the combination of both noise filling sorts together. 10 As far as the noise floor insertion is concerned, noise filler 16 could operate as follows. In particular, noise filler 16 could employ artificial noise generation such as a pseudorandom number generator or some other source of randomness in order to fill spectral lines, the spectral line coefficients of which were zero. The level of the noise floor 54 thus inserted 15 at the zero-quantized spectral lines could be set according to an explicit signaling within data stream 30 for the current frame or the current spectrum 46. The “level” of noise floor 54 could be determined using a root-mean-square (RMS) or energy measure for example. The noise floor insertion thus represents a kind of pre-filling for those scale factor bands 20 having been identified as zero-quantized ones such as scale factor band 50d in Fig. 4. It also affects other scale factor bands beyond the zero-quantized ones, but the latter are further subject to the following inter-channel noise filling. As described below, the interchannel noise filling process is to fill-up zero-quantized scale factor bands up to a level which is controlled via the scale factor of the respective zero-quantized scale factor band. 25 The latter may be directly used to this end due to all spectral lines of the respective zeroquantized scale factor band being quantized to zero. Nevertheless, data stream 30 may contain an additional signalization of a parameter, for each frame or each spectrum 46, which commonly applies to the scale factors of all zero-quantized scale factor bands of the corresponding frame or spectrum 46 and results, when applied onto the scale factors 30 of the zero-quantized scale factor bands by the noise filler 16, in a respective fill-up level which is individual for the zero-quantized scale factor bands. That is, noise filler 16 may modify, using the same modification function, for each zero-quantized scale factor band of spectrum 46, the scale factor of the respective scale factor band using the just mentioned parameter contained in data stream 30 for that spectrum 46 of the current frame so as to 35 obtain a fill-up target level for the respective zero-quantized scale factor band measuring, in terms of energy or RMS, for example, the level up to which the inter-channel noise 18 filling process shall fill up the respective zero-quantized scale factor band with (optionally) additional noise (in addition to the noise floor 54). In particular, in order to perform the inter-channel noise filling 56, noise filler 16 obtains a 5 spectrally co-located portion of the other channel’s spectrum 48, in a state already largely or fully decoded, and copies the obtained portion of spectrum 48 into the zero-quantized scale factor band to which this portion was spectrally co-located, scaled in such a manner that the resulting overall noise level within that zero-quantized scale factor band – derived by an integration over the spectral lines of the respective scale factor band – equals the 10 aforementioned fill-up target level obtained from the zero-quantized scale factor band’s scale factor. By this measure, the tonality of the noise filled into the respective zeroquantized scale factor band is improved in comparison to artificially generated noise such as the one forming the basis of the noise floor 54, and is also better than an uncontrolled spectral copying/replication from very-low-frequency lines within the same spectrum 46. 15 To be even more precise, the noise filler 16 locates, for a current band such as 50d, a spectrally co-located portion within spectrum 48 of the other channel, scales the spectral lines thereof depending on the scale factor of the zero-quantized scale factor band 50d in a manner just described involving, optionally, some additional offset or noise factor 20 parameter contained in data stream 30 for the current frame or spectrum 46, so that the result thereof fills up the respective zero-quantized scale factor band 50d up to the desired level as defined by the scale factor of the zero-quantized scale factor band 50d. In the present embodiment, this means that the filling-up is done in an additive manner relative to the noise floor 54. 25 In accordance with a simplified embodiment, the resulting noise-filled spectrum 46 would directly be input into the input of inverse transformer 18 so as to obtain, for each transform window to which the spectral line coefficients of spectrum 46 belong, a time-domain portion of the respective channel audio time-signal, whereupon (not shown in Fig. 2) an 30 overlap-add process may combine these time-domain portions. That is, if spectrum 46 is a non-interleaved spectrum, the spectral line coefficients of which merely belong to one transform, then inverse transformer 18 subjects that transform so as to result in one timedomain portion and the preceding and trailing ends of which would be subject to an overlap-add process with preceding and trailing time-domain portions obtained by inverse 35 transforming preceding and succeeding inverse transforms so as to realize, for example, time-domain aliasing cancelation. If, however, the spectrum 46 has interleaved there-into 19 spectral line coefficients of more than one consecutive transform, then inverse transformer 18 would subject same to separate inverse transformations so as to obtain one timedomain portion per inverse transformation, and in accordance with the temporal order defined thereamong, these time-domain portions would be subject to an overlap-add 5 process therebetween, as well as with respect to preceding and succeeding time-domain portions of other spectra or frames. However, for the sake of completeness it must be noted that further processing may be performed onto the noise-filled spectrum. As shown in Fig. 2, the inverse TNS filter may 10 perform an inverse TNS filtering onto the noise-filled spectrum. That is, controlled via TNS filter coefficients for the current frame or spectrum 46, the spectrum obtained so far is subject to a linear filtering along spectral direction. With or without inverse TNS filtering, complex stereo predictor 24 could then treat the 15 spectrum as a prediction residual of an inter-channel prediction. More specifically, interchannel predictor 24 could use a spectrally co-located portion of the other channel to predict the spectrum 46 or at least a subset of the scale factor bands 50 thereof. The complex prediction process is illustrated in Fig. 4 with dashed box 58 in relation to scale factor band 50b. That is, data stream 30 may contain inter-channel prediction parameters 20 controlling, for example, which of the scale factor bands 50 shall be inter-channel predicted and which shall not be predicted in such a manner. Further, the inter-channel prediction parameters in data stream 30 may further comprise complex inter-channel prediction factors applied by inter-channel predictor 24 so as to obtain the inter-channel prediction result. These factors may be contained in data stream 30 individually for each 25 scale factor band, or alternatively each group of one or more scale factor bands, for which inter-channel prediction is activated or signaled to be activated in data stream 30. The source of inter-channel prediction may, as indicated in Fig. 4, be the spectrum 48 of the other channel. To be more precise, the source of inter-channel prediction may be the 30 spectrally co-located portion of spectrum 48, co-located to the scale factor band 50b to be inter-channel predicted, extended by an estimation of its imaginary part. The estimation of the imaginary part may be performed based on the spectrally co-located portion 60 of spectrum 48 itself, and/or may use a downmix of the already decoded channels of the previous frame, i.e. the frame immediately preceding the currently decoded frame to 35 which spectrum 46 belongs. In effect, inter-channel predictor 24 adds to the scale factor 20 bands to be inter-channel predicted such as scale factor band 50b in Fig. 4, the prediction signal obtained as just-described. As already noted in the preceding description, the channel to which spectrum 46 belongs 5 may be an MS coded channel, or may be a loudspeaker related channel, such as a left or right channel of a stereo audio signal. Accordingly, optionally an MS decoder 26 subjects the optionally inter-channel predicted spectrum 46 to MS decoding, in that same performs, per spectral line or spectrum 46, an addition or subtraction with spectrally corresponding spectral lines of the other channel corresponding to spectrum 48. For example, although 10 not shown in Fig. 2, spectrum 48 as shown in Fig. 4 has been obtained by way of portion 34 of decoder 10 in a manner analogous to the description brought forward above with respect to the channel to which spectrum 46 belongs, and the MS decoding module 26, in performing MS decoding, subjects the spectra 46 and 48 to spectral line-wise addition or spectral line-wise subtraction, with both spectra 46 and 48 being at the same stage within 15 the processing line, meaning, both have just been obtained by inter-channel prediction, for example, or both have just been obtained by noise filling or inverse TNS filtering. It is noted that, optionally, the MS decoding may be performed in a manner globally concerning the whole spectrum 46, or being individually activatable by data stream 30 in 20 units of, for example, scale factor bands 50. In other words, MS decoding may be switched on or off using respective signalization in data stream 30 in units of, for example, frames or some finer spectrotemporal resolution such as, for example, individually for the scale factor bands of the spectra 46 and/or 48 of the spectrograms 40 and/or 42, wherein it is assumed that identical boundaries of both channels’ scale factor bands are defined. 25 As illustrated in Fig. 2, the inverse TNS filtering by inverse TNS filter 28 could also be performed after any inter-channel processing such as inter-channel prediction 58 or the MS decoding by MS decoder 26. The performance in front of, or downstream of, the interchannel processing could be fixed or could be controlled via a respective signalization for 30 each frame in data stream 30 or at some other level of granularity. Wherever inverse TNS filtering is performed, respective TNS filter coefficients present in the data stream for the current spectrum 46 control a TNS filter, i.e. a linear prediction filter running along spectral direction so as to linearly filter the spectrum inbound into the respective inverse TNS filter module 28a and/or 28b. 35 21 Thus, the spectrum 46 arriving at the input of inverse transformer 18 may have been subject to further processing as just described. Again, the above description is not meant to be understood in such a manner that all of these optional tools are to be present either concurrently or not. These tools may be present in decoder 10 partially or collectively. 5 In any case, the resulting spectrum at the inverse transformer’s input represents the final reconstruction of the channel’s output signal and forms the basis of the aforementioned downmix for the current frame which serves, as described with respect to the complex prediction 58, as the basis for the potential imaginary part estimation for the next frame to 10 be decoded. It may further serve as the final reconstruction for inter-channel predicting another channel than the one which the elements except 34 in Fig. 2 relate to. The respective downmix is formed by downmix provider 31 by combining this final spectrum 46 with the respective final version of spectrum 48. The latter entity, i.e. the 15 respective final version of spectrum 48, formed the basis for the complex inter-channel prediction in predictor 24. Fig. 5 shows an alternative relative to Fig. 2 insofar as the basis for inter-channel noise filling is represented by the downmix of spectrally co-located spectral lines of a previous 20 frame so that, in the optional case of using complex inter-channel prediction, the source of this complex inter-channel prediction is used twice, as a source for the inter-channel noise filling as well as a source for the imaginary part estimation in the complex inter-channel prediction. Fig. 5 shows a decoder 10 including the portion 70 pertaining to the decoding of the first channel to which spectrum 46 belongs, as well as the internal structure of the 25 aforementioned other portion 34, which is involved in the decoding of the other channel comprising spectrum 48. The same reference sign has been used for the internal elements of portion 70 on the one hand and 34 on the other hand. As can be seen, the construction is the same. At output 32, one channel of the stereo audio signal is output, and at the output of the inverse transformer 18 of second decoder portion 34, the other 30 (output) channel of the stereo audio signal results, with this output being indicated by reference sign 74. Again, the embodiments described above may be easily transferred to a case of using more than two channels. The downmix provider 31 is co-used by both portions 70 and 34 and receives temporally 35 co-located spectra 48 and 46 of spectrograms 40 and 42 so as to form a downmix based thereon by summing up these spectra on a spectral line by spectral line basis, potentially 22 with forming the average therefrom by dividing the sum at each spectral line by the number of channels downmixed, i.e. two in the case of Fig. 5. At the downmix provider’s 31 output, the downmix of the previous frame results by this measure. It is noted in this regard that in case of the previous frame containing more than one spectrum in either one 5 of spectrograms 40 and 42, different possibilities exist as to how downmix provider 31 operates in that case. For example, in that case downmix provider 31 may use the spectrum of the trailing transforms of the current frame, or may use an interleaving result of interleaving all spectral line coefficients of the current frame of spectrogram 40 and 42. The delay element 74 shown in Fig. 5 as connected to the downmix provider’s 31 output, 10 shows that the downmix thus provided at downmix provider’s 31 output forms the downmix of the previous frame 76 (see Fig. 4 with respect to the inter-channel noise filling 56 and complex prediction 58, respectively). Thus, the output of delay element 74 is connected to the inputs of inter-channel predictors 24 of decoder portions 34 and 70 on the one hand, and the inputs of noise fillers 16 of decoder portions 70 and 34, on the other hand. 15 That is, while in Fig. 2, the noise filler 16 receives the other channel’s finally reconstructed temporally co-located spectrum 48 of the same current frame as a basis of the interchannel noise filling, in Fig. 5 the inter-channel noise filling is performed instead based on the downmix of the previous frame as provided by downmix provider 31. The way in which 20 the inter-channel noise filling is performed, remains the same. That is, the inter-channel noise filler 16 grabs out a spectrally co-located portion out of the respective spectrum of the other channel’s spectrum of the current frame, in case of Fig. 2, and the largely or fully decoded, final spectrum as obtained from the previous frame representing the downmix of the previous frame, in case of Fig. 5, and adds same “source” portion to the spectral lines 25 within the scale factor band to be noise filled, such as 50d in Fig. 4, scaled according to a target noise level determined by the respective scale factor band’s scale factor. Concluding the above discussion of embodiments describing inter-channel noise filling in an audio decoder, it should be evident to readers skilled in the art that, before adding the 30 grabbed-out spectrally or temporally co-located portion of the “source” spectrum to the spectral lines of the “target” scale factor band, a certain pre-processing may be applied to the “source” spectral lines without digressing from the general concept of the inter-channel filling. In particular, it may be beneficial to apply a filtering operation such as, for example, a spectral flattening, or tilt removal, to the spectral lines of the “source” region to be added 35 to the “target” scale factor band, like 50d in Fig. 4, in order to improve the audio quality of the inter-channel noise filling process. Likewise, and as an example of a largely (instead 23 of fully) decoded spectrum, the aforementioned “source” portion may be obtained from a spectrum which has not yet been filtered by an available inverse (i.e. synthesis) TNS filter. Thus, the above embodiments concerned a concept of an inter-channel noise filling. In the 5 following, a possibility is described how the above concept of inter-channel noise filling may be built into an existing codec, namely xHE-AAC, in a semi-backward compatible manner. In particular, hereinafter a preferred implementation of the above embodiments is described, according to which a stereo filling tool is built into an xHE-AAC based audio codec in a semi-backward compatible signaling manner. By use of the implementation 10 described further below, for certain stereo signals, stereo filling of transform coefficients in either one of the two channels in an audio codec based on an MPEG-D xHE-AAC (USAC) is feasible, thereby improving the coding quality of certain audio signals especially at low bitrates. The stereo filling tool is signaled semi-backward-compatibly such that legacy xHE-AAC decoders can parse and decode the bitstreams without obvious audio errors or 15 drop-outs. As was already described above, a better overall quality can be attained if an audio coder can use a combination of previously decoded/quantized coefficients of two stereo channels to reconstruct zero-quantized (non-transmitted) coefficients of either one of the currently decoded channels. It is therefore desirable to allow such stereo filling (from previous to present channel coefficients) in addition to spectral band replication 20 (from low- to high-frequency channel coefficients) and noise filling (from an uncorrelated pseudorandom source) in audio coders, especially xHE-AAC or coders based on it. To allow coded bitstreams with stereo filling to be read and parsed by legacy xHE-AAC decoders, the desired stereo filling tool shall be used in a semi-backward compatible way: 25 its presence should not cause legacy decoders to stop – or not even start – decoding. Readability of the bitstream by xHE-AAC infrastructure can also facilitate market adoption. To achieve the aforementioned wish for semi-backward compatibility for a stereo filling tool in the context of xHE-AAC or its potential derivatives, the following implementation 30 involves the functionality of stereo filling as well as the ability to signal the same via syntax in the data stream actually concerned with noise filling. The stereo filling tool would work in line with the above description. In a channel pair with common window configuration, a coefficient of a zero-quantized scale factor band is, when the stereo filling tool is activated, as an alternative (or, as described, in addition) to noise filling, reconstructed by a sum or 35 difference of the previous frame’s coefficients in either one of the two channels, preferably the right channel. Stereo filling is performed similar to noise filling. The signaling would be 24 done via the noise filling signaling of xHE-AAC. Stereo filling is conveyed by means of the 8-bit noise filling side information. This is feasible because the MPEG-D USAC standard [3] states that all 8 bits are transmitted even if the noise level to be applied is zero. In that situation, some of the noise-fill bits can be reused for the stereo filling tool. 5 Semi-backward-compatibility regarding bitstream parsing and playback by legacy xHEAAC decoders is ensured as follows. Stereo filling is signaled via a noise level of zero (i.e. the first three noise-fill bits all having a value of zero) followed by five non-zero bits (which traditionally represent a noise offset) containing side information for the stereo filling tool 10 as well as the missing noise level. Since a legacy xHE-AAC decoder disregards the value of the 5-bit noise offset if the 3-bit noise level is zero, the presence of the stereo filling tool signaling only has an effect on the noise filling in the legacy decoder: noise filling is turned off since the first three bits are zero, and the remainder of the decoding operation runs as intended. In particular, stereo filling is not performed due to the fact that it is operated like 15 the noise-fill process, which is deactivated. Hence, a legacy decoder still offers “graceful” decoding of the enhanced bitstream 30 because it does not need to mute the output signal or even abort the decoding upon reaching a frame with stereo filling switched on. Naturally, it is however unable to provide a correct, intended reconstruction of stereo-filled line coefficients, leading to a deteriorated quality in affected frames in comparison with 20 decoding by an appropriate decoder capable of appropriately dealing with the new stereo filling tool. Nonetheless, assuming the stereo filling tool is used as intended, i.e. only on stereo input at low bitrates, the quality through xHE-AAC decoders should be better than if the affected frames would drop out due to muting or lead to other obvious playback errors. 25 In the following, a detailed description is presented how a stereo filling tool may be built into, as an extension, the xHE-AAC codec. When built into the standard, the stereo filling tool could be described as follows. In particular, such a stereo filling (SF) tool would represent a new tool in the frequency30 domain (FD) part of MPEG-H 3D-audio. In line with the above discussion, the aim of such a stereo filling tool would be the parametric reconstruction of MDCT spectral coefficients at low bitrates, similar to what already can be achieved with noise filling according to section 7.2 of the standard described in [3]. However, unlike noise filling, which employs a pseudorandom noise source for generating MDCT spectral values of any FD channel, SF 35 would be available also to reconstruct the MDCT values of the right channel of a jointly coded stereo pair of channels using a downmix of the left and right MDCT spectra of the 25 previous frame. SF, in accordance with the implementation set forth below, is signaled semi-backward-compatibly by means of the noise filling side information which can be parsed correctly by a legacy MPEG-D USAC decoder. 5 The tool description could be as follows. When SF is active in a joint-stereo FD frame, the MDCT coefficients of empty (i.e. fully zero-quantized) scale factor bands of the right (second) channel, such as 50d, are replaced by a sum or difference of the corresponding decoded left and right channels’ MDCT coefficients of the previous frame (if FD). If legacy noise filling is active for the second channel, pseudorandom values are also added to 10 each coefficient. The resulting coefficients of each scale factor band are then scaled such that the RMS (root of the mean coefficient square) of each band matches the value transmitted by way of that band’s scale factor. See section 7.3 of the standard in [3]. Some operational constraints could be provided for the use of the new SF tool in the 15 MPEG-D USAC standard. For example, the SF tool may be available for use only in the right FD channel of a common FD channel pair, i.e. a channel pair element transmitting a StereoCoreToolInfo( ) with common_window == 1. Besides, due to the semi-backwardcompatible signaling, the SF tool may be available for use only when noiseFilling == 1 in the syntax container UsacCoreConfig( ). If either of the channels in the pair is in LPD 20 core_mode, the SF tool may not be used, even if the right channel is in the FD mode. The following terms and definitions are used hereafter in order to more clearly describe the extension of the standard as described in [3]. 25 In particular, as far as the data elements are concerned, the following data element is newly introduced: stereo_filling binary flag indicating whether SF is utilized in the current frame and channel 30 Further, new help elements are introduced: noise_offset noise-fill offset to modify the scale factors of zero-quantized bands (section 7.2) 35 noise_level noise-fill level representing the amplitude of added spectrum noise (section 7.2) 26 downmix_prev[ ] downmix (i.e. sum or difference) of the previous frame’s left and right channels sf_index[g][sfb] scale factor index (i.e. transmitted integer) for window group g and band sfb 5 The decoding process of the standard would be extended in the following manner. In particular, the decoding of a joint-stereo coded FD channel with the SF tool being activated is executed in three sequential steps as follows: 10 First of all, the decoding of the stereo_filling flag would take place. stereo_filling does not represent an independent bit-stream element but is derived from the noise-fill elements, noise_offset and noise_level, in a UsacChannelPairElement() and the common_window flag in StereoCoreToolInfo(). If noiseFilling == 0 or common_window 15 == 0 or the current channel is the left (first) channel in the element, stereo_filling is 0, and the stereo filling process ends. Otherwise, if ((noiseFilling != 0) && (common_window != 0) && (noise_level == 0)) { stereo_filling = (noise_offset & 16) / 16; 20 noise_level = (noise_offset & 14) / 2; noise_offset = (noise_offset & 1) * 16; } else { stereo_filling = 0; 25 } In other words, if noise_level == 0, noise_offset contains the stereo_filling flag followed by 4 bits of noise filling data, which are then rearranged. Since this operation alters the values of noise_level and noise_offset, it needs to be performed before the noise filling 30 process of section 7.2. Moreover, the above pseudo-code is not executed in the left (first) channel of a UsacChannelPairElement( ) or any other element. Then, the calculation of downmix_prev would take place. 35 downmix_prev[ ], the spectral downmix which is to be used for stereo filling, is identical to the dmx_re_prev[ ] used for the MDST spectrum estimation in complex stereo prediction (section 7.7.2.3). This means that 27  All coefficients of downmix_prev[ ] must be zero if any of the channels of the frame and element with which the downmixing is performed – i.e. the frame before the currently decoded one – use core_mode == 1 (LPD) or the channels use unequal 5 transform lengths (split_transform == 1 or block switching to window_sequence == EIGHT_SHORT_SEQUENCE in only one channel) or usacIndependencyFlag == 1.  All coefficients of downmix_prev[ ] must be zero during the stereo filling process if the channel’s transform length changed from the last to the current frame (i.e. 10 split_transform == 1 preceded by split_transform == 0, or window_sequence == EIGHT_SHORT_SEQUENCE preceded by window_sequence != EIGHT_SHORT_SEQUENCE, or vice versa resp.) in the current element.  If transform splitting is applied in the channels of the previous or current frame, downmix_prev[ ] represents a line-by-line interleaved spectral downmix. See the 15 transform splitting tool for details.  If complex stereo prediction is not utilized in the current frame and element, pred_dir equals 0. 20 Consequently, the previous downmix only has to be computed once for both tools, saving complexity. The only difference between downmix_prev[ ] and dmx_re_prev[ ] in section 7.7.2 is the behavior when complex stereo prediction is not currently used, or when it is active but use_prev_frame == 0. In that case, downmix_prev[ ] is computed for stereo filling decoding according to section 7.7.2.3 even though dmx_re_prev[ ] is not needed for 25 complex stereo prediction decoding and is, therefore, undefined/zero. Thereinafter, the stereo filling of empty scale factor bands would be performed. If stereo_filling == 1, the following procedure is carried out after the noise filling process in 30 all initially empty scale factor bands sfb[ ] below max_sfb_ste, i.e. all bands in which all MDCT lines were quantized to zero. First, the energies of the given sfb[ ] and the corresponding lines in downmix_prev[ ] are computed via sums of the line squares. Then, given sfbWidth containing the number of lines per sfb[ ], 35 if (energy[sfb] < sfbWidth[sfb]) { /* noise level isn't maximum, or band starts below noise-fill region */ 28 facDmx = sqrt((sfbWidth[sfb] – energy[sfb]) / energy_dmx[sfb]); factor = 0.0; /* if the previous downmix isn't empty, add the scaled downmix lines such that band reaches unity energy */ 5 for (index = swb_offset[sfb]; index < swb_offset[sfb+1]; index++) { spectrum[window][index] += downmix_prev[window][index] * facDmx; factor += spectrum[window][index] * spectrum[window][index]; } if ((factor != sfbWidth[sfb]) && (factor > 0)) { /* unity energy isn't reached, so 10 modify band */ factor = sqrt(sfbWidth[sfb] / (factor + 1e-8)); for (index = swb_offset[sfb]; index < swb_offset[sfb+1]; index++) { spectrum[window][index] *= factor; } 15 } } for the spectrum of each group window. Then the scale factors are applied onto the resulting spectrum as in section 7.3, with the scale factors of the empty bands being 20 processed like regular scale factors. An alternative to the above extension of the xHE-AAC standard would use an implicit semi-backward compatible signaling method. 25 The above implementation in the xHE-AAC code framework describes an approach which employs one bit in a bitstream to signal usage of the new stereo filling tool, contained in stereo_filling, to a decoder in accordance with Fig. 2. More precisely, such signaling (let’s call it explicit semi-backward-compatible signaling) allows the following legacy bitstream data – here the noise filling side information – to be used independently of the SF 30 signalization: In the present embodiment, the noise filling data does not depend on the stereo filling information, and vice versa. For example, noise filling data consisting of allzeros (noise_level = noise_offset = 0) may be transmitted while stereo_filling may signal any possible value (being a binary flag, either 0 or 1). 35 In cases where strict independence between the legacy and the inventive bitstream data is not required and the inventive signal is a binary decision, the explicit transmission of a signaling bit can be avoided, and said binary decision can be signaled by the presence or 29 absence of what may be called implicit semi-backward-compatible signaling. Taking again the above embodiment as an example, the usage of stereo filling could be transmitted by simply employing the new signaling: If noise_level is zero and, at the same time, noise_offset is not zero, the stereo_filling flag is set equal to 1. If both noise_level and 5 noise_offset are not zero, stereo_filling is equal to 0. A dependent of this implicit signal on the legacy noise-fill signal occurs when both noise_level and noise_offset are zero. In this case, it is unclear whether legacy or new SF implicit signaling is being used. To avoid such ambiguity, the value of stereo_filling must be defined in advance. In the present example, it is appropriate to define stereo_filling = 0 if the noise filling data consists of all10 zeros, since this is what legacy encoders without stereo filling capability signal when noise filling is not to be applied in a frame. The issue which remains to be solved in the case of implicit semi-backward-compatible signaling is how to signal stereo_filling == 1 and no noise filling at the same time. As 15 explained, the noise filling data must not be all-zero, and if a noise magnitude of zero is requested, noise_level ((noise_offset & 14)/2 as mentioned above) must equal 0. This leaves only a noise_offset ((noise_offset & 1)*16 as mentioned above) greater than 0 as a solution. The noise_offset, however, is considered in case of stereo filling when applying the scale factors, even if noise_level is zero. Fortunately, an encoder can compensate for 20 the fact that a noise_offset of zero might not be transmittable by altering the affected scale factors such that upon bitstream writing, they contain an offset which is undone in the decoder via noise_offset. This allows said implicit signaling in the above embodiment at the cost of a potential increase in scale factor data rate. Hence, the signaling of stereo filling in the pseudo-code of the above description could be changed as follows, using the 25 saved SF signaling bit to transmit noise_offset with 2 bits (4 values) instead of 1 bit: if ((noiseFilling) && (common_window) && (noise_level == 0) && (noise_offset > 0)) { stereo_filling = 1; 30 noise_level = (noise_offset & 28) / 4; noise_offset = (noise_offset & 3) * 8; } else { stereo_filling = 0; 35 } 30 For the sake of completeness, Fig. 6 shows a parametric audio encoder in accordance with an embodiment of the present application. First of all, the encoder of Fig. 6 which is generally indicated using reference sign 90 comprises a transformer 92 for performing the transformation of the original, non-distorted version of the audio signal reconstructed at 5 the output 32 of Fig. 2. As described with respect to Fig. 3, a lapped transform may be used with a switching between different transform lengths with corresponding transform windows in units of frames 44. The different transform length and corresponding transform windows are illustrated in Fig. 3 using reference sign 104. In a manner similar to Fig. 2, Fig. 6 concentrates on a portion of encoder 90 responsible for encoding one channel of 10 the multichannel audio signal, whereas another channel domain portion of decoder 90 is generally indicated using reference sign 96 in Fig. 6. At the output of transformer 92 the spectral lines and scale factors are unquantized and substantially no coding loss has occurred yet. The spectrogram output by transformer 92 15 enters a quantizer 98, which is configured to quantize the spectral lines of the spectrogram output by transformer 92, spectrum by spectrum, setting and using preliminary scale factors of the scale factor bands. That is, at the output of quantizer 98, preliminary scale factors and corresponding spectral line coefficients result, and a sequence of a noise filler 16’, an optional inverse TNS filter 28a’, inter-channel predictor 24’, MS decoder 26’ and 20 inverse TNS filter 28b’ are sequentially connected so as to provide the encoder 90 of Fig. 6 with the ability to obtain a reconstructed, final version of the current spectrum as obtainable at the decoder side at the downmix provider’s input (see Fig. 2). In case of using inter-channel prediction 24’ and/or using the inter-channel noise filling in the version forming the inter-channel noise using the downmix of the previous frame, encoder 90 also 25 comprises a downmix provider 31’ so as to form a downmix of the reconstructed, final versions of the spectra of the channels of the multichannel audio signal. Of course, to save computations, instead of the final, the original, unquantized versions of said spectra of the channels may be used by downmix provider 31’ in the formation of the downmix. 30 The encoder 90 may use the information on the available reconstructed, final version of the spectra in order to perform inter-frame spectral prediction such as the aforementioned possible version of performing inter-channel prediction using an imaginary part estimation, and/or in order to perform rate control, i.e. in order to determine, within a rate control loop, that the possible parameters finally coded into data stream 30 by encoder 90 are set in a 35 rate/distortion optimal sense. 31 For example, one such parameter set in such a prediction loop and/or rate control loop of encoder 90 is, for each zero-quantized scale factor band identified by identifier 12’, the scale factor of the respective scale factor band which has merely been preliminarily set by quantizer 98. In a prediction and/or rate control loop of encoder 90, the scale factor of the 5 zero-quantized scale factor bands is set in some psychoacoustically or rate/distortion optimal sense so as to determine the aforementioned target noise level along with, as described above, an optional modification parameter also conveyed by the data stream for the corresponding frame to the decoder side. It should be noted that this scale factor may be computed using only the spectral lines of the spectrum and channel to which it belongs 10 (i.e. the “target” spectrum, as described earlier) or, alternatively, may be determined using both the spectral lines of the “target” channel spectrum and, in addition, the spectral lines of the other channel spectrum or the downmix spectrum from the previous frame (i.e. the “source” spectrum, as introduced earlier) obtained from downmix provider 31’. In particular to stabilize the target noise level and to reduce temporal level fluctuations in the decoded 15 audio channels onto which the inter-channel noise filling is applied, the target scale factor may be computed using a relation between an energy measure of the spectral lines in the “target” scale factor band, and an energy measure of the co-located spectral lines in the corresponding “source” region. Finally, as noted above, this “source” region may originate from a reconstructed, final version of another channel or the previous frame’s downmix, or 20 if the encoder complexity is to be reduced, the original, unquantized version of same other channel or the downmix of original, unquantized versions of the previous frame’s spectra. In the following, multichannel encoding and multichannel decoding according to embodiments is explained. In embodiments, the multichannel processor 204 of the 25 apparatus 201 for decoding of Fig. 1a may, e.g., be configured to conduct on or more of the technologies below that are described regarding noise multichannel decoding. At first, however, before describing multichannel decoding, multichannel encoding according to embodiments is explained with reference to Fig. 7 to Fig. 9 and, then, 30 multichannel decoding is explained with reference to Fig. 10 and Fig. 12. Now, multichannel encoding according to embodiments is explained with reference to Fig. 7 to Fig. 9 and Fig. 11: 35 Fig. 7 shows a schematic block diagram of an apparatus (encoder) 100 for encoding a multichannel signal 101 having at least three channels CH1 to CH3. 32 The apparatus 100 comprises an iteration processor 102, a channel encoder 104 and an output interface 106. The iteration processor 102 is configured to calculate, in a first iteration step, inter-channel 5 correlation values between each pair of the at least three channels CH1 to CH3 for selecting, in the first iteration step, a pair having a highest value or having a value above a threshold, and for processing the selected pair using a multichannel processing operation to derive multichannel parameters MCH_PAR1 for the selected pair and to derive first processed channels P1 and P2. In the following, such a processed channels P1 and such 10 a processed channel P2 may also be referred to as a combination channel P1 and a combination channel P2, respectively. Further, the iteration processor 102 is configured to perform the calculating, the selecting and the processing in a second iteration step using at least one of the processed channels P1 or P2 to derive multichannel parameters MCH_PAR2 and second processed channels P3 and P4. 15 For example, as indicated in Fig. 7, the iteration processor 102 may calculate in the first iteration step an inter-channel correlation value between a first pair of the at least three channels CH1 to CH3, the first pair consisting of a first channel CH1 and a second channel CH2, an inter-channel correlation value between a second pair of the at least 20 three channels CH1 to CH3, the second pair consisting of the second channel CH2 and a third channel CH3, and an inter-channel correlation value between a third pair of the at least three channels CH1 to CH3, the third pair consisting of the first channel CH1 and the third channel CH3. 25 In Fig. 7 it is assumed that in the first iteration step the third pair consisting of the first channel CH1 and the third channel CH3 comprises the highest inter-channel correlation value, such that the iteration processor 102 selects in the first iteration step the third pair having the highest inter-channel correlation value and processes the selected pair, i.e., the third pair, using a multichannel processing operation to derive multichannel 30 parameters MCH_PAR1 for the selected pair and to derive first processed channels P1 and P2. Further, the iteration processor 102 can be configured to calculate, in the second iteration step, inter-channel correlation values between each pair of the at least three channels 35 CH1 to CH3 and the processed channels P1 and P2, for selecting, in the second iteration step, a pair having a highest inter-channel correlation value or having a value above a 33 threshold. Thereby, the iteration processor 102 can be configured to not select the selected pair of the first iteration step in the second iteration step (or in any further iteration step). 5 Referring to the example shown in Fig. 7, the iteration processor 102 may further calculate an inter-channel correlation value between a fourth pair of channels consisting of the first channel CH1 and the first processed channel P1, an inter-channel correlation value between a fifth pair consisting of the first channel CH1 and the second processed channel P2, an inter-channel correlation value between a sixth pair consisting of the second 10 channel CH2 and the first processed channel P1, an inter-channel correlation value between a seventh pair consisting of the second channel CH2 and the second processed channel P2, an inter-channel correlation value between an eighth pair consisting of the third channel CH3 and the first processed channel P1, an inter-correlation value between a ninth pair consisting of the third channel CH3 and the second processed channel P2, 15 and an inter-channel correlation value between a tenth pair consisting of the first processed channel P1 and the second processed channel P2. In Fig. 7, it is assumed that in the second iteration step the sixth pair consisting of the second channel CH2 and the first processed channel P1 comprises the highest inter20 channel correlation value, such that the iteration processor 102 selects in the second iteration step the sixth pair and processes the selected pair, i.e., the sixth pair, using a multichannel processing operation to derive multichannel parameters MCH_PAR2 for the selected pair and to derive second processed channels P3 and P4. 25 The iteration processor 102 can be configured to only select a pair when the level difference of the pair is smaller than a threshold, the threshold being smaller than 40 dB, 25 dB, 12 dB or smaller than 6 dB. Thereby, the thresholds of 25 or 40 dB correspond to rotation angles of 3 or 0.5 degree. 30 The iteration processor 102 can be configured to calculate normalized integer correlation values, wherein the iteration processor 102 can be configured to select a pair, when the integer correlation value is greater than e.g. 0.2 or preferably 0.3. Further, the iteration processor 102 may provide the channels resulting from the 35 multichannel processing to the channel encoder 104. For example, referring to Fig. 7, the iteration processor 102 may provide the third processed channel P3 and the fourth 34 processed channel P4 resulting from the multichannel processing performed in the second iteration step and the second processed channel P2 resulting from the multichannel processing performed in the first iteration step to the channel encoder 104. Thereby, the iteration processor 102 may only provide those processed channels to the 5 channel encoder 104 which are not (further) processed in a subsequent iteration step. As shown in Fig. 7, the first processed channel P1 is not provided to the channel encoder 104 since it is further processed in the second iteration step. The channel encoder 104 can be configured to encode the channels P2 to P4 resulting 10 from the iteration processing (or multichannel processing) performed by the iteration processor 102 to obtain encoded channels E1 to E3. For example, the channel encoder 104 can be configured to use mono encoders (or mono boxes, or mono tools) 120_1 to 120_3 for encoding the channels P2 to P4 resulting from 15 the iteration processing (or multichannel processing). The mono boxes may be configured to encode the channels such that less bits are required for encoding a channel having less energy (or a smaller amplitude) than for encoding a channel having more energy (or a higher amplitude). The mono boxes 120_1 to 120_3 can be, for example, transformation based audio encoders. Further, the channel encoder 104 can be configured to use stereo 20 encoders (e.g., parametric stereo encoders, or lossy stereo encoders) for encoding the channels P2 to P4 resulting from the iteration processing (or multichannel processing). The output interface 106 can be configured to generate and encoded multichannel signal 107 having the encoded channels E1 to E3 and the multichannel parameters MCH_PAR1 25 and MCH_PAR2. For example, the output interface 106 can be configured to generate the encoded multichannel signal 107 as a serial signal or serial bit stream, and so that the multichannel parameters MCH_PAR2 are in the encoded signal 107 before the multichannel 30 parameters MCH_PAR1. Thus, a decoder, an embodiment of which will be described later with respect to Fig. 10, will receive the multichannel parameters MCH_PAR2 before the multichannel parameters MCH-PAR1. In Fig. 7 the iteration processor 102 exemplarily performs two multichannel processing 35 operations, a multichannel processing operation in the first iteration step and a multichannel processing operation in the second iteration step. Naturally, the iteration 35 processor 102 also can perform further multichannel processing operations in subsequent iteration steps. Thereby, the iteration processor 102 can be configured to perform iteration steps until an iteration termination criterion is reached. The iteration termination criterion can be that a maximum number of iteration steps is equal to or higher than a total number 5 of channels of the multichannel signal 101 by two, or wherein the iteration termination criterion is, when the inter-channel correlation values do not have a value greater than the threshold, the threshold preferably being greater than 0.2 or the threshold preferably being 0.3. In further embodiments, the iteration termination criterion can be that a maximum number of iteration steps is equal to or higher than a total number of channels of the 10 multichannel signal 101, or wherein the iteration termination criterion is, when the interchannel correlation values do not have a value greater than the threshold, the threshold preferably being greater than 0.2 or the threshold preferably being 0.3. For illustration purposes the multichannel processing operations performed by the 15 iteration processor 102 in the first iteration step and the second iteration step are exemplarily illustrated in Fig. 7 by processing boxes 110 and 112. The processing boxes 110 and 112 can be implemented in hardware or software. The processing boxes 110 and 112 can be stereo boxes, for example. 20 Thereby, inter-channel signal dependency can be exploited by hierarchically applying known joint stereo coding tools. In contrast to previous MPEG approaches, the signal pairs to be processed are not predetermined by a fixed signal path (e.g., stereo coding tree) but can be changed dynamically to adapt to input signal characteristics. The inputs of the actual stereo box can be (1) unprocessed channels, such as the channels CH1 to 25 CH3, (2) outputs of a preceding stereo box, such as the processed signals P1 to P4, or (3) a combination channel of an unprocessed channel and an output of a preceding stereo box. The processing inside the stereo box 110 and 112 can either be prediction based (like 30 complex prediction box in USAC) or KLT/PCA based (the input channels are rotated (e.g., via a 2x2 rotation matrix) in the encoder to maximize energy compaction, i.e., concentrate signal energy into one channel, in the decoder the rotated signals will be retransformed to the original input signal directions). 35 In a possible implementation of the encoder 100, (1) the encoder calculates an inter channel correlation between every channel pair and selects one suitable signal pair out of 36 the input signals and applies the stereo tool to the selected channels; (2) the encoder recalculates the inter channel correlation between all channels (the unprocessed channels as well as the processed intermediate output channels) and selects one suitable signal pair out of the input signals and applies the stereo tool to the selected channels; and (3) 5 the encoder repeats step (2) until all inter channel correlation is below a threshold or if a maximum number of transformations is applied. As already mentioned, the signal pairs to be processed by the encoder 100, or more precisely the iteration processor 102, are not predetermined by a fixed signal path (e.g., 10 stereo coding tree) but can be changed dynamically to adapt to input signal characteristics. Thereby, the encoder 100 (or the iteration processor 102) can be configured to construct the stereo tree in dependence on the at least three channels CH1 to CH3 of the multichannel (input) signal 101. In other words, the encoder 100 (or the iteration processor 102) can be configured to build the stereo tree based on an inter15 channel correlation (e.g., by calculating, in the first iteration step, inter-channel correlation values between each pair of the at least three channels CH1 to CH3, for selecting, in the first iteration step, a pair having the highest value or a value above a threshold, and by calculating, in a second iteration step, inter-channel correlation values between each pair of the at least three channels and previously processed channels, for selecting, in the 20 second iteration step, a pair having the highest value or a value above a threshold). According to a one step approach, a correlation matrix may be calculated for possibly each iteration containing the correlations of all, in previous iterations possibly processed, channels. 25 As indicated above, the iteration processor 102 can be configured to derive multichannel parameters MCH_PAR1 for the selected pair in the first iteration step and to derive multichannel parameters MCH_PAR2 for the selected pair in the second iteration step. The multichannel parameters MCH_PAR1 may comprise a first channel pair identification (or index) identifying (or signaling) the pair of channels selected in the first iteration step, 30 wherein the multichannel parameters MCH_PAR2 may comprise a second channel pair identification (or index) identifying (or signaling) the pair of channels selected in the second iteration step. In the following, an efficient indexing of input signals is described. For example, channel 35 pairs can be efficiently signaled using a unique index for each pair, dependent on the total 37 number of channels. For example, the indexing of pairs for six channels can be as shown in the following table: 0 1 2 3 4 5 0 0 1 2 3 4 1 5 6 7 8 2 9 10 11 3 12 13 4 14 5 5 For example, in the above table the index 5 may signal the pair consisting of the first channel and the second channel. Similarly, the index 6 may signal the pair consisting of the first channel and the third channel. The total number of possible channel pair indices for n channels can be calculated to: 10 numPairs = numChannels*(numChannels-1)/2 Hence, the number of bits needed for signaling one channel pair amount to: 15 numBits = floor(log2(numPairs-1))+1 Further, the encoder 100 may use a channel mask. The multichannel tool’s configuration may contain a channel mask indicating for which channels the tool is active. Thus, LFEs (LFE = low frequency effects/enhancement channels) can be removed from the channel 20 pair indexing, allowing for a more efficient encoding. E.g. for a 11.1 setup, this reduces the number of channel pair indices from 12*11/2=66 to 11*10/2 = 55, allowing signaling with 6 instead of 7 bit. This mechanism can also be used to exclude channels intended to be mono objects (e.g. multiple language tracks). On decoding of the channel mask (channelMask), a channel map (channelMap) can be generated to allow re-mapping of 25 channel pair indices to decoder channels. Moreover, the iteration processor 102 can be configured to derive, for a first frame, a plurality of selected pair indications, wherein the output interface 106 can be configured to 38 include, into the multichannel signal 107, for a second frame, following the first frame, a keep indicator, indicating that the second frame has the same plurality of selected pair indications as the first frame. 5 The keep indicator or the keep tree flag can be used to signal that no new tree is transmitted, but the last stereo tree shall be used. This can be used to avoid multiple transmission of the same stereo tree configuration if the channel correlation properties stay stationary for a longer time. 10 Fig. 8 shows a schematic block diagram of a stereo box 110, 112. The stereo box 110, 112 comprises inputs for a first input signal I1 and a second input signal I2, and outputs for a first output signal O1 and a second output signal O2. As indicated in Fig. 8, dependencies of the output signals O1 and O2 from the input signals I1 and I2 can be described by the s-parameters S1 to S4. 15 The iteration processor 102 can use (or comprise) stereo boxes 110,112 in order to perform the multichannel processing operations on the input channels and/or processed channels in order to derive (further) processed channels. For example, the iteration processor 102 can be configured to use generic, prediction based or KLT (Karhunen20 Loève-Transformation) based rotation stereo boxes 110,112. A generic encoder (or encoder-side stereo box) can be configured to encode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation: 25 . A generic decoder (or decoder-side stereo box) can be configured to decode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation: 30 . A prediction based encoder (or encoder-side stereo box) can be configured to encode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation 39 , wherein p is the prediction coefficient. 5 A prediction based decoder (or decoder-side stereo box) can be configured to decode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation: . 10 A KLT based rotation encoder (or encoder-side stereo box) can be configured to encode the input signals I1 to I2 to obtain the output signals O1 and O2 based on the equation: . 15 A KLT based rotation decoder (or decoder-side stereo box) can be configured to decode the input signals I1 and I2 to obtain the output signals O1 and O2 based on the equation (inverse rotation): . 20 In the following, a calculation of the rotation angle α for the KLT based rotation is described. The rotation angle for the KLT based rotation can be defined as: 25 with being the entries of a non-normalized correlation matrix, wherein , are the channel energies. 30 40 This can be implemented using the atan2 function to allow for differentiation between negative correlations in the numerator and negative energy difference in the denominator: alpha = 0.5*atan2(2*correlation[ch1][ch2], 5 (correlation[ch1][ch1] - correlation[ch2][ch2])); Further, the iteration processor 102 can be configured to calculate an inter-channel correlation using a frame of each channel comprising a plurality of bands so that a single inter-channel correlation value for the plurality of bands is obtained, wherein the iteration 10 processor 102 can be configured to perform the multichannel processing for each of the plurality of bands so that the multichannel parameters are obtained from each of the plurality of bands. Thereby, the iteration processor 102 can be configured to calculate stereo parameters in 15 the multichannel processing, wherein the iteration processor 102 can be configured to only perform a stereo processing in bands, in which a stereo parameter is higher than a quantized-to-zero threshold defined by a stereo quantizer (e.g., KLT based rotation encoder). The stereo parameters can be, for example, MS On/Off or rotation angles or prediction coefficients). 20 For example, the iteration processor 102 can be configured to calculate rotation angles in the multichannel processing, wherein the iteration processor 102 can be configured to only perform a rotation processing in bands, in which a rotation angle is higher than a quantized-to-zero threshold defined by a rotation angle quantizer (e.g., KLT based rotation 25 encoder). Thus, the encoder 100 (or output interface 106) can be configured to transmit the transformation/rotation information either as one parameter for the complete spectrum (full band box) or as multiple frequency dependent parameters for parts of the spectrum. 30 The encoder 100 can be configured to generate the bit stream 107 based on the following tables: Table 1 — Syntax of mpegh3daExtElementConfig() Syntax No. of bits Mnemonic 41 mpegh3daExtElementConfig() { usacExtElementType = escapedValue(4, 8, 16); usacExtElementConfigLength = escapedValue(4, 8, 16); if (usacExtElementDefaultLengthPresent) { 1 uimsbf usacExtElementDefaultLength = escapedValue(8, 16, 0) + 1; } else { usacExtElementDefaultLength = 0; } usacExtElementPayloadFrag; 1 uimsbf switch (usacExtElementType) { case ID_EXT_ELE_FILL: /* No configuration element */ break; case ID_EXT_ELE_MPEGS: SpatialSpecificConfig(); break; case ID_EXT_ELE_SAOC: SAOCSpecificConfig(); break; case ID_EXT_ELE_AUDIOPREROLL: /* No configuration element */ break; 42 case ID_EXT_ELE_UNI_DRC: mpegh3daUniDrcConfig(); break; case ID_EXT_ELE_OBJ_METADATA: ObjectMetadataConfig(); break; case ID_EXT_ELE_SAOC_3D: SAOC3DSpecificConfig(); break; case ID_EXT_ELE_HOA: HOAConfig(); break; case ID_EXT_ELE_MCC: /* multi channel coding */ MCCConfig(grp); break; case ID_EXT_ELE_FMT_CNVRTR /* No configuration element */ break; default: NOTE while (usacExtElementConfigLength--) { tmp; 8 uimsbf } break; } } NOTE: The default entry for the usacExtElementType is used for unknown extElementTypes so that legacy decoders can cope with future extensions. Table 21 — Syntax of MCCConfig(), Syntax No. of bits Mnemonic MCCConfig(grp) { nChannels = 0 for(chan=0;chan < bsNumberOfSignals[grp]; chan++) chanMask[chan] 1 if(chanMask[chan] > 0) { 43 mctChannelMap[nChannels]=chan; nChannels++; } } } NOTE: The corresponding ID_USAC_EXT element shall be prior to any audio element of the certain signal group grp. Table 32 — Syntax of MultichannelCodingBoxBandWise() Syntax No. of bits Mnemonic MultichannelCodingBoxBandWise() { for(pair=0; pair 0) { delta_code_time = 0; } else { delta_code_time; 1 } if (hasBandwiseAngles == 0) { hcod_angle[dpcm_alpha[pair][0]]; 1..10 vlclbf } else { for(j=0;j< numMaskBands;j++) { if (msMask[pair][j] ==1) { hcod_angle[dpcm_alpha[pair][j]]; 1..10 vlclbf } } } } } NOTE 1) nBits = floor(log2(nChannels*(nChannels-1)/2 – 1))+1 Table 4 — Syntax of MultichannelCodingBoxFullband() Syntax No. of bits Mnemonic MultichannelCodingBoxFullband() { for (pair=0; pair 0) { lastVal = alpha_prev_fullband[pair]; } else { lastVal = DEFAULT_ALPHA; } 35 newAlpha = lastVal + dpcm_alpha[pair][0]; if(newAlpha >= 64) { newAlpha -= 64; } 40 for (band=0; band < numMaskBands; band++){ /* set all angles to fullband angle */ 54 pairAlpha[pair][band] = newAlpha; /* set previous angles according to mctMask */ if(mctMask[pair][band] > 0) { 5 alpha_prev_frame[pair][band%mctBandsPerWindow] = newAlpha; } else { alpha_prev_frame[pair][band%mctBandsPerWindow] = DEFAULT_ALPHA; 10 } } alpha_prev_fullband[pair] = newAlpha; for(band=bandsPerWindow ; band 0) { lastVal = alpha_prev_frame[pair][band%mctBandsPerWindow]; 25 } else { if ((band % mctBandsPerWindow) == 0) { lastVal = DEFAULT_ALPHA; } 30 } if (msMask[pair][band] > 0 ) { newAlpha = lastVal + dpcm_alpha[pair][band]; if(newAlpha >= 64) { 35 newAlpha -= 64; } pairAlpha[pair][band] = newAlpha; alpha_prev_frame[pair][band%mctBandsPerWindow] = newAlpha; lastVal = newAlpha; 40 } else { alpha_prev_frame[pair][band%mctBandsPerWindow] = DEFAULT_ALPHA; /* -45° */ } 45 /* reset fullband angle */ alpha_prev_fullband[pair] = DEFAULT_ALPHA; } for(band=bandsPerWindow ; bandnumPairs; pair++) { mctBandOffset = 0; 50 /* inverse MCT rotation */ for (win = 0, group = 0; group MCCSignalingType == 0) { } else if (self->MCCSignalingType == 1) { /* apply fullband box */ 30 if (!self->bHasBandwiseAngles[pair] && !self- >bHasMctMask[pair]) { apply_mct_rotation(dmx, res, alphaSfb[0], nSamples); } else { 35 /* apply bandwise processing */ for (i = 0; i< mctBandsPerWindow; i++) { if (mctMask[i] == 1) { startLine = swb_offset [sfb]; stopLine = (sfb+2= totalSfb) { 50 break; } } } 57 } else if (self->MCCSignalingType == 2) { } else if (self->MCCSignalingType == 3) { 5 apply_mct_rotation(dmx, res, alpha, nSamples); } } For an application of KLT rotation the following c-code can be used. 10 apply_mct_rotation(*dmx, *res, alpha, nSamples) { for (n=0;n