Description

The present invention is concerned with a codec supporting a time-domain aliasing cancellation transform coding mode and a time-domain coding mode as well as forward aliasing cancellation for switching between both modes.

It is favorable to mix different coding modes in order to code general audio signals representing a mix of audio signals of different types such as speech, music or the like. The individual coding modes may be adapted for particular audio types, and thus, a multi-mode audio encoder may take advantage of changing the encoding mode over time corresponding to the change of the audio content type. In other words, the multi-mode audio encoder may decide, for example, to encode portions of the audio signal having speech content, using a coding mode especially dedicated for coding speech, and to use another coding mode in order encode different portions of the audio content representing non-speech content such as music. Time-domain coding modes such as codebook excitation linear prediction coding modes, tend to be more suitable for coding speech contents, whereas transform coding modes tend to outperform time-domain coding modes as far as the coding of music is concerned, for example.

There have already been solutions for addressing the problem of coping with the coexistence of different audio types within one audio signal. The currently emerging USAC, for example, suggests switching between a frequency domain coding mode largely complying with the AAC standard, and two further linear prediction modes similar to sub-frame modes of the AMR-WB plus standard, namely a MDCT (Modified Discrete Cosine Transformation) based variant of the TCX (TCX = transform coded excitation) mode and an ACELP (adaptive codebook excitation linear prediction) mode. To be more precise, in the AMR-WB+ standard, TCX is based on a DFT transform, but in USAC TCX has a MDCT transform base. A certain framing structure is used in order to switch between FD coding domain similar to AAC and the linear prediction domain similar to AMR-WB+. The AMR-WB+ standard itself uses an own framing structure forming a sub-framing structure relative to the USAC standard. The AMR-WB+ standard allows for a certain sub-division configuration sub-dividing the AMR-WB+ frames into smaller TCX and/or ACELP frames. Similarly, the AAC standard uses a basis framing structure, but allows for the use of different window lengths in order to transform code the frame content. For example, either a long window and an associated long transform length may be used, or eight short windows with associated transformations of shorter length.

MDCT causes aliasing. This is, thus, true, at TXC and FD frame boundaries. In other words, just as any frequency domain coder using MDCT, aliasing occurs at the window overlap regions, that is cancelled by the help of the neighbouring frames. That is, for any transitions between two FD frames or between two TCX (MDCT) frames or transition between either FD to TCX or TCX to FD, there is an implicit aliasing cancelation by the overlap/add procedure within the reconstruction at the decoding side. Then, there is no more aliasing after the overlap add. However, in case of transitions with ACELP, there is no inherent aliasing cancelation. Then, a new tool has to be introduced which may be called FAC (forward aliasing cancellation). FAC is to cancel the aliasing coming from the neighbouring frames if they are different from ACELP.

In other words, aliasing cancellation problems occur whenever transitions between transform coding mode and time domain coding mode, such as ACELP, occur. In order to perform the transformation from the time domain to the spectral domain as effective as possible, time-domain aliasing cancellation transform coding is used, such as MDCT, i.e. a coding mode using a overlapped transform where overlapping windowed portions of a signal are transformed using a transform according to which the number of transform coefficients per portion is less than the number of samples per portion so that aliasing occurs as far as the individual portions are concerned, with this aliasing being cancelled by time-domain aliasing cancellation, i. e. by adding the overlapping aliasing portions of neighboring re-transformed signal portions. MDCT is such a time-domain aliasing cancellation transform. Disadvantageously, the TDAC (time-domain aliasing cancellation) is not available at transitions between the TC coding mode and the time-domain coding mode.

In order to solve this problem, forward aliasing cancellation (FAC) may be used according to which the encoder signals within the data stream additional FAC data within a current frame whenever a change in the coding mode from transform coding to time-domain coding occurs. This, however, necessitates the decoder to compare the coding modes of consecutive frames in order to ascertain as to whether the currently decoded frame comprises FAC data within its syntax or not. This, in turn, means that there may be frames for which the decoder may not be sure as to whether same has to read or parse FAC data from the current frame or not. In other words, in case that one or more frames were lost during transmission, the decoder does not know for the immediately succeeding (received) frames as to whether a coding mode change occurred or not, and as to whether the bit stream of the current frame encoded data contains FAC data or not. Accordingly, the decoder has to discard the current frame and wait for the next frame. Alternatively, the decoder may parse the current frame by performing two decoding trials, one assuming that FAC data is present, and another assuming that FAC data is not present, with subsequently deciding as to whether one of both alternatives fails. The decoding process would most likely make the decoder crashing in one of the two conditions. That is, in reality, the latter possibility is not a feasible approach. The decoder should at any time know how to interpret the data and not rely on its own speculation on how to treat the data.

Accordingly, it is an object of the present invention to provide a codec which is more error robust or frame loss robust with, however, supporting switching between time-domain aliasing cancellation transform coding mode and time-domain coding mode.

This object is achieved by the subject matter of any of the independent claims attached herewith.

The present invention is based on the finding that a more error robust or frame loss robust codec supporting switching between time-domain aliasing cancellation transform coding mode and time-domain coding mode is achievable if a further syntax portion is added to the frames depending on which the parser of the decoder may select between a first action of expecting the current frame to comprise, and thus reading forward aliasing cancellation data from the current frame and a second action of not-expecting the current frame to comprise, and thus not reading forward aliasing cancellation data from the current frame. In other words, while a bit of coding efficiency is lost due to the provision of the second syntax portion, it is merely the second syntax portion which provides for the ability to use the codec in case of a communication channel with frame loss. Without the second syntax portion, the decoder would not be capable of decoding any data stream portion after a loss and will crash in trying to resume parsing. Thus, in an error prone environment, the coding efficiency is prevented from vanishing by the introduction of the second syntax portion..

Further preferred embodiments of the present invention are subject of the dependent claims. Further, preferred embodiments of the present invention are described in more detail below with regard to the figures. In particular

Figure 1 shows a schematic block diagram of a decoder according to an em- bodiment;

Figure 2 shows a schematic block diagram of an encoder according to an embodiment;

Figure 3 shows a block diagram of a possible implementation of the reconstruc- tor of Figure 2;

Figure 4 shows a block diagram of a possible implementation of the FD decoding module of Figure 3;

Figure 5 shows a block diagram of possible implementation of the LPD decoding modules of Figure 3;

Figure 6 shows schematic diagram illustrating the encoding procedure in order to generate FAC data in accordance with an embodiment;

Figure 7 shows a schematic diagram of the possible TDAC transform re- transform in accordance with an embodiment;

Figure 8, 9 show block diagrams for illustrating a path lineation of the FAC data at the encoder of a further processing in the encoder in order to test the coding mode change an optimization sense;

Figure 10, 11 show block diagrams of the decoder handling in order to arrive the FAC data figures 8 and 9 from the data stream;

Figure 12 shows a schematic diagram of the FAC based reconstruction the decod- ing side cross from boundaries frames of different coding mode;

Figures 13, 14 show schematically the processing performed at the transition handler of figure 3 in order to perform the reconstruction of figure 12;

Figure 15 to 19 show portions of a syntax structure in accordance with an embodiment;

and

Figure 20 to 22 show portions of a syntax structure in accordance with another embodiment.
Claims

Decoder (10) for decoding a data stream (12) comprising a sequence of frames into which time segments of an information signal (18) are coded, respectively, comprising

a parser (20) configured to parse the data stream (12) , wherein the parser is configured to, in parsing the data stream (12), read a first syntax portion (24) and a second syntax portion from a current frame (14b); and

a reconstructor (22) configured to reconstruct a current time segment (16b) of the information signal (18) associated with the current frame (14b) based on information (28) obtained from the current frame by the parsing, using a first selected one of a Time-Domain Aliasing Cancellation transform decoding mode and a time-domain decoding mode, the first selection depending on the first syntax portion (24),

wherein the parser (20) is configured to, in parsing the data stream (12), perform a second selected one of a first action of expecting the current frame (14b) to comprise, and thus reading forward aliasing cancellation data (34) from the current frame (14b) and a second action of not-expecting the current frame (14b) to comprise, and thus not reading forward aliasing cancellation data (34) from the current frame (14b, the second selection depending on the second syntax portion,

wherein the reconstructor (22) is configured to perform forward aliasing cancellation at a boundary between the current time segment (16b) and a previous time segment (16a) of a previous frame (14a) using the forward aliasing cancellation data (34).

Decoder (10) according to claim 1, wherein the first and second syntax portions are comprised by each frame, wherein the first syntax portion (24) associates the respective frame from which same has been read, with a first frame type or a second frame type and, if the respective frame is of the second frame type, associates sub frames of a sub division of the respective frame, composed of a number of sub frames, with a respective one of a first sub frame type and a second sub frame type, wherein the reconstructor (22) is configured to, if the first syntax portion (24) associates the respective frame with the first frame type, use frequency domain decoding as a first version of the time-domain aliasing cancellation transform decoding mode to reconstruct the time segment associated with the respective frame, and, if the first syntax portion

(24) associates the respective frame with the second frame type, use, for each sub frame of the respective frame, transform coded excitation linear prediction decoding as a second version of the time-domain aliasing cancellation transform decoding mode to reconstruct a sub portion of the time segment of the respective frame, which is associated with respected sub frame, if the first syntax portion (24) associates the respective sub frame of the respective frame with the first sub frame type, and code-book excitation linear prediction decoding as the time-domain decoding mode to reconstruct a sub portion of the time segment of the respective frame, which is associated with the respective sub frame, if the first syntax portion (24) associates the respective sub frame with a second sub frame type.

Decoder (10) according to claim 1 or 2, wherein the second syntax portion has a set of possible values each of which is uniquely associated with one of a set of possibilities comprising

the previous frame (14a) being of the first frame type,

the previous frame (14a) being of the second frame type with the last sub frame thereof being of the first sub frame type, and

the previous frame (14a) being of the second frame type with the last sub frame thereof being of the second sub frame type, and

the parser (20) is configured to perform the second selection based on a comparison between the second syntax portion of the current frame (14b) and the first syntax portion (24) of the previous frame (14a).

Decoder according to claim 3, wherein the parser (20) is configured to perform the reading of the forward aliasing cancellation data (34) from the current frame (14b), if the current frame (14b) is of the second frame type, depending on the previous frame (14a) being of the second frame type with the last sub frame thereof being of the first sub frame type or the previous frame (14a) being of the first frame type in that a forward aliasing cancellation gain is parsed from the forward aliasing cancellation data (34) in case of the previous frame (14a) being of the first frame type, and not if previous frame being of the second frame type with the last sub frame thereof being of the first sub frame type, wherein the reconstructor (22) is configured to perform the forward aliasing cancellation at an intensity which depends on the forward aliasing cancellation gain in case of the previous frame (14a) being of the first frame type.

Decoder (10) according to claim 4, wherein the parser (20) is configured to read, if the current frame (14b) is of the first frame type, a forward aliasing cancellation gain from the forward aliasing cancellation data (34) wherein the reconstructor is config- ured to perform the forward aliasing cancellation at an intensity which depends on the forward aliasing cancellation gain.

6. Decoder (10) according to claim 1 or 2, wherein the second syntax portion has a set of possible values each of which is uniquely associated with one of a set of possibilities comprising

the previous frame (14a) being of the first frame type with involving a long transform window,

the previous frame (14a) being of the first frame type with involving short transform windows,

the previous frame (14a) being of the second frame type with the last sub frame thereof being of the first sub frame type, and

the previous frame (14a) being of the second frame type with the last sub frame thereof being of the second sub frame type, and

the parser is configured to perform the second selection based on a comparison between the second syntax portion of the current frame (14b) and the first syntax portion (24) of the previous frame (14a), and perform the reading of the forward aliasing cancellation data (34) from the current frame (14b), if the previous frame (14a) is of the first frame type, depending on the previous frame (14a) involving the long transform window or short transform windows such that a amount of forward aliasing cancellation data (34) is greater if the previous frame (14a) involves the long transform window, and is lower if the previous frame (14a) involves the short transform windows.

7. Decoder (10) according to any of claims 2 to 6, wherein the reconstructor is configured to

per frame of the first frame type, perform a spectral varying de-quantization (70) of transform coefficient information within the respective frame of the first frame type based on scale factor information within the respective frame of the first frame type, and a re-transform on the de-quantized transform coefficient information to obtain a re-transformed signal segment (78) extending, in time, over and beyond the time segment associated with the respective frame of the first frame type, and

per frame of the second frame type,

per sub frame of the first sub frame type of the respective frame of the second frame type,

derive (94) a spectral weighting filter from LPC information within the respective frame of the second frame type,

spectrally weighting (96) transform coefficient information within the respective sub frame of the first sub frame type using the spectral weighting filter, and

re-transform (98) the spectrally weighted transform coefficient information to obtain a re-transformed signal segment extending, in time, over and beyond the sub portion of the time segment associated with the respective sub frame of the first sub frame type, and,

per sub frame of the second sub frame type of the respective frame of the second frame,

derive (100) an excitation signal from excitation update information within the respective sub frame of the second sub frame type and

perform LPC synthesis filtering (102) on the excitation signal using the LPC information within the respective frame of the second frame type in order to obtain an LP synthesized signal segment (110) for the sub portion of the time segment associated with the respective sub frame of the second sub frame type, and

perform time-domain aliasing cancellation within temporarily overlapping window portions at boundaries between time segments of immediately consecutive ones of frames of the first frame type and sub portions of time segments, which are associated with sub frames of the first sub frame type, to reconstruct the information signal (18) thereacross, and

if the previous frame is of the first frame type or of the second frame type with a last sub frame thereof being of the first sub frame type, and the current frame (14b) is of the second frame type with the first sub frame thereof being of the second sub frame type, derive a first forward aliasing cancellation synthesis signal from the forward aliasing cancellation data (34) and add the first forward aliasing cancellation synthesis signal to the re-transformed signal segment (78) within the previous time segment to reconstruct the information signal (18) across the boundary between the previous and current frames (14a, 14b), and

if the previous frame (14a) is of the second frame type with the first sub frame thereof being of the second sub frame type, and the current frame (14b) is of the first frame type or of the second frame type with a last sub frame thereof being of the first sub frame type, derive a second forward aliasing cancellation synthesis signal from the forward aliasing cancellation data (34) and add the second forward aliasing cancellation synthesis signal to the re-transformed signal segment within the current time segment (16b) to reconstruct the information signal (18) across the boundary between the previous and current time segments (16a, 16b).

Decoder (10) according to claim 7, wherein the reconstructor is configured to

derive the first forward aliasing cancellation synthesis signal from the forward aliasing cancellation data (34) by performing a re-transform on transform coefficient information comprised by the forward aliasing cancellation data (34) and/or

derive the second forward aliasing cancellation synthesis signal from the forward aliasing cancellation data (34) by performing a re-transform on transform coefficient information comprised by the forward aliasing cancellation data (34).

Decoder according to claim 7 or 8, wherein the second syntax portion comprises a first flag signaling as to whether forward aliasing cancellation data (34) is present or not in the respective frame, and the parser is configured to perform the second selection depending on the first flag, and wherein the second syntax portion further comprises a second flag merely within frames of the second frame type, the second flag signaling as to whether the previous frame is of the first frame type or of the second frame type with the last sub frame thereof being of the first sub frame type.

Decoder according to claim 9, wherein the parser is configured to perform the reading of the forward aliasing cancellation data (34) from the current frame (14b), if the current frame (14b) is of the second frame type, depending on the second flag in that a forward aliasing cancellation gain is parsed from the forward aliasing cancellation data (34) in case of the previous frame being of the first frame type, and not if previous frame being of the second frame type with the last sub frame thereof being of the first sub frame type, wherein the reconstructor is configured to perform the forward aliasing cancellation at an intensity which depends on the forward aliasing cancellation gain in case of the previous frame being of the first frame type.

1 1. Decoder according to claim 10, wherein the second syntax portion further comprises a third flag signaling as to whether the previous frame involves a long transform window or short transform windows, merely within frames of the second frame type if the second flag signals that the previous frame is of the first frame type, wherein the parser is configured to perform the reading of the forward aliasing cancellation data (34) from the current frame (14b) depending on the third flag such that an amount of forward aliasing cancellation data (34) is greater if the previous frame involves the long transform window, and is lower if the previous frame involves the short transform windows.

12. Decoder according to any of the claims 7 to 11, wherein the reconstructor is config- ured to, if the previous frame is of the second frame type with the last sub frame thereof being of the second sub frame type and the current frame (14b) is of the first frame type or the second frame type with the last sub frame thereof being of the first sub frame type, perform a windowing on the LP synthesis signal segment of the last sub frame of the previous frame to obtain a first aliasing cancellation signal segment and add the first aliasing cancellation signal segment to the re-transformed signal segment within the current time segment.

13. Decoder according to any of the claims 7 to 12, wherein the reconstructor is configured to, if the previous frame is of the second frame type with a last sub frame thereof being of the second sub frame type and the current frame (14b) is of the first frame type or the second frame type with the first sub frame thereof being of the first sub frame type, continue the LPC synthesis filtering performed on the excitation signal from the previous frame into the current frame, window a thus derived continuation of the LP synthesis signal segment of the previous frame within the current frame (14b) to obtain a second aliasing cancellation signal segment and add the second aliasing cancellation signal segment to the re-transformed signal segment within the current time segment.

14. Decoder according to any of the claims 1 to 13, wherein the parser (20) is configured to, in parsing the data stream (12), perform the second selection depending on the second syntax portion and independent from as to whether the current frame (14b) and the previous frame (14a) are coded using equal or different ones of the Time- Domain Aliasing Cancellation transform coding mode and the time-domain coding mode.

Encoder for encoding an information signal (18) into data stream (12) such that the data stream (12) comprises a sequence of frames into which time segments of the information signal (18) are coded, respectively, comprising

a constructor (42) configured to code a current time segment (16b) of the information signal (18) into information of the current frame (14b) using a first selected one of a Time-Domain Aliasing Cancellation transform coding mode and a time-domain coding mode; and

an inserter (44) configured to insert the information (28) into the current frame (14b) along with a first syntax portion (24) and a second syntax portion, wherein the first syntax portion (24) signals the first selection,

wherein the constructor (42) and inserter 44 are configured to

determine forward aliasing cancellation data (34)for forward aliasing cancellation at a boundary between the current time segment (16a) and a previous time segment of a previous frame and insert the forward aliasing cancellation data (34) into the current frame (14b) in case the current frame(14b) and the previous frame (14a) are encoded using different ones of the Time- Domain Aliasing Cancellation transform coding mode and the time-domain coding mode, and

refraining from inserting any forward aliasing cancellation data (34) into the current frame (14b) in case the current frame (14b) and the previous frame (14a) are encoded using equal ones of the Time-Domain Aliasing Cancellation transform coding mode and the time-domain coding mode,

wherein the second syntax portion (26) is set depending on as to whether the current frame (14b) and the previous frame (14a) are encoded using equal or different ones of the Time-Domain Aliasing Cancellation transform coding mode and the time-domain coding mode.

Encoder according to claim 15, wherein the encoder is configured to,

if the current frame (14b) and the previous frame (14a) are encoded using equal ones of the Time-Domain Aliasing Cancellation transform coding mode and the time- domain coding mode, set the second syntax portion to a first state signalling the absence of the forward aliasing cancellation data (34) in the current frame, and,

if the current frame (14b) and the previous frame (14a) are encoded using different ones of the Time-Domain Aliasing Cancellation transform coding mode and the time-domain coding mode, decide in a rate/distortion optimization sense, so as to

refrain from inserting the forward aliasing cancellation data (34) into the current frame (14b) although the current frame (14b) and the previous frame (14a) are encoded using different ones of the time-domain aliasing cancellation transform coding mode and the time-domain coding mode, with setting the second syntax portion such that same signals the absence of the forward aliasing cancellation data (34) in the current frame (14b), or

inserting the forward aliasing cancellation data (34) into the current frame (14b) with setting the second syntax portion such that same signals the insertion of the forward aliasing cancellation data (34) into the current frame (14b).

7. Method for decoding a data stream (12) comprising a sequence of frames into which time segments of an information signal (18) are coded, respectively, comprising

parsing the data stream (12), wherein parsing the data stream comprises reading a first syntax portion (24) and a second syntax portion from a current frame (14b); and

reconstructing a current time segment of the information signal (18) associated with the current frame (14b) based on information obtained from the current frame (14b) by the parsing, using a first selected one of a Time-Domain Aliasing Cancellation transform decoding mode and a time-domain decoding mode, the first selection depending on the first syntax portion (24),

wherein, in parsing the data stream (12), a second selected one of a first action of expecting the current frame (14b) to comprise, and thus reading forward aliasing cancellation data (34) from the current frame (14b) and a second action of not-expecting the current frame (14b) to comprise, and thus not reading forward aliasing cancellation data (34) from the current frame (14b) is performed, the second selection depending on the second syntax portion,

wherein the reconstructing comprises performing forward aliasing cancellation at a boundary between the current time segment and a previous time segment of a previous frame using the forward aliasing cancellation data (34).

Method for encoding an information signal (18) into data stream (12) such that the data stream (12) comprises a sequence of frames into which time segments of the information signal (18) are coded, respectively, comprising

coding a current time segment of the information signal (18) into information of the current frame (14b) using a first selected one of a Time-Domain Aliasing Cancellation transform encoding mode and a time-domain encoding mode; and

inserting the information into the current frame (14b) along with a first syntax portion (24) and a second syntax portion, wherein the first syntax portion (24) signals the first selection,

determining forward aliasing cancellation data (34) for forward aliasing cancellation at a boundary between the current time segment and a previous time segment of a previous frame and inserting the forward aliasing cancellation data (34) into the current frame (14b) in case the current frame (14b) and the previous frame are encoded using different ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode, and refraining from inserting any forward aliasing cancellation data (34) into the current frame (14b) in case the current frame (14b) and the previous frame are encoded using equal ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode,

wherein the second syntax portion is set depending on as to whether the current frame (14b) and the previous frame are encoded using equal or different ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode.

Data stream (12) comprising a sequence of frames into which time segments of an information signal (18) are coded, respectively, each frame comprising a first syntax portion (24), a second syntax portion, and information into which a time segment associated with the respective frame is coded using a first selected one of a Time-Domain Aliasing Cancellation transform coding mode and a time-domain coding

mode, the first selection depending on the first syntax portion (24) of the respective frame, wherein each frame comprises forward aliasing cancellation data (34) or not depending on the second syntax portion of the respective frame, wherein the second syntax portion indicates that the respective frame comprises forward aliasing cancel- lation data (34) of the respective frame and the previous frame are coded using different ones of the Time-Domain Aliasing Cancellation transform coding mode and the time-domain coding mode so that forward aliasing cancellation using the forward aliasing cancellation data (34) is possible at the boundary between the respective time segment and a previous time segment associated with the previous frame (14a).

20. A computer program having a program code for performing, when running on a computer, a method according to claim 17 or 18.