Coding of Spectral Coefficie: aectrum of an Audio Signal Description The present application is concerned with a coding scheme for spectral coefficients of a spectrum of an audio signal usable in, for example, various transform-based audio codecs. The context-based arithmetic coding is an efficient way of noiselessly encoding the spectral coefficients of a transform-based coder [1]. The context exploits the mutual information between a spectral coefficient and the already coded coefficients lying in its neighborhood. The context is available at both the encoder and decoder side and doesn't need any extra information to be transmitted. In this way, context-based entropy coding has the potential to provide higher gain over memoryless entropy coding. However in practice, the design of the context is seriously constrained due to amongst of others, the memory requirements, the computational complexity and the robustness to channel errors. These constrains limit the efficiency of the context-based entropy coding and engender a lower coding gain especially for tonal signals where the context has to be too limited for exploiting the harmonic structure of the signal. Moreover, in low delay audio transform ed-based coding, low-overlap windows are used to decrease the algorithmic delay. As a direct consequence, the leakage in the MDCT is important for tonal signals and results in a higher quantization noise. The tonal signals can be handled by combining the transform with prediction in frequency domain as it is done for MPEG2/4-AAC [2] or with a prediction in time-domain [3]. It would be favorable to have a coding concept at ha d which increases the coding efficiency. Accordingly, it is an object of the present invention to provide a coding concept for spectra! coefficients of a spectrum of an audio signal which increases the coding efficiency. This object is achieved by the subject matter of the pending independent claims It is a basic finding of the present application that the coding efficiency of coding spectral coefficients of a spectrum of an audio signal may be increased by en/decoding a currently to be en/decoded spectral coefficient by entropy en/decoding and, in doing so, to perform the entropy en/decoding depending, in a context-adaptive manner, on a previously en/decoded spectral coefficient, while adjusting a relative spectral distance between the previously en/decoded spectral coefficient and the currently en/decoded spectral coefficient depending on an information concerning a shape of the spectrum. The information concerning the shape of the spectrum may comprise a measure of a pitch or periodicity of the audio signal, a measure of an inter-harmonic distance of the audio signal's spectrum and/or relative locations of formants and/or valleys of a spectral envelope of the spectrum, and on the basis of this knowledge, the spectral neighborhood which is exploited in order to form the context of the currently to be en/decoded spectral coefficients may be adapted to the thus determined shape of the spectrum, thereby enhancing the entropy coding efficiency. Advantageous implementations are the subject of the dependent claims and preferred embodiments of the present application are described herein below with respect to the figures, among which Fig. 1 shows a schematic diagram illustrating a spectral coefficient encoder and its mode of operation in encoding the spectral coefficients of a spectrum of an audio signal; Fig. 2 shows a schematic diagram illustrating a spectral coefficient decoder fitting to the spectral coefficient encoder of Fig. 1; Fig. 3 shows a block diagram of a possible internal structure of the spectral coefficient encoder of Fig. 1 in accordance with an embodiment; Fig. 4 shows a block diagram of a possible internal structure of the spectral coefficient decoder of Fig. 2 in accordance with an embodiment; Fig. 5 schematically indicates a graph of a spectrum, the coefficients of which are to be encoded/decoded in order to illustrate the adaptation of the relative spectral distance depending on a measure of a pitch or periodicity of the audio signal or a measure of inter-harmonic distance; shows a schematic diagram illustrating a spectrum , the spectral coefficients of which are to be encoded /decoded in accordance with a embodiment where the spectrum is spectrally shaped according to an LP-based perceptually weighted synthesis filter, namely the inverse thereof, with illustrating the adaptation of the relative spectral distance depending on an inter-formant distance measure in accordance with an embodiment; schematically illustrates a portion of the spectrum in order to illustrate the context template surrounding the spectral coefficient to be currently coded/decoded and the adaptation of the context templates spectral spread depending on the information on the spectrum 's shape in accordance with an embodiment; shows a schematic diagram illustrating the mapping from the one or more values of the reference spectral coefficients of the context template 8 1 using a scalar function so as to derive the probability distribution estimation to be used for encoding/decoding the current spectral coefficient in accordance with an embodiment; schematically illustrates the usage of implicit signaling in order to synchronize the adaptation of the relative spectral distance between encoder and decoder; shows a schematic diagram illustrating the usage of explicit signaling in order to synchronize the adaptation of the relative spectral distance between encoder and decoder; shows a block diagram of a transform-based audio encoder in accordance with an embodiment; Fig. 10b shows a block diagram of a transform-based audio decoder fitting to the encoder of Fig. 10a; Fig 1a shows a block diagram of a transform-based audio encoder using frequency domain spectral shaping in accordance with an embodiment; Fig. 1b shows a block diagram of a transform-based audio decoder fitting to the encoder of Fig. 1 a; Fig. a shows a block diagram of a linear prediction-based transform-coded excitation audio encoder in accordance with an embodiment; Fig. 12b shows a linear-prediction based transform coded excitation audio decoder fitting to the encoder of Fig. 12a; Fig. 13 shows a block diagram of a transform-based audio encoder in accordance with a further embodiment; Fig. 14 shows a block diagram of a transform-based audio decoder fitting to the embodiment of Fig. 3; Fig. 15 shows a schematic diagram illustrating a conventional context or context template covering the neighborhood of a currently to be coded/decoded spectral coefficient; Figs. 16a-c show modified context template configurations or a mapped context in accordance with embodiments of the present application; Fig. 17 schematically illustrates a graph of a harmonic spectrum so as to illustrate the advantage of using the mapped context of any of Figs. 16a to 18c over the context template definition of Fig. 15 for a harmonic spectrum; Fig. 18 shows a f ow diagram of an algorithm for optimizing the relative spectrai distance D for the context mapping in accordance with an embodiment; Fig. 1 shows a spectral coefficient encoder accordance with an embodiment. The encoder is configured to encode spectra! coefficients of a spectrum of an audio signal. Fig. 1 illustrates sequential spectras in the f a spectrogram re precise, the spectra! coefficients 14 a e illustrated as boxes spectrotempcra!!y arranged along a temporal axis t and a frequency axis f . While it would be possible that the spectrotemporal resolution keeps constant, Fig. 1 illustrates that the spectrotemporal resolution may vary over time with one such time instant being illustrated in Fig. 1 at 16. This spectrogram 12 may be the result of a spectral decomposition transform applied to the audio signal 18 at different time instants, such as a lapped transform such as, for example, a criticallysampled transform, such as an MDCT or some other rea!-valued critically sampled transform. Insofar, spectrogram 12 may be received by spectral coefficient encoder 10 in the form of a spectrum 20 consisting of a sequence of transform coefficients each belonging to the same time instant. The spectra 20, thus respresent spectra! slices of the spectrogram and are illustrated in Fig. 1 as individual columns of spectrogram 12. Each spectrum is composed of a sequence of transform coefficients 14 and has been derived from a corresponding time frame 22 of audio signal 18 using, for example, some window function 24. In particular, the time frames 22 are sequentially arranged at the afore¬ mentioned time instances and are associated with the temporal sequence of spectra 20. They may, as illustrated in Fig. 1, overlap each other, just as the corresponding transform windows 24 may do. That is, as used herein, "spectrum" denotes spectral coefficients belonging to the same time instant and, thus, is a frequency decomposition. "Spectrogram" is a time-frequency decomposition made of consecutive spectra, wherein "Spectra" is the plural of spectrum. Sometimes, though, "spectrum" is used synonymously for spectrogram "transform coefficient" is used synonymously to "spectral coefficient", if original signal is in time domain and transformation is a frequency transformation. As just outlined, the spectral coefficient encoder 10 is for encoding the spectral coefficients 14 of spectrogram 1 of the audio signal 18 and to this end the encoder may, for example, apply a predetermined coding/decoding order which traverses, for example, the spectral coefficients 14 along a spectrotemporal path which, for example, scans the spectral coefficients 1 spectrally from ow to high frequency within one spectrum 20 and then proceeds with the spectral coefficients of the temporally succeeding spectrum 20 as outlined in Fig. 1 at 26. In a manner outlined in more detail below, the encoder 10 is configured to encode a currently to be encoded spectral coefficient, indicated using a small cross in Fig. 1, by entropy encoding depending, in a context-adaptive manner, on one or more previously encoded spectral coefficients, exemplarily indicated using a small circle in Fig. 1. In particular, the encoder 10 is configured so as to adjust a relative spectral distance between the previously encoded spectral coefficient and the currently encoded spectral coefficient depending on an information concerning a shape of the spectrum. As to the dependency and information concerning the shape of the spectrum, details are set out in the following along with considerations concerning the advantages resulting from the adaptation of the relative spectral distance 28 depending on the just mentioned information. n other words, the spectra! coefficient encoder 10 encodes the spectral coefficients 14 sequentially into a data stream 30. As wi l be outlined in more detail below, the spectral coefficient encoder 10 may be part of a transform-based encoder which, in addition to the spectral coefficients 14, encodes into data stream 30 further information so that the data stream 30 enables a reconstruction of the audio signal 18. Fig. 2 shows a spectral coefficient decoder 40 fitting to the spectral coefficient encoder 10 of Fig. 1. The functionality of the spectral coefficient decoder 40 is substantially a reversal of the spectral coefficient encoder 10 of Fig. 1: the spectral coefficient decoder 40 decodes the spectral coefficients 14 of the spectrum 1 using, for example, the decoding order 26 sequentially. In decoding a currently to be decoded spectral coefficient exemplarily indicated using the small cross in Fig. 2 by entropy decoding, spectral coefficient decoder 40 performs the entropy decoding depending, in a context-adaptive manner, on one or more previously decoded spectral coefficients also indicated by a small circle in Fig. 2 . In doing so, the spectral coefficient decoder 40 adjusts the relative spectral distance 28 between the previously decoded spectra! coefficient and the currently to be decoded spectral coefficient depending on the aforementioned information concerning the shape of the spectrum 1 . In the same manner as was indicated above, the spectral coefficient decoder 40 may be part of a transform-based decoder configured to reconstruct the audio signal 18 from data stream 30, from which spectral coefficient decoder 40 decodes the spectral coefficients 14 using entropy decoding. The latter transform-based decoder may, as a part of the reconstruction, subject the spectrum 1 to an inverse transformation such as, for example, an inverse lapped-transform, which for example results in a reconstruction of the sequence of overlapping windowed time frames 22 which, by an overlap-and-add process removes, for example, aliasing resulting from the spectral decomposition transform. As will be described in ore detail below, advantages resulting f om adjusting the relative spectral distance 28 depending on the information concerning the shape of the spectrum 12 relies on the ability to improve the probability distribution estimation used to entropy en/decode the current spectral coefficient x . The better the probability distribution estimation, the more efficient the entropy coding is, i.e. more compressed. The "probability distribution estimation" is an estimate of the actual probability distribution of the current spectral coefficient 14, i.e. a function which assigns a probability to each value of a domain of values which the current spectral coefficient may assume. Owing to the dependency of the adaptation of distance 28 on the spectrum's 12 shape, the probability distribution estimation may be determined so as to more closely correspond to the actual probability distribution, since the exploitation of the information on the spectrum's 12 shape enables to derive the probability distribution estimation from a spectral neighborhood of the current spectral coefficient x which allows a more accurate estimation of the probability distribution of the current spectral coefficient x . Details in this regard are presented below along with examples of the information on the spectrum's 12 shape. Before proceeding with specific examples of the aforementioned information on the spectrum's 12 shape, Figs. 3 and 4 show possible internal structures of spectral coefficient encoder 10 and spectral coefficient decoder 40, respectively. In particular, as shown in Fig. 3, the spectral coefficient encoder 10 may be composed of a probability distribution estimation derivator 42 and an entropy encoding engine 44, wherein, likewise, spectral coefficient decoder 40 may be composed of a probability distribution estimation derivator 52 and an entropy decoding engine 54. Probability distribution estimation derivators 42 and 52 operate in the same manner they derivate, on the basis of the value of the one or more previously decoded/encoded spectral coefficients o, the probability distribution estimation 56 for entropy decoding/encoding the current spectral coefficient x . In particular, the entropy encoding/decoding engine 44/54 receives the probability distribution estimation from derivator 42/52, and performs the entropy encoding/decoding regarding the current spectral coefficient x accordingly. The entropy encoding/decoding engine 44/54 may use, for example, variable length coding such as Huffman coding for encoding/decoding the current spectral coefficient x and in this regard, the engine 44/54 may use different VLC (variable length coding) tabies for different probability distribution estimations 56. Alternatively, engine 44/54 may use arithmetic encoding/decoding with respect to the current spectral coefficient x with the probability distribution estimation 56 controlling the probability interval subdivisioning of the current probability interval representing the arithmetic coding/decoding engines' 44/54 internal state, each partial interval being assigned to a different possible value out of a target range of values which may be assumed by the current spectral coefficient x . As will be outlined in more detail below, the entropy encoding engine and entropy decoding engine 44 and 54 may use an escape mechanism in order to map the spectral coefficient 's 14 overall value range onto a limited integer value interval, i.e. the target range, such as [Q...2 -1]. The set of integer values in the target range, i.e. {0 2 1} defines, along with an escape symbol {esc}, the symbol alphabet of the arithmetic encoding/decoding engine 44/54, i.e. {0,... ,2 esc}. For example, entropy encoding engine 44 subjects the inbound spectra! coefficient x to a division by 2 as often as needed, if any, in order to bring the spectral coefficient x into the aforementioned target interval [0...2 -1] with, for each division, encoding the escape symbol into data stream 30, followed by arithmetically encoding the division remainder - or the original spectral value in case of no division being necessary - into data stream 30. The entropy decoding engine 54, in turn, would implement the escape mechanism as follows: it would decode a current transform coefficient x from data stream 30 as a sequence of 0 , 1 or more escape symbols esc followed by a non-escape symbol, i.e. as one of sequences {a}, {esc, a}, {esc, esc, a}, ... . with a denoting the non-escape symbol. The entropy decoding engine 54 would, by arithmetically decoding the non-escape symbol, obtain a value a within the target interval [0...2 -1], for example, and would derive the coefficient value of x by computing the current spectral coefficient's value to be equal to a + 2 times the number of escape symbols. Different possibilities exist with respect to the usage of the probability distribution estimation 56 and the appliance of the same onto the sequence of symbols used to represent current spectral coefficient x : the probability distribution estimation may, for example, be applied onto any symbol conveyed within data stream 30 for spectral coefficient x , i.e. the non-escape symbol as we l as any escape symbol, if any. Alternatively, the probability distribution estimation 56 is merely used for the first or the first two or the first n= 0 ; lastNz -= 2 ) { iff ( x[2 *lndexPermutaion[lastNz/2]] != 0 ) \\ ( x[2* lndexPermutaion[lastNz/2]+ 1] != 0)) break; } lastNz += 2; lastNz/2 is coded on ceil(log2(A//2)) bits before the spectral components. Arithmetic encoder pseudo-code: put spectrum x[N] Input: contextMapping[N/2] Input: lastNz Output: coded bitstream for ( i = 0 ; i < N/2;i++) { whiie((i=lastNz/2)){ context[contextMapping[i]] = -1; ; if(i>=~N/2}{ Contrary to the previous section, two non-subsequent spectral coefficients can be gather in the same 2-tuple. Fo this reason, the context mapping for the two elements of the 2- tuple can point to two different indexes in the context table. In the preferred embodiment, we select the mapped context with the lowest index but one can also have a different rule, like averaging the two mapped contexts. For the same reason the update of the context should also be handled differently. If the 2 elements are consecut ve in the spectrum, we use the conventional way of computing the context. Otherwise, the context is updated separately for the 2 elements considering only its own magnitude. The decoding consists of the following steps: • Decode the flag to know if context mapping is performed • Decode the context mapping, by decoding either Dopt or the parameter adjustment parameters for getting Dopt for DO. • Decode lastNz • Decode the quantized spectrum as follows: Input: lastNz Input: contextMapping[N] Input: coded bitstream local: context[N/2] Output quantized spectrum x[N] for ( k=0, i = 0 ; k < lastnz ; k+=2){ a=b=0; / * Next coefficient to code*/ while(contextMapping[i]>=lastnz) x[ i++]=0; a1_i-i++: / * Next coefficient to code*/ while(contexiMapping[i]>=lasinz) x[ i++]=0; b1J=i++; / *Get context for the lowest index*/ i_min=min( contextMapping[a 1JJ, contextMapping[b 1J ]) t = contexf[(i_min/2)-2]«6 + context[(i_min/2)-1]; / * Store decoded data */ x[a 1 J = a; x[b1_i] = b; } Thus, above embodiments, inter alias revealed a , for example, pitch-based context mapping for entropy, such as arithmetic, coding of tonal signals. Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of the method steps may be executed by (or using) a hardware apparatus, like for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some one or more of the most important method steps may be executed by such an apparatus. The inventive encoded audio signal can be stored on a digital storage medium or can be transmitted on a transmission medium such as a wireless transmission medium or a wired transmission medium such as the Internet. Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example a floppy disk, a DVD, a Blu-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable. Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed. Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine readable carrier. Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier. In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer. A further embodiment of the inventive methods is. therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or nontransitionary. A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may for example be configured to be transferred via a data communication connection, for example via the Internet. A further embodiment comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein. A further embodiment according to the invention comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program for performing one of the methods described herein to a receiver. The receiver may, for example, be a computer, a mobile device, a memory device or the like. The apparatus or system may, for exam ple, comprise a file server for transferring the computer program to the receiver . In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus. The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein. References [1] Fuchs, G.; Subbaraman, V.; Multrus, M., "Efficient context adaptive entropy coding for real-time applications," Acoustics, Speech and Signal Processing (ICASSP), 201 1 IEEE International Conference on , vol., no., pp.493,496, 22-27 May 201 1 [2] ISO/IEC 13818, Part 7 , MPEG-2 AAC [3] Juin-Hwey Chen; Dongmei Wang, 'Transform predictive coding of wideband speech signals," Acoustics, Speech, and Signal Processing, 1996. ICASSP-96. Conference Proceedings., 1996 EEE International Conference on , vol.1 , no., pp.275,278 vol. 1, 7-10 May 1996 Claims 1 Decoder (40) configured to decode spectral coefficients (14) of a spectrum ( 12) of an audio signal (18), the decoder being configured to decode a currently to be decoded spectra! coefficient (x) by entropy decoding depending, in a contextadaptive manner, on a previously decoded spectral coefficient (o), with adjusting a relative spectral distance (28) between the previously decoded spectral coefficient (o) and the currently to be decoded spectral coefficient (x) depending on an information concerning a shape of the spectrum. 2 Decoder according to claim 1, wherein the information concerning a shape of the spectrum comprises at least one of a measure (60) of a pitch or periodicity of the audio signal (18); a measure of an inter-harmonic distance of the audio signal's spectrum (12); relative locations of formants (70) and/or valleys (72) of a spectral envelope of the spectrum. 3 Decoder according to claim 1 o 2, wherein the decoder (40) is configured to derive the information concerning the shape of the spectrum from explicit signalization. 4 Decoder according to claim 1 or 2 , wherein the decoder (40) is configured to derive the information concerning the shape of the spectrum from previously decoded spectral coefficients (o) or the previously decoded LPC-based spectral envelope of the spectrum. 5 Decoder according to any of the previous claims, wherein the decoder (40) is configured such that the dependence of the entropy decoding involves a plurality of previously decoded spectral coefficients (o), a spectra! spread of spectral positions of which is adjusted depending on the information concerning the shape of the spectrum. Decoder according to any of the previous claims, wherein the decoder (40) is configured such that the information concerning the shape of the spectrum is a measure (60) of a pitch of the audio signal and the decoder is configured to adjust the relative spectral distance (28) between the previously decoded spectral coefficient (o) and the currently to be decoded spectral coefficient (x) depending on the measure of the pitch such that the relative spectral distance increases with increasing pitch, or the information concerning the shape of the spectrum is a measure (60) of a periodicity of the audio signal and the decoder is configured to adjust the relative spectral distance (28) between the previously decoded spectral coefficient (o) and the currently to be decoded spectral coefficient (x) depending on the measure of periodicity such that the relative spectral distance decreases with increasing periodicity, or the information concerning the shape of the spectrum is a measure of an interharmonic distance of the audio signal's spectrum (12), and the decoder (40) is configured to adjust the relative spectral distance between the previously decoded spectral coefficient (o) and the currently to be decoded spectral coefficient (x) depending on the measure of the inter-harmonic distance such that the relative spectral distance increases with increasing inter-harmonic distance, or the information concerning the shape of the spectrum comprises relative locations of formants (70) and/or valleys (72) of a spectral envelope of the spectrum, and the decoder is configured to adjust the relative spectral distance between the previously decoded spectral coefficient and the currently to be decoded spectral coefficient depending on the location such that the relative spectral distance increases with increasing spectral distance (74) between the valleys in the spectral envelope and/or between the formants in the spectral envelope. Decoder according to any of the previous claims, wherein the decoder is configured to, in decoding the currently to be decoded spectral coefficient by entropy decoding, derive a probability distribution estimation (56) for the currently to be decoded spectral coefficient by subjecting the previously decoded spectral coefficient to a scalar function (82) and use the probability distribution estimation for the entropy decoding. Decoder according to any of the previous claims, wherein the decoder is configured to use arithmetic decoding as entropy decoding 9 . Decoder according to any of the previous claims, wherein the decoder is configured to decode the currently to be decoded spectral coefficient by spectrally and/or temporally predicting the currently to be decoded spectral coefficient and correcting the spectral and/or temporal prediction by a prediction residual obtained via the entropy decoding. 10. Transform-based audio decoder comprising a decoder configured to decode spectral coefficients of a spectrum of an audio signal according to any of the previous claims. 11. Transform-based audio decoder according to claim 10, wherein the decoder is configured to spectrally shape the spectrum by scaling the spectrum using scale factors ( 1 14). 12. Transform-based audio decoder according to claim 11, configured to determine the scale factors ( 114) based on linear prediction coefficient information so that the scale factors represent a transfer function depending on a linear prediction synthesis filter defined by the linear prediction coefficient information. 13. Transform-based audio decoder according to claim 12, wherein the transfer function's dependency on the linear prediction synthesis filter defined by the linear prediction coefficient information is such that the transfer function is perceptually weighted. 14 Transform-based audio decoder according to claim 13, wherein the transfer function's dependency on the linear prediction synthesis filter, 1/A(z), defined by the linear prediction information, is such that the transfer function is a transfer function of 1/A(k • z), where k nstant. Transform-based audio decoder according to any of claims 10 to 14, wherein the transform-based audio decoder supports ong term prediction harmonic or post filtering controlled via explicitly signaled long term prediction parameters, wherein the transform-based audio decoder is configured to derive the information concerning the shape of the spectrum from the explicitly signaled ong term prediction parameters. Encoder (10) configured to encode spectral coefficients (14) of a spectrum (12) of an audio signal (18), the encoder being configured to encode a currently to be encoded spectral coefficient (x) by entropy encoding depending, in a contextadaptive manner, on a previously encoded spectral coefficient (o), with adjusting a relative spectral distance (28) between the previously encoded spectral coefficient and the currently encoded spectral coefficient depending on an information concerning a shape of the spectrum. Method for decoding spectral coefficients (14) of a spectrum (12) of an audio signal (18), the method comprising decoding a currently to be decoded spectral coefficient (x) by entropy decoding depending, in a context-adaptive manner, on a previously decoded spectral coefficient (o), with adjusting a relative spectral distance (28) between the previously decoded spectral coefficient (o) and the currently to be decoded spectral coefficient (x) depending on an information concerning a shape of the spectrum. Method for encoding spectral coefficients (14) of a spectrum (12) of an audio signal (18), the method comprising encoding a currently to be encoded spectra! coefficient (x) by entropy encoding depending, in a context-adaptive manner, on a previously encoded spectral coefficient (o), with adjusting a relative spectral distance (28) between the previously encoded spectral coefficient and the currently encoded spectral coefficient depending on an information concerning a shape of the spectrum. Computer program having a program code for performing, when running on a computer, a method according to els or 1 .