Patent Description:
Generally speaking, embodiments according to the invention are related to a noise filling.

Audio coding concepts often encode an audio signal in the frequency domain. For example, the so-called "advanced audio coding" (AAC) concept encodes the contents of different spectral bins (or frequency bins), taking into consideration a psychoacoustic model. For this purpose, intensity information for different spectral bins is encoded. However, the resolution used for encoding intensities in different spectral bins is adapted in accordance with the psychoacoustic relevances of the different spectral bins. Thus, some spectral bins, which are considered as being of low psychoacoustic relevance, are encoded with a very low intensity resolution, such that some of the spectral bins considered to be of low psychoacoustic relevance, or even a dominant number thereof, are quantized to zero. Quantizing the intensity of a spectral bin to zero brings along the advantage that the quantized zero-value can be encoded in a very bit-saving manner, which helps to keep the bit rate as small as possible. Nevertheless, spectral bins quantized to zero sometimes result in audible artifacts, even if the psychoacoustic model indicates that the spectral bins are of low psychoacoustic relevance.

Therefore, there is a desire to deal with spectral bins quantized to zero, both in an audio encoder and an audio decoder.

Different approaches are known for dealing with spectral bins encoded to zero in transform-domain audio coding systems and also in speech coders.

For example, the MPEG-<NUM> "AAC" (advanced audio coding) uses the concept of perceptual noise substitution (PNS). The perceptional noise substitution fills complete scale factor bands with noise only. Details regarding the MPEG-<NUM> AAC may, for example, be found in the International Standard ISO/IEC <NUM>-<NUM> (Information Technology - Coding of Audio-Visual Objects - Part <NUM>: Audio). Furthermore, the AMR-WB+ speech coder replaces vector quantization vectors (VQ vectors) quantized to zero with a random noise vector, where each complex spectral value has a constant amplitude, but a random phase. The amplitude is controlled by one noise value transmitted with the bitstream. Details regarding the AMR-WB+ speech coder may, for example, be found in the technical specification entitled "Third Generation Partnership Project; Technical Specification Group Services and System Aspects; Audio Codec Processing Functions; Extended Adaptive Multi-Rate-Wide Band (AMR-WB+) Codec; Transcoding Functions (Release Six)", which is also known as "<NPL>".

Further, <CIT> describes an audio coding concept. The publication describes a means by which selected frequency bands of information from an original audio signal, which are audible, but which are perceptionally less relevant, need not be encoded, but may be replaced by a noise filling parameter. Those signal bands having content, which is perceptionally more relevant are, in contrast, fully encoded. Encoding bits are saved in this manner without leaving voids in the frequency spectrum of the received signal. The noise filling parameter is a measure of the RMS signal value within the band in question and is used at the reception end by a decoding algorithm to indicate the amount of noise to inject in the frequency band in question.

Further approaches provide for a non-guided noise insertion in the decoder, taking into account the tonality of the transmitted spectrum.

However, the conventional concepts typically bring along the problem that they either comprise a poor resolution regarding the granularity of the noise filling, which typically degrades the hearing impression, or require a comparatively large amount of noise filling side information, which requires extra bit rate.

The approaches known from <NPL>, and from <NPL>, address the aforementioned problem and allow for noise filling of individual spectral lines on the basis of a single value representing quantization noise or a mean quantization error.

In view of the above, there is the need for an improved concept of noise filling, which provides for an improved trade-off between the achievable hearing impression and the required bit rate.

An embodiment according to the invention creates a method for providing a decoded representation of an audio signal as set forth in claim <NUM>. Another embodiment according to the invention creates a computer program for performing said method, according to claim <NUM>.

The method is based on the finding that a single multi-band noise intensity value can be applied for a noise filling with good results if separate frequency band gain information is associated with the different frequency bands. Accordingly, an individual scaling of noise introduced in the different frequency bands is performed on the basis of the frequency band gain information, such that, for example, the single common multi-band noise intensity value provides, when taken in combination with separate frequency band gain information, sufficient information to introduce noise in a way adapted to human psychoacoustics. Thus, the concept described herein allows to apply a noise filling in the quantized (but non-rescaled) domain. The noise added in the method can be scaled with the psychoacoustic relevance of the band without requiring additional side information (beyond the side information, which is, anyway, required to scale the non-noise audio content of the frequency bands in accordance with the psychoacoustic relevance of the frequency bands).

<FIG> shows a block schematic diagram of an encoder for providing an audio stream on the basis of the transform-domain representation of an input audio signal, which can be used in connection with embodiments of the invention.

The encoder <NUM> of <FIG> comprises a quantization error calculator <NUM> and an audio stream provider <NUM>. The quantization error calculator <NUM> is configured to receive an information <NUM> regarding a first frequency band, for which a first frequency band gain information is available, and an information <NUM> about a second frequency band, for which a second frequency band gain information is available. The quantization error calculator is configured to determine a multi-band quantization error over a plurality of frequency bands of the input audio signal, for which separate band gain information is available. For example, the quantization error calculator <NUM> is configured to determine the multi-band quantization error over the first frequency band and the second frequency band using the information <NUM>, <NUM>. Accordingly, the quantization error calculator <NUM> is configured to provide the information <NUM> describing the multi-band quantization error to the audio stream provider <NUM>. The audio stream provider <NUM> is configured to also receive an information <NUM> describing the first frequency band and an information <NUM> describing the second frequency band. In addition, the audio stream provider <NUM> is configured to provide an audio stream <NUM>, such that the audio stream <NUM> comprises a representation of the information <NUM> and also a representation of the audio content of the first frequency band and of the second frequency band.

Accordingly, the encoder <NUM> provides an audio stream <NUM>, comprising an information content, which allows for an efficient decoding of the audio content of the frequency band using a noise filling. In particular, the audio stream <NUM> provided by the encoder brings along a good trade-off between bit rate and noise-filling-decoding-flexibility.

In the following, an improved audio coder, which can be used in connection with embodiments of the invention, will be described, which is based on the audio encoder described in the <NPL>.

The audio encoder <NUM> according to <FIG> is specifically based on the audio encoder described in <NPL>.

The audio encoder <NUM> may, for example, be configured to receive an input time signal <NUM> and to provide, on the basis thereof, a coded audio stream <NUM>. A signal processing path may comprise an optional downsampler <NUM>, an optional AAC gain control <NUM>, a block-switching filterbank <NUM>, an optional signal processing <NUM>, an extended AAC encoder <NUM> and a bit stream payload formatter <NUM>. However, the encoder <NUM> typically comprises a psychoacoustic model <NUM>.

In a very simple case, the encoder <NUM> only comprises the blockswitching/filter bank <NUM>, the extended AAC encoder <NUM>, the bit stream payload formatter <NUM> and the psychoacoustic model <NUM>, while the other components (in particular, components <NUM>, <NUM>, <NUM>) should be considered as merely optional.

In a simple case, the block-switching/filter bank <NUM>, receives the input time signal <NUM> (optionally downsampled by the downsampler <NUM>, and optionally scaled in gain by the AAC gain controller <NUM>), and provides, on the basis thereof, a frequency domain representation 224a. The frequency domain representation 224a may, for example, comprise an information describing intensities (for example, amplitudes or energies) of spectral bins of the input time signal <NUM>. For example, the block-switching/filter bank <NUM>, may be configured to perform a modified discrete cosine transform (MDCT) to derive the frequency domain values from the input time signal <NUM>. The frequency domain representation 224a may be logically split in different frequency bands, which are also designated as "scale factor bands". For example, it is assumed that the block-switching/ filter bank <NUM>, provides spectral values (also designated as frequency bin values) for a large number of different frequency bins. The number of frequency bins is determined, among others, by the length of a window input into the filterbank <NUM>, and also dependent on the sampling (and bit) rate. However, the frequency bands or scale factor bands define sub-sets of the spectral values provided by the block-switching/filterbank. Details regarding the definition of the scale factor bands are known to the man skilled in the art, and also described in ISO/IEC <NUM>-<NUM>: <NUM>(E), Part <NUM>, Sub-part <NUM>.

The extended AAC encoder <NUM> receives the spectral values 224a provided by the block-switching/filterbank <NUM> on the basis of the input time signal <NUM> (or a pre-processed version thereof) as an input information 228a. As can be seen from <FIG>, the input information 228a of the extended AAC encoder <NUM> may be derived from the spectral values 224a using one or more of the processing steps of the optional spectral processing <NUM>. For details regarding the optional pre-processing steps of the spectral processing <NUM>, reference is made to ISO/IEC <NUM>-<NUM>: <NUM>(E), and to further Standards referenced therein.

The extended AAC encoder <NUM> is configured to receive the input information 228a in the form of spectral values for a plurality of spectral bins and to provide, on the basis thereof, a quantized and noiselessly coded representation 228b of the spectrum. For this purpose, the extended AAC encoder <NUM> may, for example, use information derived from the input audio signal <NUM> (or a pre-processed version thereof) using the psychoacoustic model <NUM>. Generally speaking, the extended AAC encoder <NUM> may use an information provided by the psychoacoustic model <NUM> to decide which accuracy should be applied for the encoding of different frequency bands (or scale factor bands) of the spectral input information 228a. Thus, the extended AAC encoder <NUM> may generally adapt its quantization accuracy for different frequency bands to the specific characteristics of the input time signal <NUM>, and also to the available number of bits. Thus, the extended AAC encoder may, for example, adjust its quantization accuracies, such that the information representing the quantized and noiselessly coded spectrum comprises an appropriate bit rate (or average bit rate).

The bit stream payload formatter <NUM> is configured to include the information 228b representing the quantized and noiselessly coded spectra into the coded audio stream <NUM> according to a predetermined syntax.

For further details regarding the functionality of the encoder components described here, reference is made to ISO/IEC <NUM>-<NUM>: <NUM>(E) (including annex <NUM>. B thereof), and also to ISO/IEC <NUM>-<NUM>: <NUM>.

Further, reference is made to ISO/IEC <NUM>-<NUM>: <NUM>, Sub-clauses C1 to C9.

Furthermore, specific reference regarding the terminology is made to ISO/IEC <NUM>-<NUM>: <NUM>(E), Part <NUM>: Audio, Sub-part <NUM>: Main.

In addition, specific reference is made to ISO/IEC <NUM>-<NUM>: <NUM>(E), Part <NUM>: Audio, Sub-part <NUM>: General Audio Coding (GA) - AAC, Twin VQ, BSAC.

In the following, details regarding the encoder will be described taking reference to <FIG>, <FIG>, <FIG> and <FIG>.

<FIG> and <FIG> show a block schematic diagram of an extended AAC encoder which can be used in connection with embodiments of the invention. The extended AAC decoder is designated with <NUM> and can take the place of the extended AAC encoder <NUM> of <FIG>. The extended AAC encoder <NUM> is configured to receive, as an input information 228a, a vector of magnitudes of spectral lines, wherein the vector of spectral lines is sometimes designated with mdct_line (<NUM>. The extended AAC encoder <NUM> also receives a codec threshold information 228c, which describes a maximum allowed error energy on a MDCT level. The codec threshold information 228c is typically provided individually for different scale factor bands and is generated using the psychoacoustic model <NUM>. The codec threshold information <NUM> is sometimes designated with xmin (sb), wherein the parameter sb indicates the scale factor band dependency. The extended AAC encoder <NUM> also receives a bit number information 228d, which describes a number of available bits for encoding the spectrum represented by the vector 228a of magnitudes of spectral values. For example, the bit number information 228d may comprise a mean bit information (designated with mean bits) and an additional bit information (designated with more_bits). The extended AAC encoder <NUM> is also configured to receive a scale factor band information 228e, which describes, for example, a number and width of scale factor bands.

The extended AAC encoder comprises a spectral value quantizer <NUM>, which is configured to provide a vector <NUM> of quantized values of spectral lines, which is also designated with x_quant (<NUM>. The spectral value quantizer <NUM>, which includes a scaling, is also configured to provide a scale factor information <NUM>, which may represent one scale factor for each scale factor band and also a common scale factor information. Further, the spectral value quantizer <NUM> may be configured to provide a bit usage information <NUM>, which may describe a number of bits used for quantizing the vector 228a of magnitudes of spectral values. Indeed, the spectral value quantizer <NUM> is configured to quantize different spectral values of the vector 228a with different accuracies depending on the psychoacoustic relevance of the different spectral values. For this purpose, the spectral value quantizer <NUM> scales the spectral values of the vector 228a using different, scale-factor-band-dependent scale factors and quantizes the resulting scaled spectral values. Typically, spectral values associated with psychoacoustically important scale factor bands will be scaled with large scale factors, such that the scaled spectral values of psychoacoustically important scale factor bands cover a large range of values. In contrast, the spectral values of psychoacoustically less important scale factor bands are scaled with smaller scale factors, such that the scaled spectral values of the psychoacoustically less important scale factor bands cover a smaller range of values only. The scaled spectral values are then quantized, for example, to an integral value. In this quantization, many of the scaled spectral values of the psychoacoustically less important scale factor bands are quantized to zero, because the spectral values of the psychoacoustically less important scale factor bands are scaled with a small scale factor only.

As a result, it can be said that spectral values of psychoacoustically more relevant scale factor bands are quantized with high accuracy (because the scaled spectral lines of said more relevant scale factor bands cover a large range of values and, therefore, many quantization steps), while the spectral values of the psychoacoustically less important scale factor bands are quantized with lower quantization accuracy (because the scaled spectral values of the less important scale factor bands cover a smaller range of values and are, therefore, quantized to less different quantization steps).

The spectral value quantizer <NUM> is typically configured to determine appropriate scaling factors using the codec threshold 228c and the bit number information 228d. Typically, the spectral value quantizer <NUM> is also configured to determine the appropriate scale factors by itself. Details regarding a possible implementation of the spectral value quantizer <NUM> are described in <NPL>. In addition, the implementation of the spectral value quantizer is well known to a man skilled in the art of MPEG4 encoding.

The extended AAC encoder <NUM> also comprises a multi-band quantization error calculator <NUM>, which is configured to receive, for example, the vector 228a of magnitudes of spectral values, the vector <NUM> of quantized-values of spectral lines and the scale factor information314. The multi-band quantization error calculator <NUM> is, for example, configured to determine a deviation between a non-quantized scaled version of the spectral values of the vector 228a (for example, scaled using a non-linear scaling operation and a scale factor) and a scaled-and-quantized version (for example, scaled using a non-linear scaling operation and a scale factor, and quantized using an "integer" rounding operation) of the spectral values. In addition, the multi-band quantization error calculator <NUM> may be configured to calculate an average quantization error over a plurality of scale factor bands. It should be noted that the multi-band quantization error calculator <NUM> preferably calculates the multi-band quantization error in a quantized domain (more precisely in a psychoacoustically scaled domain), such that a quantization error in psychoacoustically relevant scale factor bands is emphasized in weight when compared to a quantization error in psychoacoustically less relevant scale factor bands. Details regarding the operation of the multi-band quantization error calculator will subsequently be described taking reference to <FIG> and <FIG>.

The extended AAC encoder <NUM> also comprises a scale factor adaptor <NUM>, which is configured to receive the vector <NUM> of quantized values, the scale factor information <NUM> and also the multi-band quantization error information <NUM>, provided by the multi-band quantization error calculator <NUM>. The scale factor adaptor <NUM> is configured to identify scale factor bands, which are "quantized to zero", i.e. scale factor bands for which all the spectral values (or spectral lines) are quantized to zero. For such scale factor bands quantized entirely to zero, the scale factor adaptor <NUM> adapts the respective scale factor. For example, the scale factor adaptor <NUM> may set the scale factor of a scale factor band quantized entirely to zero to a value, which represents a ratio between a residual energy (before quantization) of the respective scale factor band and an energy of the multi-band quantization error <NUM>. Accordingly, the scale factor adaptor <NUM> provides adapted scale factors <NUM>. It should be noted that both the scale factors provided by the spectral value quantizer <NUM> and the adapted scale factors provided by the scale factor adaptor are designated with "scale factor (sb)", "scf[band]", "sf[g][sfb]", "scf[g][sfb]" in the literature and also within this application. Details regarding the operation of the scale factor adaptor <NUM> will subsequently be described taking reference to <FIG> and <FIG>.

The extended AAC encoder <NUM> also comprises a noiseless coding <NUM>, which is, for example, explained in <NPL>. In brief, the noiseless coding <NUM> receives the vector of quantized values of spectral lines (also designated as "quantized values of the spectra") <NUM>, the integer representation <NUM> of the scale factors (either as provided by the spectral value quantizer <NUM>, or as adapted by the scale factor adaptor <NUM>), and also a noise filling parameter <NUM> (for example, in the form of a noise level information) provided by the multi-band quantization error calculator <NUM>.

The noiseless coding <NUM> comprises a spectral coefficient encoding 350a to encode the quantized values <NUM> of the spectral lines, and to provide quantized and encoded values <NUM> of the spectral lines. Details regarding the spectral coefficient encoding are, for example, described in sections <NUM>. <NUM>, <NUM>. <NUM>, <NUM>. <NUM> and <NUM>. <NUM> of ISO/IEC <NUM>-<NUM>: <NUM>. The noiseless coding <NUM> also comprises a scale factor encoding 350b for encoding the integer representation <NUM> of the scale factor to obtain an encoded scale factor information <NUM>. The noiseless coding <NUM> also comprises a noise filling parameter encoding 350c to encode the one or more noise filling parameters <NUM>, to obtain one or more encoded noise filling parameters <NUM>. Consequently, the extended AAC encoder provides an information describing the quantized as noiselessly encoded spectra, wherein this information comprises quantized and encoded values of the spectral lines, encoded scale factor information and encoded noise filling parameter information.

In the following, the functionality of the multi-band quantization error calculator <NUM> and of the scale factor adaptor <NUM>, which are key components of the inventive extended AAC encoder <NUM> will be described, taking reference to <FIG> and <FIG>. For this purpose, <FIG> shows a program listing of an algorithm performed by the multi-band quantization error calculator <NUM> and the scale factor adaptor <NUM>.

A first part of the algorithm, represented by lines <NUM> to <NUM> of the pseudo code of <FIG>, comprises a calculation of a mean quantization error, which is performed by the multi-band quantization error calculator <NUM>. The calculation of the mean quantization error is performed, for example, over all scale factor bands, except for those which are quantized to zero. If a scale factor band is entirely quantized to zero (i.e. all spectral lines of the scale factor band are quantized to zero), said scale factor band is skipped for the calculation of the mean quantization error. If, however, a scale factor band is not entirely quantized to zero (i.e. comprises at least one spectral line, which is not quantized to zero), all the spectral lines of said scale factor band are considered for the calculation of the mean quantization error. The mean quantization error is calculated in a quantized domain (or, more precisely, in a scaled domain). The calculation of a contribution to the average error can be seen in line <NUM> of the pseudo code of <FIG>. In particular, line <NUM> shows the contribution of a single spectral line to the average error, wherein the averaging is performed over all the spectral lines (wherein nLines indicates the number of total considered lines).

As can be seen in line <NUM> of the pseudo code, the contribution of a spectral line to the average error is the absolute value ("fabs"- operator) of a difference between a non-quantized, scaled spectral line magnitude value and a quantized, scaled spectral line magnitude value. In the non-quantized, scaled spectral line magnitude value, the magnitude value "line" (which may be equal to mdct_line) is non-linearly scaled using a power function (pow(line, <NUM>) = line<NUM>) and using a scale factor (e.g. a scale factor <NUM> provided by the spectral value quantizer <NUM>). In the calculation of the quantized, scaled spectral line magnitude value, the spectral line magnitude value "line" may be non-linearly scaled using the above-mentioned power functions and scaled using the above-mentioned scale factor. The result of this non-linear and linear scaling may be quantized using an integer operator "(INT)". Using the calculation as indicated in line <NUM> of the pseudo code, the different impact of the quantization on the psychoacoustically more important and the psychoacoustically less important frequency bands is considered.

Following the calculation of the (average) multi-band quantization error (avgError), the average quantization error may optionally be quantized, as shown in lines <NUM> and <NUM> of the pseudo code. It should be noted that the quantization of the multi-band quantization error as shown here is specifically adapted to the expected range of values and statistical characteristics of the quantization error, such that the quantization error can be represented in a bit-efficient way. However, other quantizations of the multi-band quantization error can be applied.

A third part of the algorithm, which is represented in lines <NUM> to <NUM>, may be executed by the scale factor adaptor <NUM>. The third part of the algorithm serves to set scale factors of scale factor frequency bands, which have been entirely quantized to zero, to a well-defined value, which allows for a simple noise filling, which brings along a good hearing impression. The third part of the algorithm optionally comprises an inverse quantization of the noise level (e.g. represented by the multi-band quantization error <NUM>). The third part of the algorithm also comprises a calculation of a replacement scale factor value for scale factor bands quantized to zero (while scale factors of scale factor bands not quantized to zero will be left unaffected). For example, the replacement scale factor value for a certain scale factor band ("band") is calculated using the equation shown in line <NUM> of the algorithm of <FIG>. In this equation, "(INT)" represents an integer operator, "<NUM>. f" represents the number "<NUM>" in a floating point representation, "log" designates a logarithm operator, "energy" designates an energy of the scale factor band under consideration (before quantization), "(float)" designates a floating point operator, "sfbWidth" designates a width of the certain scale factor band in terms of spectral lines (or spectral bins), and "noiseVal" designates a noise value describing the multi-band quantization error. Consequently, the replacement scale factor describes a ratio between an average per-frequency-bin energy (energy/sfbWidth) of the certain scale factor bands under consideration, and an energy (noiseVal<NUM>) of the multi-band quantization error.

This specification describes an encoder having a new type of noise level calculation. The noise level is calculated in the quantized domain based on the average quantization error.

Calculating the quantization error in the quantized domain brings along significant advantages, for example, because the psychoacoustic relevance of different frequency bands (scale factor bands) is considered. The quantization error per line (i.e. per spectral line, or spectral bin) in the quantized domain is typically in the range [-<NUM>; <NUM>] (<NUM> quantization level) with an average absolute error of <NUM> (for normal distributed input values that are usually larger than <NUM>). Using an encoder, which provides information about a multi-band quantization error, the advantages of noise filling in the quantized domain can be exploited in an encoder, as will subsequently be described.

Noise level calculation and noise substitution detection in the encoder may comprise the following steps:.

An appropriate noise level quantization may help to produce the number of bits required for transporting the information describing the multi-band quantization error. For example, the noise level may be quantized in <NUM> quantization levels in the logarithmic domain, taking into account human perception of loudness. For instance, the algorithm shown in <FIG> may be used, wherein "(INT)" designates an integer operator, wherein "LD" designates a logarithm operation for a base of <NUM>, and wherein "meanLineError" designates a quantization error per frequency line. )" designates a minimum value operator, and "max(. )" designates a maximum value operator.

<FIG> shows a block schematic diagram of a decoder in which the method according to an embodiment of the invention can be performed. The decoder <NUM> is configured to receive an encoded audio information, for example, in the form of an encoded audio stream <NUM>, and to provide, on the basis thereof, a decoded representation of the audio signal, for example, on the basis of spectral components <NUM> of a first frequency band and spectral components <NUM> of a second frequency band. The decoder <NUM> comprises a noise filler <NUM>, which is configured to receive a representation <NUM> of spectral components of a first frequency band, to which first frequency band gain information is associated, and a representation <NUM> of spectral components of a second frequency band, to which second frequency band gain information is associated. Further, the noise filler <NUM> is configured to receive a representation <NUM> of a multi-band noise intensity value. Further, the noise filler is configured to introduce noise into spectral components (e.g. into spectral line values or spectral bin values) of a plurality of frequency bands to which separate frequency band gain information (in the form of scale factors) is associated on the basis of the common multi-band noise intensity value <NUM>. For example, the noise filler <NUM> may be configured to introduce noise into the spectral components <NUM> of the first frequency band to obtain the noise-affected spectral components <NUM> of the first frequency band, and also to introduce noise into the spectral components <NUM> of the second frequency band to obtain the noise-affected spectral components <NUM> of the second frequency band.

Thus, starting from a noise filling start line, the decoder is configured to replace every spectral line quantized to zero with a replacement value, which is an indicated noise value, a magnitude of which is determined by the multi-band noise intensity value, with a random sign, to perform the noise filling in a quantized domain.

The decoder is configured to scale the replacement value with a scale factor transmitted for an actual scale factor band, to perform an individual scaling of noise introduced into different frequency bands on the basis of the frequency band gain information.

By applying noise described by a single multi-band noise intensity value <NUM> to spectral components of different frequency bands to which different frequency band gain information is associated, noise can be introduced into the different frequency bands in a very fine-tuned way, taking into account the different psychoacoustic relevance of a different frequency bands, which is expressed by the frequency band gain information. Thus, the decoder <NUM> is able to perform a time-tuned noise filling on the basis of a very small (bit-efficient) noise filling side information.

In other words, the decoder is based on the finding that a single multi-band noise intensity value can be applied for a noise filling with good results if separate frequency band gain information is associated with the different frequency bands. Accordingly, an individual scaling of noise introduced in the different frequency bands is possible on the basis of the frequency band gain information.

<FIG> shows a block schematic diagram of a decoder <NUM> in which the invention can be implemented.

The decoder <NUM> is similar to the decoder disclosed in <NPL>), such that reference is made to this International Standard. The decoder <NUM> is configured to receive a coded audio stream <NUM> and to provide, on the basis thereof, output time signals <NUM>. The coded audio stream may comprise some or all of the information described in <NPL>), and additionally comprises information describing a multi-band noise intensity value. The decoder <NUM> further comprises a bitstream payload deformatter <NUM>, which is configured to extract from the coded audio stream <NUM> a plurality of encoded audio parameters, some of which will be explained in detail in the following. The decoder <NUM> further comprises an extended "advanced audio coding" (AAC) decoder <NUM>, the functionality of which will be described in detail, taking reference to <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>. The extended AAC decoder <NUM> is configured to receive an input information 630a, which comprises, for example, a quantized and encoded spectral line information, an encoded scale factor information and an encoded noise filling parameter information. For example, input information 630a of the extended AAC encoder <NUM> may be identical to the output information 228b provided by the extended AAC encoder 220a described with reference to <FIG>.

The extended AAC decoder <NUM> may be configured to provide, on the basis of the input information 630a, a representation 630b of a scaled and inversely quantized spectrum, for example, in the form of scaled, inversely quantized spectral line values for a plurality of frequency bins (for example, for <NUM> frequency bins).

Optionally, the decoder <NUM> may comprise additional spectrum decoders, like, for example, a TwinVQ spectrum decoder and/or a BSAC spectrum decoder, which may be used alternatively to the extended AAC spectrum decoder <NUM> in some cases.

The decoder <NUM> may optionally comprise a spectrum processing <NUM>, which is configured to process the output information 630b of the extended AAC decoder <NUM> in order to obtain an input information 640a of a block switching/filterbank <NUM>. The optional spectral processing <NUM> may comprise one or more, or even all, of the functionalities M/S, PNS, prediction, intensity, long-term prediction, dependently-switched coupling, TNS, dependently-switched coupling, which functionalities are described in detail in <NPL>) and the documents referenced therein. If, however, the spectral processing <NUM> is omitted, the output information 630b of the extended AAC decoder <NUM> may serve directly as input information 640a of the block-switching/filterbank <NUM>. Thus, the extended AAC decoder <NUM> may provide, as the output information 630b, scaled and inversely quantized spectra. The block-switching/filterbank <NUM> uses, as the input information 640a, the (optionally pre-processed) inversely-quantized spectra and provides, on the basis thereof, one or more time domain reconstructed audio signals as an output information 640b. The filterbank/block-switching may, for example, be configured to apply the inverse of the frequency mapping that was carried out in the encoder (for example, in the block-switching/filterbank <NUM>). For example, an inverse modified discrete cosine transform (IMDCT) may be used by the filterbank. For instance, the IMDCT may be configured to support either one set of <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>, or four sets of <NUM> or <NUM> spectral coefficients.

For details, reference is made, for example, to the <NPL>). The decoder <NUM> may optionally further comprise an AAC gain control <NUM>, a SBR decoder <NUM> and an independently-switched coupling <NUM>, to derive the output time signal <NUM> from the output signal 640b of the block-switching/filterbank <NUM>.

However, the output signal 640b of the block-switching/filterbank <NUM> may also serve as the output time signal <NUM> in the absence of the functionality <NUM>, <NUM>, <NUM>.

In the following, details regarding the extended AAC decoder will be described, taking reference to <FIG> and <FIG>. <FIG> and <FIG> show a block schematic diagram of the AAC decoder <NUM> of <FIG> in combination with the bitstream payload deformatter <NUM> of <FIG>.

The bitstream payload deformatter <NUM> receives a decoded audio stream <NUM>, which may, for example, comprise an encoded audio data stream comprising a syntax element entitled "ac_raw_data_block", which is an audio coder raw data block. However, the bit stream payload formatter <NUM> is configured to provide to the extended AAC decoder <NUM> a quantized and noiselessly coded spectrum or a representation, which comprises a quantized and arithmetically coded spectral line information 630aa (e.g. designated as ac_spectral_data), a scale factor information 630ab (e.g. designated as scale_factor_data) and a noise filling parameter information 630ac. The noise filling parameter information 630ac comprises, for example, a noise offset value (designated with noise_offset) and a noise level value (designated with noise_level).

Regarding the extended AAC decoder, it should be noted that the extended AAC decoder <NUM> is very similar to the AAC decoder of the <NPL>), such that reference is made to the detailed description in said Standard.

The extended AAC decoder <NUM> comprises a scale factor decoder <NUM> (also designated as scale factor noiseless decoding tool), which is configured to receive the scale factor information 630ab and to provide on the basis thereof, a decoded integer representation <NUM> of the scale factors (which is also designated as sf[g] [sfb] or scf[g] [sfb]). Regarding the scale factor decoder <NUM>, reference is made to <NPL>. It should be noted that the decoded integer representation <NUM> of the scale factors reflects a quantization accuracy with which different frequency bands (also designated as scale factor bands) of an audio signal are quantized. Larger scale factors indicate that the corresponding scale factor bands have been quantized with high accuracy, and smaller scale factors indicate that the corresponding scale factor bands have been quantized with low accuracy.

The extended AAC decoder <NUM> also comprises a spectral decoder <NUM>, which is configured to receive the quantized and entropy coded (e.g. Huffman coded or arithmetically coded) spectral line information 630aa and to provide, on the basis thereof, quantized values <NUM> of the one or more spectra (e.g. designated as x_ac_quant or x_quant). Regarding the spectral decoder, reference is made, for example, to section <NUM>. <NUM> of the above-mentioned International Standard. However, alternative implementations of the spectral decoder may naturally be applied. For example, the Huffman decoder of ISO/IEC <NUM>-<NUM>: <NUM> may be replaced by an arithmetical decoder if the spectral line information 630aa is arithmetically coded.

The extended AAC decoder <NUM> further comprises an inverse quantizer <NUM>, which may be a non-uniform inverse quantizer. For example, the inverse quantizer <NUM> may provide un-scaled inversely quantized spectral values <NUM> (for example, designated with x_ac_invquant, or x_invquant). For instance, the inverse quantizer <NUM> may comprise the functionality described in <NPL>. Alternatively, the inverse quantizer <NUM> may comprise the functionality described with reference to <FIG>.

The extended AAC decoder <NUM> also comprises a noise filler <NUM> (also designated as noise filling tool), which receives the decoded integer representation <NUM> of the scale factors from the scale factor decoder <NUM>, the un-scaled inversely quantized spectral values <NUM> from the inverse quantizer <NUM> and the noise filling parameter information 630ac from the bitstream payload deformatter <NUM>. The noise filler is configured to provide, on the basis thereof, the modified (typically integer) representation <NUM> of the scale factors, which is also designated herein with sf[g] [sfb] or scf[g] [sfb]. The noise filler <NUM> is also configured to provide un-scaled, inversely quantized spectral values <NUM>, also designated as x_ac_invquant or x_invquant on the basis of its input information. Details regarding the functionality of the noise filler will subsequently be described, taking reference to <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, <FIG>.

The extended AAC decoder <NUM> also comprises a rescaler <NUM>, which is configured to receive the modified integer representation of the scale factors <NUM> and the un-scaled inversely quantized spectral values <NUM>, and to provide, on the basis thereof, scaled, inversely quantized spectral values <NUM>, which may also be designated as x_rescal, and which may serve as the output information 630b of the extended AAC decoder <NUM>. The rescaler <NUM> may, for example, comprise the functionality as described in <NPL>.

In the following, the functionality of the inverse quantizer <NUM> will be described, taking reference to <FIG> shows a representation of an equation for deriving the un-scaled inversely quantized spectral values <NUM> from the quantized spectral values <NUM>. In the alternative equations of <FIG>, "sign(. )" designates a sign operator, and ". " designates an absolute value operator. <FIG> shows a pseudo program code representing the functionality of the inverse quantizer <NUM>. As can be seen, the inverse quantization according to the mathematical mapping rule shown in <FIG> is performed for all window groups (designated by running variable g), for all scale factor bands (designated by running variable sfb), for all windows (designated by running index win) and all spectral lines (or spectral bins) (designated by running variable bin). <FIG> shows a flow chart representation of the algorithm of <FIG>. For scale factor bands below a predetermined maximum scale factor band (designated with max_sfb), un-scaled inversely quantized spectral values are obtained as a function of un-scaled quantized spectral values. A non-linear inverse quantization rule is applied.

<FIG> shows a block schematic diagram of a noise filler <NUM>. The noise filler <NUM> may, for example, take the place of the noise filler <NUM> described with reference to <FIG> and <FIG>.

The noise filler <NUM> receives the decoded integer representation <NUM> of the scale factors, which may be considered as frequency band gain values. The noise filler <NUM> also receives the un-scaled inversely quantized spectral values <NUM>. Further, the noise filler <NUM> receives the noise filling parameter information 630ac, for example, comprising noise filling parameters noise_value and noise_offset. The noise filler <NUM> further provides the modified integer representation <NUM> of the scale factors and the un-scaled inversely quantized spectral values <NUM>. The noise filler <NUM> comprises a spectral-line-quantized-to-zero detector <NUM>, which is configured to determine whether a spectral line (or spectral bin) is quantized to zero (and possibly fulfills further noise filling requirements). For this purpose, the spectral-line-quantized-to-zero detector <NUM> directly receives the un-scaled inversely quantized spectra <NUM> as input information. The noise filler <NUM> further comprises a selective spectral line replacer <NUM>, which is configured to selectively replace spectral values of the input information <NUM> by spectral line replacement values <NUM> in dependence on the decision of the spectral-line-quantized-to-zero detector <NUM>. Thus, if the spectral-line-quantized-to-zero detector <NUM> indicates that a certain spectral line of the input information <NUM> should be replaced by a replacement value, then the selective spectral line replacer <NUM> replaces the certain spectral line with the spectral line replacement value <NUM> to obtain the output information <NUM>. Otherwise, the selective spectral line replacer <NUM> forwards the certain spectral line value without change to obtain the output information <NUM>. The noise filler <NUM> also comprises a selective scale factor modifier <NUM>, which is configured to selectively modify scale factors of the input information <NUM>. For example, the selective scale factor modifier <NUM> is configured to increase scale factors of scale factor frequency bands, which have been quantized to zero by a predetermined value, which is designated as "noise_offset". Thus, in the output information <NUM>, scale factors of frequency bands quantized to zero are increased when compared to corresponding scale factor values within the input information <NUM>. In contrast, corresponding scale factor values of scale factor frequency bands, which are not quantized to zero, are identical in the input information <NUM> and in the output information <NUM>.

For determining whether a scale factor frequency band is quantized to zero, the noise filler <NUM> also comprises a band-quantized-to-zero detector <NUM>, which is configured to control the selective scale factor modifier <NUM> by providing an "enable scale factor modification" signal or flag <NUM> on the basis of the input information <NUM>. For example, the band-quantized-to-zero detector <NUM> may provide a signal or flag indicating the need for an increase of a scale factor to the selective scale factor modifier <NUM> if all the frequency bins (also designated as spectral bins) of a scale factor band are quantized to zero.

It should be noted here that the selective scale factor modifier can also take the form of a selective scale factor replacer, which is configured to set scale factors of scale factor bands quantized entirely to zero to a predetermined value, irrespective of the input information <NUM>.

In the following, a re-scaler <NUM> will be described, which may take the function of the re-scaler <NUM>. The re-scaler <NUM> is configured to receive the modified integer representation <NUM> of the scale factors provided by the noise filler and also for the un-scaled, inversely quantized spectral values <NUM> provided by the noise filler. The re-scaler <NUM> comprises a scale factor gain computer <NUM>, which is configured to receive one integer representation of the scale factor per scale factor band and to provide one gain value per scale factor band. For example, the scale factor gain computer <NUM> may be configured to compute a gain value <NUM> for an i-th frequency band on the basis of a modified integer representation <NUM> of the scale factor for the i-th scale factor band. Thus, the scale factor gain computer <NUM> provides individual gain values for the different scale factor bands. The re-scaler <NUM> also comprises a multiplier <NUM>, which is configured to receive the gain values <NUM> and the un-scaled, inversely quantized spectral values <NUM>. It should be noted that each of the un-scaled, inversely quantized spectral values <NUM> is associated with a scale factor frequency band (sfb). Accordingly, the multiplier <NUM> is configured to scale each of the un-scaled, inversely quantized spectral values <NUM> with a corresponding gain value associated with the same scale factor band. In other words, all the un-scaled, inversely quantized spectral values <NUM> associated with a given scale factor band are scaled with the gain value associated with the given scale factor band. Accordingly, un-scaled, inversely quantized spectral values associated with different scale factor bands are scaled with typically different gain values associated with the different scale factor bands.

Thus, different of the un-scaled, inversely quantized spectral values are scaled with different gain values depending on which scale factor bands they are associated to.

In the following, the functionality of the noise filler <NUM> will be described taking reference to <FIG> and <FIG>, which show a pseudo program code representation (<FIG>) and a corresponding legend (<FIG>). Comments start with.

The noise filling algorithm represented by the pseudo code program listing of <FIG> comprises a first part (lines <NUM> to <NUM>) of deriving a noise value (noiseVal) from a noise level representation (noise_level). In addition, a noise offset (noise_offset) is derived. Deriving the noise value from the noise level comprises a non-linear scaling, wherein the noise value is computed according to <MAT>.

In addition, a range shift of the noise offset value is performed such that the range-shifted noise offset value can take positive and negative values.

A second part of the algorithm (lines <NUM> to <NUM>) is responsible for a selective replacement of un-scaled, inversely quantized spectral values with spectral line replacement values and for a selective modification of the scale factors. As can be seen from the pseudo program code, the algorithm may be executed for all available window groups (for-loop from lines <NUM> to <NUM>). In addition, all scale factor bands between zero and a maximum scale factor band (max_sfb) may be processed even though the processing may be different for different scale factor bands (for-loop between lines <NUM> and <NUM>). One important aspect is the fact that it is generally assumed that a scale factor band is quantized to zero unless it is found that the scale factor band is not quantized to zero (confer line <NUM>). However, the check whether a scale factor band is quantized to zero or not is only executed for scale factor bands, a starting frequency line (swb_offset[sfb]) of which is above a predetermined spectral coefficient index (noiseFillingStartOffset). A conditional routine between lines <NUM> and <NUM> is only executed if an index of the lowest spectral coefficients of scale factor band sfb is larger than noise filling start offset. In contrast, for any scale factor bands for which an index of the lowest spectral coefficient (swb_offset[sfb]) is smaller than or equal to a predetermined value (noiseFillingStartOffset), it is assumed that the bands are not quantized to zero, independent from the actual spectral line values (see lines 24a,24b and 24c).

If, however, the index of the lowest spectral coefficients of a certain scale factor band is larger than the predetermined value (noiseFillingStartOffset), then the certain scale factor band is considered as being quantized to zero only if all spectral lines of the certain scale factor band are quantized to zero (the flag "band_quantized_to_zero" is reset by the for-loop between lines <NUM> and <NUM> if a single spectral bin of the scale factor band is not quantized to zero.

Consequently, a scale factor of a given scale factor band is modified using the noise offset if the flag "band_quantized_to_zero", which is initially set by default (line <NUM>) is not deleted during the execution of the program code between lines <NUM> and <NUM>. As mentioned above, a reset of the flag can only occur for scale factor bands for which an index of the lowest spectral coefficient is above the predetermined value (noiseFillingStartOffset). Furthermore, the algorithm of <FIG> comprises a replacement of spectral line values with spectral line replacement values if the spectral line is quantized to zero (condition of line <NUM> and replacement operation of line <NUM>). However, said replacement is only performed for scale factor bands for which an index of the lowest spectral coefficient is above the predetermined value (noiseFillingStartOffset). For lower spectral frequency bands, the replacement of spectral values quantized to zero with replacement spectral values is omitted.

It should further be noted that the replacement values could be computed in a simple way in that a random or pseudo-random sign is added to the noise value (noiseVal) computed in the first part of the algorithm (confer line <NUM>).

It should be noted that <FIG> shows a legend of the relevant symbols used in the pseudo program code of <FIG> to facilitate a better understanding of the pseudo program code.

Important aspects of the functionality of the noise filler are illustrated in <FIG>. As can be seen, the functionality of the noise filler optionally comprises computing <NUM> a noise value on the basis of the noise level. The functionality of the noise filler also comprises replacement <NUM> of spectral line values of spectral lines quantized to zero with spectral line replacement values in dependence on the noise value to obtain replaced spectral line values. However, the replacement <NUM> is only performed for scale factor bands having a lowest spectral coefficient above a predetermined spectral coefficient index.

The functionality of the noise filler also comprises modifying <NUM> a band scale factor in dependence on the noise offset value if, and only if, the scale factor band is quantized to zero. However, the modification <NUM> is executed in that form for scale factor bands having a lowest spectral coefficient above the predetermined spectral coefficient index.

The noise filler also comprises a functionality of leaving <NUM> band scale factors unaffected, independent from whether the scale factor band is quantized to zero, for scale factor bands having a lowest spectral coefficient below the predetermined spectral coefficient index.

Furthermore, the re-scaler comprises a functionality <NUM> of applying unmodified or modified (whichever is available) band scale factors to un-replaced or replaced (whichever is available) spectral line values to obtain scaled and inversely quantized spectra.

<FIG> shows a schematic representation of the concept described with reference to <FIG>, <FIG> and <FIG>. In particular, the different functionalities are represented in dependence on a scale factor band start bin.

<FIG> show pseudo code program listings of algorithms, which may be performed in an alternative implementation of the noise filler <NUM>. <FIG> describes an algorithm for deriving a noise value (for use within the noise filler) from a noise level information, which may be represented by the noise filling parameter information 630ac.

As the mean quantization error is approximately <NUM> most of the time, the noiseVal range [<NUM>, <NUM>] is rather large and can be optimized.

<FIG> represents an algorithm, which may be formed by the noise filler <NUM>. The algorithm of <FIG> comprises a first portion of determining the noise value (designated with "noiseValue" or "noiseVal" - line s <NUM> to <NUM>). A second portion of the algorithm comprises a selective modification of a scale factor (lines <NUM> to <NUM>) and a selective replacement of spectral line values with spectral line replacement values (lines <NUM> to <NUM>).

However, according to the algorithm of <FIG>, the scale factor (scf) is modified using the noise offset (noise_offset) whenever a band is quantized to zero (see line <NUM>). No difference is made between lower frequency bands and higher frequency bands in this embodiment. Furthermore, noise is introduced into spectral lines quantized to zero only for higher frequency bands (if the line is above a certain predetermined threshold "noiseFillingStartOffset").

To summarize, a decoder, which performs the method according to embodiments of the present invention, comprises the following features:.

In the following, an audio stream which can be used in connection with embodiments of the invention will be described. In the following, a so-called "usac bitstream payload" will be described. The "usac bitstream payload" carries payload information to represent one or more single channels (payload "single_channel_element ()) and/or one or more channel pairs (channel_pair_element ()), as can be seen from <FIG>. A single channel information (single_channel_element ()) comprises, among other optional information, a frequency domain channel stream (fd_channel_stream), as can be seen from <FIG>.

A channel pair information (channel_pair_element) comprises, in addition to additional elements, a plurality of, for example, two frequency domain channel streams (fd_channel_stream), as can be seen from <FIG>.

The data content of a frequency domain channel stream may, for example, be dependent on whether a noise filling is used or not (which may be signaled in a signaling data portion not shown here). In the following, it will be assumed that a noise filling is used. In this case, the frequency domain channel stream comprises, for example, the data elements shown in <FIG>. For example, a global gain information (global_gain), as defined in ISO/IEC <NUM>-<NUM>: <NUM> may be present. Moreover, the frequency domain channel stream may comprise a noise offset information (noise_offset) and a noise level information (noise_level), as described herein. The noise offset information may, for example, be encoded using <NUM> bits and the noise level information may, for example, be encoded using <NUM> bits.

In addition, the frequency domain channel stream may comprise encoded scale factor information (a scale_factor_data ()) and arithmetically encoded spectral data (AC_spectral_data ())as described herein and as also defined in ISO/IEC <NUM>-<NUM>.

Optionally, the frequency domain channel stream also comprises temporal noise shaping data (tns_data) ()), as defined in ISO/IEC <NUM>-<NUM>.

Naturally, the frequency domain channel stream may comprise other information, if required.

<FIG> shows a schematic representation of the syntax of a channel stream representing an individual channel (individual_channel_stream ()).

The individual channel stream may comprise a global gain information (global_gain) encoded using, for example, <NUM> bits, noise offset information (noise_offset) encoded using, for example, <NUM> bits and a noise level information (noise_level) encoded using, for example, <NUM> bits.

The individual channel stream further comprises section data (section_data ()), scale factor data (scale_factor_data ()) and spectral data (spectral_data ()).

In addition, the individual channel stream may comprise further optional information, as can be seen from <FIG>.

To summarize, a bitstream is disclosed in which the following bitstream syntax elements are used:.

In low bit rate coding, noise filling can be used for two purposes:.

The newly proposed noise filling coding scheme described herein efficiently combines the above purposes into a single application.

As a comparison, in MPEG-<NUM> audio, the perceptual noise substitution (PNS) is used to only transmit a parameterized information of noise-like signal parts and to reproduce these signal parts perceptionally equivalent in the decoder.

As a further comparison, in AMR-WB+, vector quantization vectors (VQ-vectors) quantized to zero are replaced with a random noise vector where each complex spectral value has constant amplitude, but random phase. The amplitude is controlled by one noise value transmitted with the bitstream.

However, the comparison concepts provide significant disadvantages. PNS can only be used to fill complete scale factor bands with noise, whereas AMR-WB+ only tries to mask artifacts in the decoded signal resulting from large parts of the signal being quantized to zero. In contrast, the proposed noise filling coding scheme efficiently combines both aspects of noise filling into a single application.

According to an aspect, the present invention comprises a new form of noise level calculation. The noise level is calculated in the quantized domain based on the average quantization error.

The quantization error in the quantized domain differs from other forms of quantization error. The quantization error per line in the quantized domain is in the range [-<NUM>; <NUM>] (<NUM> quantization level) with an average absolute error of <NUM> (for normal distributed input values that are usually larger than <NUM>).

In the following, some advantages of noise filling in the quantized domain will be summarized. The advantage of adding noise in the quantized domain is the fact that noise added in the decoder is scaled, not only with the average energy in a given band, but also the psychoacoustic relevance of a band.

Usually, the perceptually most relevant (tonal) bands will be the bands quantized most accurately, meaning multiple quantization levels (quantized values larger than <NUM>) will be used in these bands. Now adding noise with a level of the average quantization error in these bands will have only very limited influence on the perception of such a band.

Bands that are perceptually not as relevant or more noise-like, may be quantized with a lower number of quantization levels. Although much more spectral lines in the band will be quantized to zero, the resulting average quantization error will be the same as for the fine quantized bands (assuming a normal distributed quantization error in both bands), while the relative error in the band may be much higher.

In these coarse quantized bands, the noise filling will help to perceptually mask artifacts resulting from the spectral holes due to the coarse quantization.

A consideration of the noise filling in the quantized domain can be achieved by the above-described encoder and also by the above-described decoder.

Claim 1:
A method for providing a decoded representation (<NUM>; <NUM>: 630b) of an audio signal on the basis of an encoded audio stream (<NUM>; <NUM>) representing spectral components of frequency bands of the audio signal and a multi-band noise intensity value (<NUM>), the method comprising:
introducing noise into spectral components of a plurality of frequency bands, to which separate frequency band gain information, namely scale factors, is associated, on the basis of a common multi-band noise intensity value;
wherein starting from a noise filling start line every spectral line quantized to zero is replaced with a replacement value, which is an indicated noise value, a magnitude of which is determined by the multi-band noise intensity value, with a random sign, to perform a noise filling in a quantized domain; and
wherein the method comprises scaling a replacement value using a scale factor transmitted for an actual scale factor band,
to perform an individual scaling of noise introduced into different frequency bands on the basis of the frequency band gain information;
wherein spectral bin values of spectral bins quantized to zero are replaced with spectral bin noise values, magnitudes of which spectral bin noise values are dependent on the multi-band noise intensity value (<NUM>), to obtain replaced spectral bin values, only for frequency bands having a lowest spectral bin index above a predetermined spectral bin index, leaving spectral bin values of frequency bands having a lowest spectral bin index below the predelormined spectral bin index unaftected;
wherein the method comprises selectively modifying, for the frequency bands having a lowest spectral bin index above the predetermined spectral bin index, a band gain value of a given frequency band in dependence on a noise offset value, if the given frequency band is entirely quantized to zero; and
wherein the selectively-modified or unmodified band gain values are applied to the selectively-replaced or un-replaced spectral bin values, to obtain a scaled spectral information, which represents the audio signal.