Frequency-domain audio coding supporting transform length switching
A frequency-domain audio codec is provided with the ability to additionally support a certain transform length in a backward-compatible manner, by the following: the frequency-domain coefficients of a respective frame are transmitted in an interleaved manner irrespective of the signalization signaling for the frames as to which transform length actually applies, and additionally the frequency-domain coefficient extraction and the scale factor extraction operate independent from the signalization. By this measure, old-fashioned frequency-domain audio coders/decoders, insensitive for the signalization, would be able to nevertheless operate without faults and with reproducing a reasonable quality. Concurrently, frequency-domain audio coders/decoders able to support the additional transform length would offer even better quality despite the backward compatibility. As far as coding efficiency penalties due to the coding of the frequency domain coefficients in a manner transparent for older decoders are concerned, same are of comparatively minor nature due to the interleaving.
1. Frequency-domain audio decoder supporting transform length switching, comprising
a frequency-domain coefficient extractor configured to extract frequency-domain coefficients of frames of an audio signal from a data stream;
a scale factor extractor configured to extract scale factors from the data stream;
an inverse transformer configured to subject the frequency-domain coefficients of the frames, scaled according to the scale factors, to inverse transformation to acquire time-domain portions of the audio signal;
a combiner configured to combine the time-domain portions to acquire the audio signal,
wherein the inverse transformer is responsive to a signalization within the frames of the audio signal so as to, depending on the signalization,
form one transform by sequentially arranging the frequency-domain coefficients of a respective frame, scaled according to the scale factors, in a non-de-interleaved manner and subject the one transform to an inverse transformation of a first transform length, or
form more than one transform by de-interleaving the frequency-domain coefficients of the respective frame, scaled according to the scale factors, and subject each of the more than one transforms to an inverse transformation of a second transform length, shorter than the first transform length,
wherein the frequency-domain coefficient extractor and the scale factor extractor operate independent from the signalization,
wherein the inverse transformer is configured to
perform inverse temporal noise shaping filtering onto a sequence of N coefficients irrespective of the signalization by applying a filter, a transfer function of which is set according to TNS coefficients onto the sequence of N coefficients, with
in the formation of the one transform, applying the inverse temporal noise shaping filtering using the frequency-domain coefficients sequentially arranged in a non-de-interleaved manner as the sequence of N coefficients, and
in the formation of the more than one transforms, applying the inverse temporal noise shaping filtering on the frequency-domain coefficients using the frequency-domain coefficients sequentially arranged in a de-interleaved manner according to which the more than one transforms are concatenated spectrally as the sequence of N coefficients.
2. Frequency-domain audio decoder according to claim 1 , wherein the scale factor extractor is configured to extract the scale factors from the data stream at a spectro-temporal resolution which is independent from the signalization.
3. Frequency-domain audio decoder according to claim 1 , wherein the frequency-domain coefficient extractor uses context- or codebook-based entropy decoding to extract the frequency-domain coefficients from the data stream, with assigning, for each frequency-domain coefficient, the same context or codebook to the respective frequency-domain coefficient irrespective of the signalization.
4. Frequency-domain audio decoder according to claim 1 , wherein the inverse transformer is configured to subject the frequency-domain coefficients to scaling according to the scale factors at a spectral resolution independent from the signalization.
5. Frequency-domain audio decoder according to claim 1 , wherein the inverse transformer is configured to subject the frequency-domain coefficients to noise filling, with the frequency-domain coefficients sequentially arranged in a non-de-interleaved manner, and at a spectral resolution independent from the signalization.
6. Frequency-domain audio decoder according to claim 1 , wherein the inverse transformer is configured to support joint-stereo coding with or without inter-channel stereo prediction and to use the frequency-domain coefficients as a sum (mid) or difference (side) spectrum or prediction residual of the inter-channel stereo prediction, with the frequency-domain coefficients arranged in a non-de-interleaved manner, irrespective of the signalization.
7. Frequency-domain audio decoder according to claim 1 , wherein the number of the more than one transforms equals 2, and the first transform length is twice the second transform length.
8. Frequency-domain audio decoder according to claim 1 , wherein the inverse transformation is an inverse modified discrete cosine transform, MDCT (IMDCT).
9. Method for frequency-domain audio decoding supporting transform length switching, comprising extracting frequency-domain coefficients of frames of an audio signal from a data stream;
extracting scale factors from the data stream;
subjecting the frequency-domain coefficients of the frames, scaled according to scale factors, to inverse transformation to acquire time-domain portions of the audio signal;
combining the time-domain portions to acquire the audio signal,
wherein the subjection to inverse transformation is responsive to a signalization within the frames of the audio signal so as to, depending on the signalization, comprise
forming one transform by sequentially arranging the frequency-domain coefficients of a respective frame in a non-de-interleaved manner and subjecting the one transform to an inverse transformation of a first transform length, or
forming more than one transform by de-interleaving the frequency-domain coefficients of the respective frame and subjecting each of the more than one transforms to an inverse transformation of a second transform length, shorter than the first transform length,
wherein the extraction of the frequency-domain coefficients and the extraction of the scale factors are independent from the signalization,
wherein the subjecting to the inverse transformation comprises
performing inverse temporal noise shaping filtering onto a sequence of N coefficients irrespective of the signalization by applying a filter, a transfer function of which is set according to TNS coefficients onto the sequence of N coefficients, with
in the formation of the one transform, applying the inverse temporal noise shaping filtering using the frequency-domain coefficients sequentially arranged in a non-de-interleaved manner as the sequence of N coefficients, and
in the formation of the more than one transforms, applying the inverse temporal noise shaping filtering on the frequency-domain coefficients using the frequency-domain coefficients sequentially arranged in a de-interleaved manner according to which the more than one transforms are concatenated spectrally as the sequence of N coefficients.
10. Non-transitory digital storage medium having computer-readable code stored thereon to perform, when running on a computer, the method according to claim 9 .