Lossless and near lossless scalable audio codec
A scalable audio codec encodes an input audio signal as a base layer at a high compression ratio and one or more residual signals as an enhancement layer of a compressed bitstream, which permits a lossless or near lossless reconstruction of the input audio signal at decoding. The scalable audio codec uses perceptual transform coding to encode the base layer. The residual is calculated in a transform domain, which includes a frequency and possibly also multi-channel transform of the input audio. For lossless reconstruction, the frequency and multi-channel transforms are reversible.
1. A method of scalable audio encoding, comprising:
processing an input audio signal using perceptual transform coding to produce a base compressed audio substream;
partially reconstructing compressed audio from the base compressed audio substream to a transform domain;
processing the input audio signal using at least one transform into a transformed audio in the transform domain;
taking a difference between the partially reconstructed compressed audio and the transformed audio to produce an audio residual;
coding the audio residual into an enhancement audio substream; and
packing the base compressed audio substream and the enhancement audio substream into a compressed audio bitstream.
2. The method of claim 1 , wherein the at least one transform comprises a frequency transform.
3. The method of claim 2 , wherein the frequency transform is a modulated lapped transform.
4. The method of claim 2 , wherein the frequency transform is reversible and the compressed audio bitstream permits a lossless reconstruction of the input audio signal.
5. The method of claim 2 , wherein the at least one transform further comprises a multi-channel transform.
6. The method of claim 5 , wherein the frequency transform and the multi-channel transform are each reversible and the compressed audio bitstream permits a lossless reconstruction of the input audio signal.
7. A method of decoding a scalable audio compressed bitstream having a base layer and an enhancement layer, the method comprising:
performing entropy decoding of a base audio from the base layer and a residual audio from the enhancement layer of the scalable audio compressed bitstream;
partially reconstructing the base audio to a transform domain representation;
combining the base audio and residual audio;
processing the combined base and residual audio using at least one inverse transform to complete reconstruction of an output audio signal; and
producing the output audio signal.
8. The method of claim 7 wherein the at least one inverse transform comprises an inverse frequency transform.
9. The method of claim 8 wherein the inverse frequency transform is an inverse modulated lapped transform.
10. The method of claim 8 wherein the at least one inverse transform further comprises an inverse multi-channel transform.
11. The method of claim 10 wherein the inverse frequency transform and the inverse multi-channel transform are reversible transforms.
12. The method of claim 8 further comprising:
processing the residual audio using an inverse channel transform prior to said combining.
13. The method of claim 12 wherein the inverse multi-channel transform is a reversible transform.
14. A scalable audio decoder, comprising:
an input for receiving a compressed audio bitstream containing a compressed audio base layer and at least one residual enhancement layer;
a first entropy decoder for decoding a base audio from the compressed audio base layer of the compressed audio bitstream;
a second entropy decoder for decoding a residual from the at least one residual enhancement layer of the compressed audio bitstream;
a partial reconstructor for applying at least one inverse perceptual transform coding process to partially reconstruct the base audio to a transform domain representation;
a summer for summing the base audio and residual in the transform domain; and
an inverse transformer for applying at least one inverse transform to the summed base audio and residual to produce a reconstructed audio signal in the time domain; and
an audio output for output of the reconstructed audio signal.
15. The scalable audio decoder of claim 14 wherein the at least one inverse transform comprises an inverse frequency transform.
16. The scalable audio decoder of claim 15 wherein the inverse frequency transform is an inverse modulated lapped transform.
17. The scalable audio decoder of claim 15 wherein the at least one inverse transform further comprises an inverse multi-channel transform.
18. The scalable audio decoder of claim 15 wherein the inverse frequency transform and the inverse multi-channel transform are reversible.
19. The scalable audio decoder of claim 14 further comprising:
an inverse multi-channel transformer for performing an inverse multi-channel transform of the residual prior to summing with the base audio by said summer.