Audio encoder and decoder
The present disclosure provides methods, devices and computer program products for encoding and decoding a multi-channel audio signal based on an input signal. According to the disclosure, a hybrid approach of using both parametric stereo coding and discrete representation of the processed multi-channel audio signal is used which may improve the quality of the encoded and decoded audio for certain bitrates.
1. A decoding method in a multi-channel audio processing system for reconstructing M encoded channels, wherein M>2, comprising the steps of:
receiving N waveform-coded downmix signals comprising spectral coefficients corresponding to frequencies between a first and a second cross-over frequency, wherein 1<N<M;
receiving M waveform-coded signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency, each of the M waveform-coded signals corresponding to a respective one of the M encoded channels;
downmixing the M waveform-coded signals into N downmix signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency;
combining each of the N waveform-coded downmix signals comprising spectral coefficients corresponding to frequencies between a first and a second cross-over frequency with a corresponding one of the N downmix signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency into N combined downmix signals;
extending each of the N combined downmix signals to a frequency range above the second cross-over frequency by performing high frequency reconstruction, whereby each extended downmix signal comprises spectral coefficients corresponding to a range extending below the first cross-over frequency and above the second cross-over frequency;
performing a parametric upmix of the N frequency extended combined downmix signals into M upmix signals comprising spectral coefficients corresponding to frequencies above the first cross-over frequency, each of the M upmix signals corresponding to one of the M encoded channels; and
combining the M upmix signals comprising spectral coefficients corresponding to frequencies above the first cross-over frequency with the M waveform-coded signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency.
2. The decoding method of claim 1 wherein the step of combining each of the N waveform-coded downmix signals comprising spectral coefficients corresponding to frequencies between a first and a second cross-over frequency with a corresponding one of the N downmix signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency into N combined downmix is performed in a frequency domain.
3. The decoding method of claim 1 , wherein the step of extending each of the N combined downmix signals to a frequency range above the second cross-over frequency is performed in a frequency domain.
4. The decoding method of claim 1 , wherein the step of combining the M upmix signals comprising spectral coefficients corresponding to frequencies above the first cross-over frequency with the M waveform-coded signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency is performed in a frequency domain.
5. The decoding method of claim 1 , wherein the step of performing a parametric upmix of the N frequency extended combined downmix signals into M upmix signals is performed in a frequency domain.
6. The decoding method of claim 1 , wherein the step of downmixing the M waveform-coded signals into N downmix signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency is performed in a frequency domain.
7. The decoding method of claim 2 , wherein the frequency domain is a Quadrature Mirror Filters, QMF, domain.
8. The decoding method of claim 1 , wherein the step of downmixing the M waveform-coded signals into N downmix signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency is performed in the time domain.
9. The decoding method of claim 1 , wherein the first cross-over frequency depends on a bit transmission rate of the multi-channel audio processing system.
10. The decoding method of claim 1 , wherein the step of extending each of the N combined downmix signals to a frequency range above the second cross-over frequency by performing high frequency reconstruction:
receiving high frequency reconstruction parameters; and
extending each of the N combined downmix signals to a frequency range above the second cross-over frequency by performing high frequency reconstruction using the high frequency reconstruction parameters.
11. The decoding method of claim 1 , wherein the step of extending each of the N combined downmix signals to a frequency range above the second cross-over frequency by performing high frequency reconstruction comprises performing spectral band replication, SBR.
12. The decoding method of claim 1 , wherein the step of performing a parametric upmix of the N frequency extended combined downmix signals into M upmix signals comprises:
receiving upmix parameters;
generating decorrelated versions of the N frequency extended combined downmix signals; and
subjecting the N frequency extended combined downmix signals and the decorrelated versions of the N frequency extended combined downmix signals to a matrix operation, wherein the parameters of the matrix operation are given by the upmix parameters.
13. A computer program product comprising a computer-readable medium with instructions for performing the method of claim 1 .
14. A decoder for a multi-channel audio processing system for reconstructing M encoded channels, wherein M>2, comprising:
a first receiving stage configured to receive N waveform-coded downmix signals comprising spectral coefficients corresponding to frequencies between a first and a second cross-over frequency, wherein 1<N<M;
a second receiving stage configured to receive M waveform-coded signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency, each of the M waveform-coded signals corresponding to a respective one of the M encoded channels;
a downmix stage downstreams of the second receiving stage configured to downmix the M waveform-coded signals into N downmix signals comprising spectral coefficients corresponding to frequencies up to the first cross-over frequency;
a first combining stage downstreams of the first receiving stage and the downmix stage configured to combine each of the N downmix signals received by the first receiving stage with a corresponding one of the N downmix signals from the downmix stage into N combined downmix signals;
a high frequency reconstructing stage downstreams of the first combining stage configured to extend each of the N combined downmix signals from the combining stage to a frequency range above the second cross-over frequency by performing high frequency reconstruction, whereby each extended downmix signal comprises spectral coefficients corresponding to a range extending below the first cross-over frequency and above the second cross-over frequency;
an upmix stage downstreams of the high frequency reconstructing stage configured to perform a parametric upmix of the N frequency extended signals from the high frequency reconstructing stage into M upmix signals comprising spectral coefficients corresponding to frequencies above the first cross-over frequency, each of the M upmix signals corresponding to one of the M encoded channels; and
a second combining stage downstreams of the upmix stage and the second receiving stage configured to combine the M upmix signals from the upmix stage with the M waveform-coded signals received by the second receiving stage.
15. An encoding method for a multi-channel audio processing system for encoding M channels, wherein M>2, comprising the steps of:
receiving M signals corresponding to the M channels to be encoded;
generating M waveform-coded signals by individually waveform-coding the M signals for a frequency range corresponding to frequencies up to a first cross-over frequency, whereby the M waveform-coded signals comprise spectral coefficients corresponding to frequencies up to the first cross-over frequency;
downmixing the M signals, each of which comprises spectral coefficients corresponding to a range extending below the first cross-over frequency and above a second cross-over frequency, into N downmix signals, wherein 1<N<M;
subjecting the N downmix signals to high frequency reconstruction encoding, whereby high frequency reconstruction parameters are extracted which enable high frequency reconstruction of the N downmix signals above the second cross-over frequency;
subjecting the M signals to parametric encoding for the frequency range corresponding to frequencies above the first cross-over frequency, whereby upmix parameters are extracted which enable upmixing of the N downmix signals into M reconstructed signals corresponding to the M channels for the frequency range above the first cross-over frequency;
generating N waveform-coded downmix signals by waveform-coding the N downmix signals for a frequency range corresponding to frequencies between the first and the second cross-over frequency, whereby the N waveform-coded downmix signals comprise spectral coefficients corresponding to frequencies between the first cross-over frequency and the second cross-over frequency.
16. The encoding method of claim 15 , wherein the step of subjecting the N downmix signals to high frequency reconstruction encoding is performed in a frequency domain, preferably a Quadrature Mirror Filters, QMF, domain.
17. The encoding method of any one of claim 15 , wherein the step of subjecting the M signals to parametric encoding is performed in a frequency domain, preferably a Quadrature Mirror Filters, QMF, domain.
18. The encoding method of any one of claim 15 , wherein the step of generating M waveform-coded signals by individually waveform-coding the M signals, comprises applying an overlapping windowed transform to the M signals, wherein different overlapping window sequences are used for at least two of the M signals.
19. A computer program product comprising a computer-readable medium with instructions for performing the method of any one of claim 15 .
20. An encoder for a multi-channel audio processing system for encoding M channels, wherein M>2, comprising the steps of:
a receiving stage configured to receive M signals corresponding to the M channels to be encoded;
a first waveform-coding stage configured to receive the M signals from the receiving stage and to generate M waveform-coded signals by individually waveform-coding the M signals for a frequency range corresponding to frequencies up to a first cross-over frequency, whereby the M waveform-coded signals comprise spectral coefficients corresponding to frequencies up to the first cross-over frequency;
a downmixing stage configured to receive the M signals from the receiving stage, each of the M received downmix signals comprising spectral coefficients corresponding to a range extending below the first cross-over frequency and above a second cross-over frequency, and to downmix the M signals into N downmix signals, wherein 1<N<M;
a high frequency reconstruction encoding stage configured to receive the N downmix signals from the downmixing stage and to subject the N downmix signals to high frequency reconstruction encoding, whereby the high frequency reconstruction encoding stage is configured to extract high frequency reconstruction parameters which enable high frequency reconstruction of the N downmix signals above the second cross-over frequency;
a parametric encoding stage configured to receive the M signals from the receiving stage, and to subject the M signals to parametric encoding for the frequency range corresponding to frequencies above the first cross-over frequency, whereby the parametric encoding stage is configured to extract upmix parameters which enable upmixing of the N downmix signals into M reconstructed signals corresponding to the M channels for the frequency range above the first cross-over frequency; and
a second waveform-coding stage configured to receive the N downmix signals from the downmixing stage and to generate N waveform-coded downmix signals by waveform-coding the N downmix signals for a frequency range corresponding to frequencies between the first and the second cross-over frequency, whereby the N waveform-coded downmix signals comprise spectral coefficients corresponding to frequencies between the first cross-over frequency and the second cross-over frequency.