Spectral translation/folding in the subband domain
The present invention relates to a new method and apparatus for improvement of High Frequency Reconstruction (HFR) techniques using frequency translation or folding or a combination thereof. The proposed invention is applicable to audio source coding systems, and offers significantly reduced computational complexity. This is accomplished by means of frequency translation or folding in the subband domain, preferably integrated with spectral envelope adjustment in the same domain. The concept of dissonance guard-band filtering is further presented. The proposed invention offers a low-complexity, intermediate quality HFR method useful in speech and natural audio coding applications.
1. A method for decoding coded signals, the coded signals comprising a coded lowband audio signal, comprising:
separating the coded lowband audio signal from the coded signals;
audio decoding the coded lowband audio signal to obtain a decoded audio signal;
obtaining an envelope adjusted and frequency-translated signal, comprising:
filtering the decoded audio signal using an analysis filterbank to obtain complex-valued subband signals within a source range, wherein each complex-valued subband signal is represented by a real-valued component and an imaginary-valued component;
patching the real-valued component and the imaginary-valued component of a complex-valued subband signal with index i within the source range to a complex-valued subband signal with index j within a reconstruction range, wherein the source range comprises frequencies lower than frequencies in the reconstruction range;
patching the real-valued component and the imaginary-valued component of a complex-valued subband signal with index i+1 within the source range to a complex-valued subband signal with index j+1 within a reconstruction range; applying an envelope adjustment to the patched complex-valued subband signals within the reconstruction range; and
filtering the patched and envelope adjusted complex-valued subband signals within the reconstruction range using a synthesis filterbank to obtain the envelope adjusted and frequency translated signal.
2. A method according to claim 1 , wherein the analysis filterbank and the synthesis filterbank are obtained by cosine or sine modulation of a lowpass prototype filter.
3. A method according to claim 1 , wherein the analysis filterbank and the synthesis filterbank are obtained by complex-exponential-modulation of a lowpass prototype filter.
4. A method according to claim 2 , wherein the lowpass prototype filter is designed so that a transition band of channels of the analysis filterbank and the synthesis filterbank overlaps a passband of neighbouring channels only.
5. A method according to claim 1 , in which the synthesis filterbank comprises a dissonance guard band, the dissonance guard band being positioned between synthesis filterbank channels in the source range and synthesis filterbank channels in the reconstruction range.
6. A method according to claim 5 , in which one or several of the channels in the dissonance guard band are fed with zeros or gaussian noise; whereby dissonance related artifacts are attenuated.
7. A method according to claim 5 , in which a bandwidth of the dissonance guard band is approximately one half Bark.
8. A method according to claim 1 , in which the step of patching implements a first iteration step, and in which the method further comprises another step of patching implementing a second iteration step, wherein in the second iteration step, subband signals within the source range for the second iteration step comprise the subband signals within the reconstruction range for the first iteration step.
9. A decoder for decoding coded signals, the coded signals comprising a coded lowband audio signal, comprising:
a separator for separating the coded lowband audio signal from the coded signals;
an audio decoder for audio decoding the coded lowband audio signal to obtain a decoded audio signal;
an apparatus for obtaining an envelope adjusted and frequency-translated signal, comprising:
an analysis filterbank for filtering the decoded audio signal to obtain complex-valued subband signals within a source range, wherein each complex-valued subband signal is represented by a real-valued component and an imaginary-valued component,
a high frequency reconstruction/envelope adjustment unit for:
patching the real-valued component and the imaginary-valued component of a complex-valued subband signal with index i within the source range to a complex-valued subband signal with index j within a reconstruction range, wherein the source range comprises frequencies lower than frequencies in the reconstruction range;
patching the real-valued component and the imaginary-valued component of a complex-valued subband signal with index i+1 within the source range to a complex-valued subband signal with index j+1 within a reconstruction range; and
applying an envelope adjustment to the patched complex-valued subband signals within the reconstruction range; and
a synthesis filterbank for filtering the patched and envelope adjusted complex-valued subband signals within the reconstruction range using a synthesis filterbank to obtain the envelope adjusted and frequency translated signal.
10. A decoder according to claim 9 , in which the coded signals further comprise envelope data,
in which the separator is further arranged to separate the envelope data from the coded signals,
wherein the decoder further comprises an envelope decoder for decoding the envelope data to obtain spectral envelope information,
wherein the spectral envelope information is fed to the apparatus for obtaining an envelope adjusted and frequency-translated signal and is used to apply the spectral envelope adjustment.
11. A non-transitory computer readable storage medium comprising a sequence of instructions which, when executed by a processing device, cause the processing device to perform a method for decoding coded signals, the coded signals comprising a coded lowband audio signal, comprising:
separating the coded lowband audio signal from the coded signals;
audio decoding the coded lowband audio signal to obtain a decoded audio signal;
obtaining an envelope adjusted and frequency-translated signal, comprising:
filtering the decoded audio signal using an analysis filterbank to obtain complex-valued subband signals within a source range, wherein each complex-valued subband signal is represented by a real-valued component and an imaginary-valued component;
patching the real-valued component and the imaginary-valued component of a complex-valued subband signal with index i within the source range to a complex-valued subband signal with index j within a reconstruction range, wherein the source range comprises frequencies lower than frequencies in the reconstruction range;
patching the real-valued component and the imaginary-valued component of a complex-valued subband signal with index i+1 within the source range to a complex-valued subband signal with index j+1 within a reconstruction range;
applying an envelope adjustment to the patched complex-valued subband signals within the reconstruction range; and
filtering the patched and envelope adjusted complex-valued subband signals within the reconstruction range using a synthesis filterbank to obtain the envelope adjusted and frequency translated signal.