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 obtaining an envelope adjusted and frequency-translated signal, comprising:
filtering a lowband 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 the 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 complex-exponential-modulation of a lowpass prototype filter.
3. 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.
4. 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.
5. A method according to claim 4 , 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.
6. A method according to claim 4 , in which a bandwidth of the dissonance guard band is approximately one half Bark.
7. 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, complex-valued subband signals within the source range for the second iteration step comprise the complex-valued subband signals within the reconstruction range for the first iteration step.
8. An apparatus for obtaining an envelope adjusted and frequency-translated signal, comprising:
an analysis filterbank for filtering a lowband 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 signals with index i within the source range to a complex-valued subband signal with index j within a 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 the reconstruction range, and for applying an envelope adjustment to the patched complex-valued subband signals within the reconstruction range, wherein the source range comprises frequencies lower than frequencies in the reconstruction range; and
a synthesis filterbank for filtering the patched and envelope adjusted complex-valued subband signals within the reconstruction range to obtain the envelope adjusted and frequency translated signal.