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 decoder for generating an analogue output signal from digital coded signals, the digital coded signals comprising a digital coded lowband audio signal, the decoder comprising:
a separator configured to separate the digital coded lowband audio signal from the digital coded signals;
an audio decoder configured to decode the digital coded lowband audio signal to obtain a digital decoded audio signal;
a digital analysis filterbank configured to generate a plurality of digital source range subband signals;
a high frequency reconstruction/envelope adjustment unit configured to generate a plurality of consecutive digital reconstruction range subband signals by frequency-translating at least a subset of the digital source range subband signals and to apply an envelope correction,
wherein frequencies of the digital reconstruction range subband signals are higher than frequencies of the digital source range subband signals, and
wherein a digital source range subband signal having a subband index i is frequency-translated to a digital reconstruction range subband signal having a subband index j, and wherein a digital source range subband signal having a subband index i+1 is frequency-translated to a digital reconstruction range subband signal having a subband index j+1;
a digital synthesis filterbank configured to generate a wideband digital output signal by filtering the digital source range subband signals and the digital reconstruction range subband signals; and
a digital to analogue converter configured to convert the wideband digital output signal to the analogue output signal.
2. The decoder according to claim 1 , wherein the digital coded signals further comprise digital coded envelope data, and
the separator is further configured to separate the digital coded envelope data from the digital coded signals,
wherein the decoder further comprises an envelope decoder configured to decode the digital coded envelope data to obtain digital envelope information, and
wherein the spectral envelope information is provided to the high frequency reconstruction/envelope adjustment unit and is used in applying the envelope correction.
3. The decoder according to claim 1 , wherein one or more of the digital analysis filterbank and the digital synthesis filterbank is obtained by cosine or sine modulation of a lowpass prototype filter.
4. The decoder according to claim 3 , wherein the lowpass prototype filter is designed so that a transition band of a subband of said digital filterbank overlaps a passband of neighbouring subbands only.
5. The decoder according to claim 1 , wherein one or more of the digital analysis filterbank and the digital synthesis filterbank is obtained by complex-exponential-modulation of a lowpass prototype filter.
6. The decoder according to claim 5 , wherein the lowpass prototype filter is designed so that a transition band of a subband of said digital filterbank overlaps a passband of neighbouring subbands only.
7. The decoder according to claim 1 , wherein one or more digital dissonance guardband subband signals are positioned between the digital source range subband signals and the digital reconstruction range subband signals.
8. The decoder according to claim 7 , in which one or more of the digital dissonance guard band subband signals comprises zeros or gaussian noise.
9. The decoder according to claim 7 , in which a combined bandwidth of the digital dissonance guard band subband signals is approximately one half Bark.
10. The decoder according to claim 1 , wherein the high frequency reconstruction/envelope adjustment unit is further configured to generate additional consecutive digital reconstruction range subband signals by frequency translating and envelope adjusting one or more of the consecutive digital reconstruction range subband signals, and wherein generating the wideband digital output signal further comprises filtering the additional consecutive digital reconstruction range subband signals.
11. A method for generating an analogue output signal from digital coded signals, the digital coded signals comprising a digital coded lowband audio signal, the method comprising:
separating the digital coded lowband audio signal from the digital coded signals;
audio decoding the digital coded lowband audio signal to obtain a digital decoded audio signal;
generating a plurality of digital source range subband signals by filtering the digital decoded audio signal using a digital analysis filterbank;
generating a plurality of consecutive digital reconstruction range subband signals by frequency-translating at least a subset of the digital source range subband signals and applying an envelope correction,
wherein frequencies of the digital reconstruction range subband signals are higher than frequencies of the digital source range subband signals, and
wherein a digital source range subband signal having a subband index i is frequency-translated to a digital reconstruction range subband signal having a subband index j, and wherein a digital source range subband signal having a subband index i+1 is frequency-translated to a digital reconstruction range subband signal having a subband index j+1;
generating a wideband digital output signal by filtering the digital source range subband signals and the digital reconstruction range subband signals using a digital synthesis filterbank; and
generating the analogue output signal by applying a digital-to-analogue conversion to the wideband digital output signal.
12. The method according to claim 11 , in which the digital coded signals further comprise digital coded envelope data, and the method further comprises:
separating the digital coded envelope data from the digital coded signals,
decoding the digital coded envelope data to obtain digital envelope information, and
using the digital envelope information for applying the envelope correction.
13. The method according to claim 11 , wherein one or more of the digital analysis filterbank and the digital synthesis filterbank is obtained by cosine or sine modulation of a lowpass prototype filter.
14. The method according to claim 13 , wherein the lowpass prototype filter is designed so that a transition band of a subband of said digital filterbank overlaps a passband of neighbouring subbands only.
15. The method according to claim 11 , wherein one or more of the digital analysis filterbank and the digital synthesis filterbank is obtained by complex-exponential-modulation of a lowpass prototype filter.
16. The method according to claim 15 , wherein the lowpass prototype filter is designed so that a transition band of a subband of said digital filterbank overlaps a passband of neighbouring subbands only.
17. The method according to claim 11 , wherein one or more digital dissonance guardband subband signals are positioned between the digital source range subband signals and the digital reconstruction range subband signals.
18. The method according to claim 17 , in which one or more of the digital dissonance guard band subband signals comprises zeros or gaussian noise.
19. The method according to claim 17 , in which a total bandwidth of the one or more digital dissonance guard band subband signals is approximately one half Bark.
20. The method according to claim 11 , further comprising generating additional consecutive digital reconstruction range subband signals by frequency translating and envelope adjusting one or more of the consecutive digital reconstruction range subband signals, wherein generating the wideband digital output signal further comprises filtering the additional consecutive digital reconstruction range subband signals.