Spectral Translation/Folding in the Subband Domain

Abstract

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 . An apparatus for reconstructing a high frequency portion of an audio signal, the apparatus comprising:
 a complex exponential modulated analysis filterbank having L channels for filtering a low frequency portion of the audio signal to produce L low frequency complex-valued subband signals, wherein the complex exponential modulated analysis filterbank includes a plurality of decimators;   a high frequency reconstructor that reconstructs the high frequency portion of the audio signal by patching both a real and an imaginary part of a consecutive number of the L low frequency complex-valued subband signals to consecutive subbands of the high frequency portion; and   a complex exponential modulated synthesis filterbank having Q*L channels for generating a wideband audio signal by combining the reconstructed high frequency portion of the audio signal with the low frequency portion of the audio signal, wherein the complex exponential modulated synthesis filterbank includes a plurality of interpolators,   wherein the high frequency reconstructor comprises an envelope adjuster that adjusts an envelope of the high frequency portion of the audio signal, and   wherein Q is an upsampling factor and Q*L is an integer value.   
     
     
         2 . The apparatus of  claim 1  wherein the high frequency reconstructor uses a first parameter indicating a quantity of the consecutive number of the plurality of low frequency complex-valued subband signals and a second parameter indicating a reconstruction range start channel. 
     
     
         3 . The apparatus of  claim 1  wherein the high frequency reconstructor is configured to reconstruct the high frequency portion of the audio signal with multiple patches. 
     
     
         4 . The apparatus of  claim 1  wherein the Q=2 or Q=4. 
     
     
         5 . The apparatus of  claim 1  wherein the the low frequency portion of the audio signal has a sampling frequency f s . 
     
     
         6 . The apparatus of  claim 1  wherein the wideband audio signal has a sampling frequency 2*f s . 
     
     
         7 . A method for reconstructing a high frequency portion of an audio signal, the method comprising:
 filtering a low frequency portion of the audio signal with a complex exponential modulated analysis filterbank having L channels to produce L low frequency complex-valued subband signals, wherein the complex exponential modulated analysis filterbank includes a plurality of decimators;   reconstructing the high frequency portion of the audio signal by patching both a real and an imaginary part of a consecutive number of the L low frequency complex-valued subband signals to consecutive subbands of the high frequency portion; and   generating a wideband audio signal with a complex exponential modulated synthesis filterbank having Q*L channels by combining the reconstructed high frequency portion of the audio signal with the low frequency portion of the audio signal, wherein the complex exponential modulated synthesis filterbank includes a plurality of interpolators,   wherein the reconstructing comprises adjusting an envelope of the high frequency portion of the audio signal, and   wherein Q is an upsampling factor and Q*L is an integer value.   
     
     
         8 . A non-transitory computer readable medium containing instructions that when executed by a processor perform the method of  claim 7 .