Harmonic transposition in an audio coding method and system
The present invention relates to transposing signals in time and/or frequency and in particular to coding of audio signals. More particular, the present invention relates to high frequency reconstruction (HFR) methods including a frequency domain harmonic transposer. A method and system for generating a transposed output signal from an input signal using a transposition factor T is described. The system comprises an analysis window of length L a , extracting a frame of the input signal, and an analysis transformation unit of order M transforming the samples into M complex coefficients. M is a function of the transposition factor T. The system further comprises a nonlinear processing unit altering the phase of the complex coefficients by using the transposition factor T, a synthesis transformation unit of order M transforming the altered coefficients into M altered samples, and a synthesis window of length L s , generating a frame of the output signal.
1. An audio signal processing device for transposing an input audio signal by a transposition factor T to generate an output audio signal, the audio signal processing device comprising one or more components that:
extract a frame of L time-domain samples of the input audio signal using an analysis window of length L,
convert the L time-domain samples into M complex frequency-domain coefficients;
alter a phase of the complex frequency-domain coefficients by converting one or more of the complex frequency-domain coefficients to a polar representation, and multiplying the phase of the polar representation by the transposition factor T;
convert the altered frequency-domain coefficients into M altered time-domain samples; and
create a frame of L time-domain output samples of the output audio signal from the M altered time-domain samples using a synthesis window;
wherein M=F*L, with F being a frequency domain oversampling factor determined in response to frequency domain oversampling information received in an encoded bitstream; and
wherein the frame of L time-domain output samples of the output audio signal comprises a plurality of high frequency components not present in the frame of L time-domain samples of the input audio signal, at least one of the high frequency components is generated using the transposition factor T, and at least one other of the high frequency components is generated using a second transposition factor T 2 , wherein T is not equal to T 2 .
2. The audio signal processing device of claim 1 , wherein the oversampling factor F is greater or equal to (T+1)/2, and wherein the transposition factor T is an integer greater than 1.
3. The audio signal processing device of claim 1 , wherein the analysis window has a length L with zero padding by additional (F−1)*L zeros.
4. The audio signal processing device of claim 1 , wherein the one or more components further:
shift the analysis window by an analysis stride along the input audio signal to generate successive frames of the input audio signal;
shift successive frames of L time-domain output samples by a synthesis stride; and
overlap and add the successive shifted frames of L time-domain output samples to generate the output signal.
5. The audio signal processing device of claim 4 , wherein the one or more components further increase the sampling rate of the output signal by the transposition order T to yield a transposed output signal.
6. The audio signal processing device of claim 5 , wherein the synthesis stride is T times the analysis stride.
7. A method, performed by an audio signal processing device, for transposing an input audio signal by a transposition factor T to generate an output audio signal, the method comprising:
extracting a frame of L time-domain samples of the input audio signal using an analysis window of length L,
transforming the L time-domain samples into M complex frequency-domain coefficients,
altering a phase of the complex frequency-domain coefficients by converting one or more of the complex frequency-domain coefficients to a polar representation, and multiplying the phase of the polar representation by the transposition factor T;
transforming the altered frequency-domain coefficients into M altered time-domain samples; and
generating a frame of L time-domain output samples of the output audio signal from the M altered time-domain samples using a synthesis window;
wherein M=F*L, with F being a frequency domain oversampling factor determined in response to frequency domain oversampling information received in an encoded bitstream; and
wherein the frame of L time-domain output samples of the output audio signal comprises a plurality of high frequency components not present in the frame of L time-domain samples of the input audio signal, at least one of the high frequency components is generated using the transposition factor T, and at least one other of the high frequency components is generated using a second transposition factor T 2 , wherein T is not equal to T 2 .
8. A non-transitory computer readable medium comprising instructions for execution on an audio signal processing device, wherein, when executed by the audio signal processing device, the instructions cause the audio signal processing device to perform the method of claim 7 .