Oversampling in a combined transposer filterbank
The present invention relates to coding of audio signals, and in particular to high frequency reconstruction methods including a frequency domain harmonic transposer. A system and method for generating a high frequency component of a signal from a low frequency component of the signal is described. The system comprises an analysis filter bank ( 501 ) comprising an analysis transformation unit ( 601 ) having a frequency resolution of Δf; and an analysis window ( 611 ) having a duration of D A ; the analysis filter bank ( 501 ) being configured to provide a set of analysis subband signals from the low frequency component of the signal; a nonlinear processing unit ( 502, 650 ) configured to determine a set of synthesis subband signals based on a portion of the set of analysis subband signals, wherein the portion of the set of analysis subband signals is phase shifted by a transposition order T; and a synthesis filter bank ( 504 ) comprising a synthesis transformation unit ( 602 ) having a frequency resolution of QΔf; and a synthesis window ( 612 ) having a duration of D S ; the synthesis filter bank ( 504 ) being configured to generate the high frequency component of the signal from the set of synthesis subband signals; wherein Q is a frequency resolution factor with Q≥1 and smaller than the transposition order T; and wherein the value of the product of the frequency resolution Δf and the duration D A of the analysis filter bank is selected based on the frequency resolution factor Q.
1. A system configured to generate a high frequency component of an audio signal from a low frequency component of the audio signal, comprising:
an analysis filter bank comprising an analysis transformation unit having a frequency resolution of Δf; and an analysis window having a duration of D A ; the analysis filter bank being configured to provide a set of analysis subband signals from the low frequency component of the signal;
a nonlinear processing unit configured to determine a set of synthesis subband signals based on a portion of the set of analysis subband signals, wherein phases of the portion of the set of analysis subband signals are modified based on a transposition order T, and wherein magnitudes of the portion of the set of analysis subband signals are modified based on the transposition order T; and
a synthesis filter bank comprising a synthesis transformation unit having a frequency resolution of QΔf; and a synthesis window having a duration of D S ; the synthesis filter bank being configured to generate the high frequency component of the signal from the set of synthesis subband signals;
wherein Q is a frequency resolution factor with Q≥1 and independent of the transposition order T; and wherein the value of the product of the frequency resolution Δf and the duration D A of the analysis filter bank equals the value of the product of the frequency resolution factor Q, the frequency resolution Δf and the duration D S of the synthesis filter bank.
2. The system of claim 1 , wherein the value of the product ΔfD A is proportional to
1
Q
+
1
.
3. The system of claim 2 , wherein the value of the product ΔfD A is smaller or equal to
2
Q
+
1
.
4. The system of claim 1 , wherein the value of the product ΔfD A is greater than
2
T
+
1
.
5. The system of claim 1 , wherein
the analysis filter bank and the synthesis filter bank are evenly stacked such that a center frequency of an analysis subband is given by kΔf and a center frequency of a synthesis subband is given by nQΔf.
6. The system of claim 1 , wherein
the analysis filter bank and the synthesis filter bank are oddly stacked such that a center frequency of an analysis subband is given by
(
k
+
1
2
)
Δ
f
and a center frequency of a synthesis subband is given by
(
n
+
1
2
)
Q
Δ
f
;
and
the difference between the transposition order T and the resolution factor Q is even.
7. A method for generating a high frequency component of an audio signal from a low frequency component of the audio signal, the method comprising:
providing a set of analysis subband signals from the low frequency component of the signal using an analysis filter bank comprising an analysis transformation unit having a frequency resolution of Δf and an analysis window having a duration of D A ;
determining a set of synthesis subband signals based on a portion of the set of analysis subband signals, wherein phases of the portion of the set of analysis subband signals are modified based on a transposition order T, and wherein magnitudes of the portion of the set of analysis subband signals are modified based on the transposition order T; and
generating the high frequency component of the signal from the set of synthesis subband signals using a synthesis filter bank comprising a synthesis transformation unit having a frequency resolution of QΔf and a synthesis window having a duration of D S ;
wherein Q is a resolution factor with Q≥1 and independent of the transposition order T; and wherein the value of the product of the frequency resolution Δf and the duration D A of the analysis filter bank equals the value of the product of the frequency resolution factor Q, the frequency resolution Δf and the duration D S of the synthesis filter bank.
8. A non-transitory computer-readable storage medium comprising a sequence of instructions, wherein, when executed by one or more processors, the sequence of instructions causes the one or more processors to perform a method for generating a high frequency component of an audio signal from a low frequency component of the audio signal, the method comprising:
providing a set of analysis subband signals from the low frequency component of the signal using an analysis filter bank comprising an analysis transformation unit having a frequency resolution of Δf and an analysis window having a duration of D A ;
determining a set of synthesis subband signals based on a portion of the set of analysis subband signals, wherein phases of the portion of the set of analysis subband signals are modified based on a transposition order T, and wherein magnitudes of the portion of the set of analysis subband signals are modified based on the transposition order T; and
generating the high frequency component of the signal from the set of synthesis subband signals using a synthesis filter bank comprising a synthesis transformation unit having a frequency resolution of QΔf and a synthesis window having a duration of D S ;
wherein Q is a resolution factor with Q≥1 and independent of the transposition order T; and wherein the value of the product of the frequency resolution Δf and the duration D A of the analysis filter bank equals the value of the product of the frequency resolution factor Q, the frequency resolution Δf and the duration D S of the synthesis filter bank.