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 to generate a high frequency component of an audio signal, comprising:
an analysis filter bank having a frequency resolution of Δf and an analysis window of duration D A , the analysis filter bank providing a plurality of analysis subband signals from the audio signal;
a nonlinear processing unit determining a synthesis subband signal based on at least some of the plurality of analysis subband signals, wherein the at least some of the plurality of analysis subband signals are phase shifted by a transposition order T; and
a synthesis filter bank having a frequency resolution of Q·Δf, the synthesis filter bank generating the high frequency component from the synthesis subband signal,
wherein the value of the product of the frequency resolution Δf and the duration D A of the analysis filter bank is determined based on the frequency resolution Q·Δf of the synthesis filter bank.
2. The system of claim 1 , wherein
Δ
f
≤
2
Q
(
D
A
+
D
S
)
,
wherein the synthesis filter bank has a synthesis window of duration D S .
3. The system of claim 1 , wherein the nonlinear processing unit determines the synthesis subband signal from two consecutive analysis subband signals.
4. The system of claim 1 , wherein the value of the product Δf·D A satisfies:
2
T
+
1
<
Δ
f
·
D
A
≤
2
Q
+
1
.
5. The system of claim 1 , wherein the system comprises only a single analysis filter bank and a single synthesis filter bank for generating the high frequency component.
6. The system of claim 1 , wherein the value of the product ΔfD A of the analysis filter bank equals the value of the product QΔfD S of the synthesis filter bank, wherein the synthesis filter bank has a synthesis window of duration D S .
7. The system of claim 1 , wherein
the analysis filter bank comprises an analysis transformation unit to perform one of a Fourier Transform, a Fast Fourier Transform, a Discrete Fourier Transform, a Wavelet Transform; and
the synthesis filter bank comprises a synthesis transformation unit to perform the corresponding inverse transform.
8. The system of claim 1 , wherein the analysis and/or synthesis window is one of:
Gaussian window;
cosine window;
Hamming window;
Hann window;
rectangular window;
Bartlett window;
Blackman window.
9. The system of claim 1 , further comprising:
a second nonlinear processing unit configured to determine a second set of synthesis subband signals from the set of analysis subband signals using a second transposition order T 2 ; wherein the second set of synthesis subband signals is determined based on a portion of the set of analysis subband signals, phase shifted by the second transposition order T 2 ; wherein the transposition order T and the second transposition order T 2 are different; and
a combining unit configured to combine the set of synthesis subband signals and the second set of synthesis subband signals; thereby yielding a combined set of synthesis subband signals as an input to the synthesis filter bank.
10. The system of claim 9 , wherein
the combining unit is configured to superpose synthesis subband signals of the set of synthesis subband signals and the second set of synthesis subband signals corresponding to overlapping frequency ranges.
11. The system of claim 1 , wherein
the analysis filter bank has a number K A of analysis subbands, with K A >1, where k is an analysis subband index with k=0, . . . , K A −1; and
the synthesis filter bank has a number N S of synthesis subbands, with N S >0, where n is a synthesis subband index with n=0, . . . , N S −1.
12. The system of claim 11 , wherein the nonlinear processing unit is configured to determine an n th synthesis subband signal of the set of synthesis subband signals from a k th analysis subband signal and a (k+1) th analysis subband signal of the set of analysis subband signals.
13. The system of claim 12 , wherein the nonlinear processing unit is configured to
determine a phase of the n th synthesis subband signal as the sum of a shifted phase of the k th analysis subband signal and a shifted phase of the (k+1) th analysis subband signal; and/or
determine a magnitude of the n th synthesis subband signal as the product of an exponentiated magnitude of the k th analysis subband signal and an exponentiated magnitude of the (k+1) th analysis subband signal.
14. The system of claim 13 , wherein
the analysis subband index k of the analysis subband signal contributing to the synthesis subband with synthesis subband index n is given by the integer obtained by truncating the expression
Q
T
n
;
wherein a remainder r is given by
Q
T
n
-
k
.
15. The system of claim 14 , wherein the nonlinear processing unit is configured to
determine the phase of the n th synthesis subband signal as the sum of the phase of the k th analysis subband signal multiplied by T(1−r) and the phase of the (k+1) th analysis subband signal multiplied by T(r); and/or
determine the magnitude of the n th synthesis subband signal as the product of the magnitude of the k th analysis subband signal raised to the power of (1−r) and the magnitude of the (k+1) th analysis subband signal raised to the power of r.
16. 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.
17. 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.
18. The system of claim 1 , wherein
a sampling rate of the audio signal is f A ;
the analysis transformation unit is a discrete M point transformation; the analysis window has a length of L A samples; and the analysis window is shifted by an analysis hop size of Δs A samples along the audio signal;
the frequency resolution is
Δ
f
=
f
A
M
;
the duration is
D
A
=
L
A
f
A
;
a physical time stride of the analysis filter bank is
Δ
t
A
=
Δ
s
A
f
A
;
a sampling rate of the high frequency component is f S =Qf A ;
the synthesis transformation unit is a discrete M point transformation; the synthesis window has a length of L S samples; and the synthesis window is shifted by a synthesis hop size of Δs S samples along the high frequency component; and
the frequency resolution is
Q
Δ
f
=
f
s
M
;
the duration is
D
s
=
L
s
f
s
;
a physical time stride of the synthesis filter bank is
Δ
t
s
=
Δ
s
s
f
s
=
Δ
s
A
f
A
=
Δ
t
A
.
19. A method for generating a high frequency component of an audio signal, the method comprising:
applying an analysis filter bank having a frequency resolution of Δf and an analysis window of duration D A on the audio signal;
determining a synthesis subband signal based on at least two analysis subband signals, wherein the analysis subband signals are phase shifted by a transposition order T; and
applying a synthesis filter bank having a frequency resolution of Q·Δf and a synthesis window of duration D S on the synthesis subband signal;
wherein the value of the product of the frequency resolution Δf and the duration D A of the analysis filter bank is determined based on the frequency resolution of the synthesis filter bank.
20. A method for designing a harmonic transposer to generate a high frequency component of an audio signal, the method comprising:
providing an analysis filter bank having a frequency resolution of Δf and an analysis window of duration D A ;
providing a nonlinear processing unit to determine a synthesis subband signal based on at least two analysis subband signals, wherein the analysis subband signals are phase shifted by a transposition order T;
providing a synthesis filter bank having a frequency resolution of Q·Δf and a synthesis window of duration D S ; and
selecting the value of the product of the frequency resolution Δf and the duration D A of the analysis filter bank based on the frequency resolution of the synthesis filter bank.