resynchronization of the adaptive codebook in a constrained celp codec after a frame erasure
DESCRIPTION
RESYNCHRONIZATION OF THE ADAPTIVE CODEBOOK IN A CONSTRAINED CELP CODEC AFTER A FRAME ERASURE. Mohamed Chibani, Roch Lefebvre and Philippe Gournay Université de Sherbrooke, Sherbrooke, Québec, Canada. Outline. Basic CELP model Constrained optimization Resynchronization at the decoder - PowerPoint PPT PresentationTRANSCRIPT
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RESYNCHRONIZATION OF THE ADAPTIVE CODEBOOK IN A
CONSTRAINED CELP CODECAFTER A FRAME ERASURE
Mohamed Chibani, Roch Lefebvre and Philippe Gournay
Université de Sherbrooke, Sherbrooke, Québec, Canada
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SHERBROOKE
Outline
Basic CELP model Constrained optimization Resynchronization at the decoder
Open-loop search of the shift (drift) of the ACB Closed-loop search of the shift
Pitch contour modification Experimental results Conclusions
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Excitation Model in CELP Coding
ICB (ACELP)
ACB )(
1
zA
pg
cgSynthesized
speech
Codebook index
ACB delay
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Excitation Search in CELP Coding
Calculate the optimal gain of the ACB
y0(n) is the (non-scaled) ACB contribution
Determine the optimal delay of the ACB
Update the target for the ICB parameters search
Build the target signal x1(n) from the input speech
.)(
)()(
2
0
01
0
n
n
ny
nynxg
)()()( 0012 nygnxnx
)(zW
)(zW)(1 zA
)(zW)(1 zA
)(zW)(1 zA
)(nx
ICB (ACELP)
ACB
Zero input
Error minimization
)(2 nx
)(1 nx
+-
+
+
-
-cg
pg)(0 ny
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At the Encoder…
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Constrained Search of the Excitation Parameters
Evaluate the ACB contribution
Determine the ACB parameters (the gain and the delay)
)55.0(
th
th
R
RR yesno
Update the target for the ICB parameters search
n
n
ny
nynxg
)(
)()(
2
0
01
0
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nth
ny
nxRg
)(
)(
2
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)()()( 0012 nygnxnx
n
n
x
p
nx
nyg
E
ER
)(
)(
2
1
2
0
2
0
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At the Decoder…
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Prelude to the Resynchronization Algorithm
After a frame erasure, both the waveform and the position of the pitch pulses in the ACB memory are erroneous.
For voiced speech, the pitch pulse waveform evolves slowly.
If the expected position of the last pitch pulse in the ACB memory can be determined, the ACB memory can be corrected.
Due to the constraint, a good approximation of the pitch pulse can be obtained using only the parameters of the current frame.
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The Excitation Signal Obtained After Setting to Zero the ACB Memory
Excitation signal e0(n)
The correct excitation signal
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Block Diagram of the Resynchronization Algorithm
Search for P(-1)
ACB memory Determination of the
potential shift d0
Set to zero the ACB memory and
build eo(n)
Estimation of P(0)
Closed-loop search for the
optimal shift dopt
P(-1)
eo(n)
P(0)
dopt
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Current frame
subframe 0
subframe 1
subframe 2
subframe 3
ACB memory
)1(P)0(P
Determination of the Expected Pitch Pulse Position in the Erroneous ACB Memory
The excitation e0(n)
The correct excitation
The last pulse in the ACB memory
P(-1)
ACB delays
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Estimation of the Shift d0
P(0) P(-1)
ACB memory
T(-1)
0d )P()P(δ 100
P(0) : The expected position of the last pitch pulse in the ACB memory
P(-1) : The actual position of the last pitch pulse in the ACB memory
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Closed-loop Search for the Optimal Shift
1_
_
2
0
1_
_0
)(
)()(
)( FRML
LFRMLn
FRML
LFRMLn
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nene
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L_FRM=256
))((maxarg dd d Copt
2,2 00 ddd
L=max(2*L_SBFR,T(3))
T(3) is the ACB delay of the 4th subframe
ed is the excitation
signal built after correcting the ACB for every shift candidate
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Example of a Resynchronized Excitation
The correct excitation
The excitation e0(n)
The excitation signal built using the erroneous ACB memory
The excitation signal built after correcting the ACB memory
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Modification of the Pitch Contour After the Resynchronization
current frame next frameprevious frame
a)
b)
c)
T(0)
T(0)+dopt
T(0)+d1
The correct excitation
The excitation after the resynchronization
The excitation after the modification of the pitch contour
1
1
pN
iiopt dd di is the shift of each interval
Np is the number of pitch periods
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The Effect of the Resynchronization Algorithm when Applied on Voiced
Speech Segment
1 8765432
Lost frame
a)
b)
c)
d)
Frame number
The frame where the resynchronization is
applied
Error-free signal
Standard codec
Constrained codec
Constr. + resynchro.
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Experimental Results
0
10
20
30
40
50
60
70
80
90
MU
SH
RA
Sco
re
0% 5% 10%
Frame erasure rate
Test features:
AMR-WB at mode 2 (12.65 kb/s)
10 listeners 14 pairs of sentences
for each condition Listening using
binaural headphones
42.3
6
29.9
9
74.8
8 Standard codec73
.67
49.1
8
36.7
0
Constrained codec73.6
7
53.9
0
40.4
0
Constr. + Resynchro.
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Conclusions
The resynchronization allows to speed up the recovery of the decoder after a frame erasure.
The method (constraint + resynchronization) needs neither extra bits nor extra delay.
The modified codec is completely interoperable with the standard (the bitstream is not modified).
Only 10 to 15% of the frames following an erased frame are resynchronized.
The only drawback is a minor loss of quality in error-free channels.
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Thank you