de-reverberation using lpc -...
TRANSCRIPT
De-Reverberation Using LPC
Presented by:Eyal Enav
LPC temporal averaging basic concept
• LPC coefficients achieved by
• Estimation error = residual excitation.
• The spatial expectation of the LPC coefficients is not affected by reverberation.
• Applying the LPC filter to reverbed signal yields reverbed excitation.
ARMSE S S
Single channel LPC
• Calculate the cross correlation vector .
• Calculate autocorrelation matrix .
• The LPC filter is
• The filter approximates the clean speech LPC in terms of spatial expectation:
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ˆ ˆspatialE b a
1ˆss ssa R r
Multichannel LPC
• For each channel calculate
• The LPC filter achieved by averaging:
• Approximates spatial averaging.
• Best results.
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1ˆxx xxb R r
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R RM
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DSB- LPC
• Perform delay and sum beamformer.• Calculate LPC based on DSB output:
• LPC estimation error (Itakura distance) :– Increases with reverberation time. – Bad results
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1 M
m m
m
x x nM
1ˆDSB xx xxb R r
Z Plane MC/DSB LPC
LPC-Single/Multi/DSB
Extracting the reverbed excitation
• Theory:
If the LPC coefficients are identical to those of the clean speech signal. (Graph Itakura for MC)
Then applying the LPC filter to the reverbed signal results in the reverbed excitation:
is the Fourier transform of the excitation at the microphone.
is the Fourier transform of the RIR from the source to the microphone.
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m mE e E e H e
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Algorithm
1. Multichannel LPC filter is calculated.
Algorithm
2. LPC filter is applied on the DSB-output.
Algorithm
3.Glottal Closure Instants locations are identified (complex algorithms)
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0.005
0.01
0.015
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0.025
GCI identification on LPC residual
Algorithm
4. Larynx Cycles are averaged (de-reverberation)
Actual
De-Reverberation
Larynx Cycles Averaging
• Presumption:There is low correlation between the reverb impulse response of the consecutive GCIs within consecutive larynx cycles.
Therefore averaging consecutive larynx cycles is expected to remove the unwanted reverb.
• In fact :– True only for large T60 > Larynx cycle length.– False for early reverberation
Larynx Cycles Averaging
Ideal:• Perform averaging only of the reverb signal in
consecutive larynx cycles.• Keep the original glottal pulses.
Should be taken to consideration:• GCI’s estimation is imperfect.• The glottal pulse is spread in time.
Larynx Cycles Averaging
Method :• Multiply each of the consecutive larynx cycles by
a Tukey window (W) and average.• Add to the average in order to keep
the original glottal pulses.
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vec j vec j vec j
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e n I W e n We n iK
( ) jI W e n
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vec j j j j Ln e n e n e n
Larynx Cycles Averaging
Larynx Cycle Averaging
Symmetric Tukey window averaging ignores:• Short, early reverberations traces are more likely to
be correlated.(T60<Larynx cycle length)
• Reverb signal tends to decay with time.
• The glottal pulse is asymmetric.
Use a window that uses prior information:• Biased Tukey window:
Emphasizing the early part of the larynx cycle.
• Glottal pulse shape oriented window:– Assuming GCI is precise.– Define a window the relates to the common shape
of a glottal pulse.
Suggested Improvement
LPC dereverbration frameworkimplemented
• RIRs created using Emanuël Habets RIR generator.
• Ideal DSB employed using known source to microphones delays.
• Multi channel LPC employed.
LPC dereverbration frameworkimplemented
• GCIs extracted from clean speech are employed for larynx cycle averaging.
• Larynx cycles are scaled to the center cycle size prior to averaging.(sync interpulation)
• Larynx cycle averaging using Tukey window.