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Aparna Ram · Guest

Dear all,

I am working on the acoustic echo cancellation.I have
implemented the time domain LMS algorithm for acoustic echo
cancellation and it is working fine only for some audio files and not
working for some other audio files which has the large eigen value
spread.So to avoid this eigen value spread now I am working on the
subband LMS algorithm in frequency domain for acoustic echo
cancellation.So now I am searching for the documents of subband LMS
algorithm and frequency domain LMS algorithm in which it is mentioned
clearly which samples are to be considered and why.But I didnt find
those details.But I understood the overall view of frequency domain LMS

algorithm from the documents that I collected and its steps are as
follows:

Step1:For every sample of a frame (ie., 10 m sec data), that sample and

its previous samples, a total of samples equal to number of weights of
the filter are taken(as a full band) and they are converted to
frequency domain using DFT.

Step2:Initially all the weights of the filter are taken as zeros in
frequency domain.

Step3:Then to get the echo sample of kth subsequent sample position its

corresponding input sample in frequency domain(farend in frequency
domain) are multiplied with its corresponding weight in frequency
domain (its corresponding weight).
N(k) = X(k) * W(k)

Step4:Then the corresponding error sample is obtained by subtracting
this echo sample from the desired sample in frequency domain.
Ek = Yk - Nk
where Yk is the desired signal sample in frequency domain.

Step5:Using this error sample in frequency domain all the weights of
the filter are updated using the following formula:

Wk+1(i) = Wk(i) + 2 * muk * Ek * Xk
where Xk is column vector containing the current sample and its
previous samples.
Wk is the column vector of all the weights of the filter.

Step6:All the above steps are repeated for each and every sample of the

frame untill the end of the frame and is repeated for all the frames of

the audio file.

My doubt is in the calculation of desired samples in
frequency domain ie.,Yk .According to logic, I think these frequency
components are calculated using all the samples of desired signal of a
frame.But it is nowhere mentioned in the documents that which samples I

need to consider for this frequncy band desired samples Okay if we
generate frequency domain desired samples equal to number of desired
samples in time domain using all these time domain samples.If I
integrate this idea with the subband LMS algorithm then again I am
getting the doubt that for each subband of farend signal(ie., obtained
by dividing a full band of farend signal consisting of current sample
of the frame and its previous samples to a particular number of
subbands)whether we need to have the same desired signal frequency
component for all the subbands or different frequency component for
each subband and how it is calculated.I have these type of doubts and
if we think of that there may be several possibilities.

So I am searching for that, but I am not getting the
required points.

I think one solution for this is converting the code of
time domain LMS algorithm to frequency domain and checking the code for

all the possibilities by trial and error method and seeing that for
which combination we will get the perfect output.But I am not getting
the output.

Here I have converted the farend signal, desired signal
and weights of the filter are converted to freqency domain and after
performing the operation the error signal is reconverted to time
domain.And the convergence weight factor is given in different ways in
different documents.I am in total confusion to proceed further.

I am very greatful to you if any one give me suggestions
and guidence regarding this.

Thanking in advance.

Regards,
Aparna Ram.K.