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Comments about the whitening filter.

Folks, the whitening filter turns out to be an IIR filter which is anticausal. This cannot be implemented with a finite delay. However, this can be implemented by filtering the sequence backwards in time. Here is a small MATLAB code that i wrote to show you how to do this.

% This is a program to see how to implement a whitening filter that is
% anticausal and stable

% define h and compute x

h = [1 0.9 0.8];
L = length(h)-1;
x = xcorr(h);
f = h; % This should be the equivalent channel including the whitening filter

% generate data

N = 1000;
I = [sign(rand(1,N)-0.5)]; % this is a random data sequence

% This is the filtering with x
y = conv(x,I);

% filter with the whitening filter
% Notice that the filter coefficient is f since implementing the filter
% with reverse time makes the equivalent filter like this. Think about this
% it is good to actually write this out on a piece of paper and see the equations
% in the time domain. Then try to compute v[n] from v[n+1], v[n+2] and y[n]
% Then you will see what is going on. dont try to compute v[n+2] from v[n+1], v[n]
% and y[n]. This is an unstable operation.

yflip = fliplr(y);
vflip = filter([1],f,yflip);
vtemp = fliplr(vflip);
v = vtemp(L+1:length(vtemp));

%-—Now we check to see if v is the same as filtering I with f
% The difference between u and v should be very small (ideally they
should be the same).

u = conv(f,I);

error = (u-v)*(u-v)'

[Comment:] It looks like this does not work when h is complex. Trying the channel coefficient we use in this project.
[Comment] Yes ,I also checked it,it is not working!
[Comment:] I believe that the whitening filter coefficients should be conj(f) rather than f.

_Question_ Could you please specify what the variable names would be in the audio_sample.dat and noisevariance.dat files, meaning how to use it in our program, can I just say load noisevariance.dat, variable_name = noise_variance?

[Comment, grader] Removed. Please follow the updated instructions at http://ecen661tamu.wikidot.com/midterm2.

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