I generate some coefficents for a filter and can inspect the frequency response as following:
%%Orginal Data N = 5000; data = cumsum(randn(N,1)); t = 252; a = 2 / (t+1); b = repmat(1-a,1 ,N).^(1:N); %b are your filter coeff b = b ./ sum(b); a = 1; %%Plot the Filter on some example data ma = filter(b, a, data); figure;plot(data); hold all; plot(ma, 'r'); %%Plot the Response figure;freqz(b,1); [h,w] = freqz(b,1);
I now explain my problem. I am now in the situation where I have a frequency response (i.e. the vector "h") and know nothing else.
I would like to estimate from this my original "b" (the filter coefficents) to allow me to estimate my variable "t".
I thought I could use invfreqs.m (or invfreqz.m) to do this, but Im afraid I dont know how.
%%Find the impluse response n = 10; % I choose a large number allowing a good approximation m = 0; % I choose 0 here as I have 1 in my orignal filter ==> the output comes out as aNew = 1; [bNew,aNew] = invfreqz(h,w,n,m); %[bNew,aNew] = invfreqs(h,w,n,m); sys = tf(bNew,aNew) %%Plot the filter coeffcients x1 = [0: 1/(size(b,2) -1) : 1]; x2 = [0: 1/(size(bNew,2) -1) : 1]; figure;plot(x1,b); hold all; plot(x2,bNew, 'r');
When I inspect the final plot, I would expect to see the red line (bNew) as a good approximation to b. It is not. not even close.
Clearly I am doing something very wrong. Please could someone with experince of how this function works, explain my mistake.
many thanks!
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invfreqz.m has some odd rules re calling it.
%%Orginal Data N = 5000; data = cumsum(randn(N,1)); t = 252; a = 2 / (t+1); b = repmat(1-a,1 ,N).^(1:N); b = b ./ sum(b); a = 1; %%Plot the Filter on some example data ma = filter(b, a, data); figure;plot(data); hold all; plot(ma, 'r'); %%Plot the Response figure;freqz(b,1, N); [h,w] = freqz(b,1,N); %%Using dflit % b = 1; a = -1; %Sanity Check. simple difference filter % Hd = dfilt.df1([b a],1); % num/ denom == a/b % fvtool(Hd); %%Find the impluse response % If n>(N-1) then you get a random answer!
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