I'm studying the IIR filter design that is described in the book: Algorithms for the constrained design of digital filters with arbitrary phase and magnitude responses.
You can get the code at page 171 (at least the main function), and here is an example of a filter design :
M=4; N=4; tau=5; om=pi*[linspace(0,0.2,20),linspace(0.4,1,60)]; D=[exp(-1i*om(1:20)*tau),zeros(1,60)]; W=ones(1,80); [b,a,e]=mpiir_l2(M,N,om,D,W,0.98);
This is the design of a pass-band linear phase lowpass filter
There is something I don't understand in this code:
exp(-1i*om(1:20)*tau) is a way to create regularly spaced points with constant phase change and magnitude = 1. But I don't understand the parameter
tau. I tried to change that parameter and I had totally different results: like a notch instead of a lowpass.
- How do you choose that number ?
- I have to design my filters according to the user desire but how to guess that parameter?
- And also how to you choose the phase angles shift ?