# Freqz function- plssssss help

Started by December 31, 2005

I am using notch filter, below are the commands for calculating
filter coeff:

f0 = 0.4 % freq to be notched

b = [1 -2*cos(2*pi*f0)
1];
% compute the filter numerator coeff "b"

r =0.995;
a = [1 -2*r*cos(2*pi*f0)
r^2];
% compute the filter denominator coeff "a"

[h,f1] =freqz(b,a,256);
figure
plot(f1/(2*pi), abs(h), 'r');

If "f0" i.e the freq to be notched is less than 0.5 then the plot
shows the notch at f0, but if its greater than 0.5 then the plot
shows some different frequency. Any one has any clue regarding this?

You're using two different conventions for relative frequency. The first (commonly used) convention is that the sampling frequency Fs  = 1. Your coefficients are based on this convention, so any notch frequency above 0.5 will exceed the Nyquist frequency Fs/2 and hence be aliased to another frequency.
The second convention (used by most MATLAB signal processing functions such as freqz) is where frequencies are relative to the Nyquist frequency , so Fs = 2.
So the f1 values in your plot range from 0 to 1, representing 0 to Fs/2, not 0 to Fs. Since the frequencies above Fs/2 are just a mirror reflection, they aren't returned by freqz at all.

Mark

From: m...@yahoogroups.com [mailto:m...@yahoogroups.com] On Behalf Of discreteworld
Sent: Saturday, December 31, 2005 1:24 PM
To: m...@yahoogroups.com
Subject: [matlab] Freqz function- plssssss help

I am using notch filter, below are the commands for calculating
filter coeff:

f0 = 0.4 % freq to be notched

b = [1 -2*cos(2*pi*f0)
1];
% compute the filter numerator coeff "b"

r =0.995;
a = [1 -2*r*cos(2*pi*f0)
r^2];
% compute the filter denominator coeff "a"

[h,f1] =freqz(b,a,256);
figure
plot(f1/(2*pi), abs(h), 'r');

If "f0" i.e the freq to be notched is less than 0.5 then the plot
shows the notch at f0, but if its greater than 0.5 then the plot
shows some different frequency. Any one has any clue regarding this?

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