How to Plot Absolute Frequency Response of Filter?
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I am currently using the freqz() command to plot the frequency response of an FIR filter designed in fdatool and exported to the workspace as a variable.This plots the correct frequency response (as seen in FDAtool), but with normalized frequency on the x-axis.I understand that the freqz() command returns 512 points from 0 to pi.I divide the index by 512 and multiply it with FS/2, which "corrects" the x-axis but when I plot this against 20*log10(abs(filter)), the amplitude is not as seen in FDA tool.
mag=(freqz(Filter1,512));
plot(((1:512)/512)*(44100/2),20*log10(abs(mag(1:512))));
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/164334/image.jpeg)
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Star Strider
am 15 Mai 2016
You can output the frequency in Hz if you give freqz the sampling frequency as an argument (at least in more recent MATLAB releases):
[mag,frq] = freqz(Filter1,2048,FS);
I’ve also noticed that fdatool and freqz give different results. I design my own filters with the individual filter functions, and analyse them with freqz. (I have nothing against using fdatool and the others, but when I learned signal processing, we had to design our own filters at every step, and I just got used to designing them that way.)
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Star Strider
am 16 Mai 2016
My pleasure.
If you use freqz with no outputs, it plots the frequency response by default:
figure(1)
freqz(Filter1,2048,FS)
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