freqz() uses normalised frequencies. If you want specific frequencies, you just have to de-normalize them.
How much help you need, probably depends on how much you have learned about frequency response and transfer functions. Do you understand what freqz() does enough so that you can "fork" it for yourself, and replace the normalised frequencies with specific Hz values?
If not I can provide a bit of code. Probably the best thing for your DSP chops, though, is to first code up your own frequency response function based on what you understand freqz() to be doing.
Message: 6
Date: Wed, 20 Apr 2016 21:44:42 -0300
From: Guilherme Ritter <address@hidden>
To: address@hidden
Subject: Plotting the frequency response of a filter in "real" Hz
Message-ID:
<address@hidden>
Content-Type: text/plain; charset="utf-8"
Hi everyone.
I've just started in filter design at college and I'm learning to use
Octave for it. I want to see the frequency response of filters I design.
I've managed to find code on the internet, but the output's x axis is in
radian frequency. I'd like it to show "actual" Hz. For example, if the
cutoff frequency is 5,5 kHz, I'd like for it to be represented in the
plot's x axis at 5500 or 5,5.
I've searched a lot but couldn't find anything, only some solutions that
work in MatLab but not in Octave. At college, I'm using Octave 4.0.0,
packages control 3.0.0 and signal 1.3.2, Windows 7 Enterprise x64. At home,
all the versions are up to date, Xubuntu 14.04 x64.
I've found the code here:
https://ccrma.stanford.edu/~jos/fp/Example_LPF_Frequency_Response.html
Can I use Octave's functions to get that plot the way I want it?
Thanks in advance.