Re: Plotting the frequency response of a filter in "real" Hz

[Sergei Steshenko]

--------------------------------------------
On Thu, 4/21/16, Sergei Steshenko <address@hidden> wrote:

 Subject: Re: Plotting the frequency response of a filter in "real" Hz
 To: address@hidden, "Guilherme Ritter" <address@hidden>
 Date: Thursday, April 21, 2016, 9:18 PM
 
 
 --------------------------------------------
 On Thu, 4/21/16, Guilherme Ritter <address@hidden>
 wrote:
 
  Subject: Plotting the frequency response of a filter in
 "real" Hz
  To: address@hidden
  Date: Thursday, April 21, 2016, 3:44 AM
  
  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.
  -----Inline Attachment Follows-----
  
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  "the output's x axis is in radian frequency" - I guess you
 are talking about 'z' domain filter.
 
 If so, then something like this;
 
 % code BEGIN
 sample_rate = 44100; % or whatever you have
 number_of_points = 1000; % the bigger number of points, the
 better is resolution of the plot
 zfrequency_range = exp(pi * i * (0:number_of_points) /
 number_of_points); % from 0Hz to Nyquist frequency
 semilogx(zfrequency_range(2:end), 20 *
 log10(abs(F(zfrequency_range(2:end))))); % 'F' is your
 filter transfer function in 'z' domain
 % code END
 
 Pay attention to "(2:end)" instead of "(1:end)", i.e.
 instead of default full index range - in 'semilogx' you
 can't display x of 0  because log(0) is -inf .
 
 --Sergei.
 
 
 
A correction:

% code BEGIN
sample_rate = 44100; % or whatever you have
number_of_points = 1000; % the bigger number of points, the better is 
resolution of the plot
zfrequency_range = exp(pi * i * (0:number_of_points) /number_of_points); % from 
0Hz to Nyquist frequency
frequency_range = 0.5 * sample_rate * (0:number_of_points) /number_of_points); 
% from 0Hz to Nyquist frequency
semilogx(frequency_range(2:end), 20 * log10(abs(F(zfrequency_range(2:end))))); 
% 'F' is your filter transfer function in 'z' domain
% code END

--Sergei.