From bug-octave-request at bevo dot che dot wisc dot edu Sun May  5 14:54:30 2002
Subject: decimate.m
From: Mirek Kwasniak <mirek at zind dot ikem dot pwr dot wroc dot pl>
To: bug-octave at bevo dot che dot wisc dot edu
Date: Sun, 5 May 2002 21:53:01 +0200

Hi,

Tried decimate and got very weird results :(

With FIR filter, you have only -10..-12 dB at new Nyquist frequency.
Whith IIR even worse, cheby1 function gives badly scaled coefficients.

You can see results for decimations factor 2:10 here:

  http://www.zind.ikem.pwr.wroc.pl/dydaktyka/mirek/dec_filters.ps

My script at end (frequency axis normalized to 1 at new Nyquist frequency).

Mirek

PS

Where report bugs agains octave-forge?

----------------------------------------------------
%/usr/share/octave/2.1.36/site/m/octave-forge/signal/fir1.m
%/usr/share/octave/2.1.36/site/m/octave-forge/signal/cheby1.m
%/usr/share/octave/2.1.36/site/m/octave-forge/FIXES/freqz.m

clf;

nsubplot(211);
n=30;
for q=2:10;
  b=fir1(n, 1/q);
  [H, W]=freqz(b,1);
  plot(W/pi*q,20*log10(abs(H).^2)); % square because decimate uses filtfilt
end;
grid('on');
title('FIR');
ax(0,2);
ay(-60,6);

nsubplot(212);
n=8;
for q=2:10;
  [b,a]=cheby1(n, 0.05, 1/q);
  [H, W]=freqz(b,1);
  plot(W/pi*q,20*log10(abs(H).^2));
end;
grid('on');
title('IIR');
ax(0,2);
drawnow 



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