From help-request at octave dot org Sun Apr 16 01:03:52 2006
Subject: Re: freqz / postpad problem
From: Quentin Spencer <qspencer at ieee dot org>
To: "Dmitri A. Sergatskov" <dasergatskov at gmail dot com>
CC: Octave Help <help at octave dot org>
Date: Sun, 16 Apr 2006 01:02:47 -0500

Dmitri A. Sergatskov wrote:
> I found a buglet in freqz (octave 2.9.4, Quentin's  rpm for Fedora Core 5).
> "test freqz" fails with the following error:
>
> octave:57> test freqz
>   ***** test # whole-half consistency
>  b = [1 1 1]/3; # 3-sample average
>  [h,w] = freqz(b,1,32,'whole');
>  assert(h(2:16),conj(h(32:-1:18)),20*eps);
>  [h2,w2] = freqz(b,1,16,'half');
>  assert(h(1:16),h2,20*eps);
>  assert(w(1:16),w2,20*eps);
> !!!!! test failed
> error: quotient: nonconformant arguments (op1 is 1x32, op2 is 32x1)
>
> The culprit is the follqwing code (lines 134 and 135 in freqz.m):
>
>
>     hb = fft (postpad (b, n));
>     ha = fft (postpad (a, n));
>
> (and later)
>
>     h = hb ./ ha;
>
> The problem here is that when b is a vector (e.g. 1:m) postpad
> and consecuetly fft return 1:n vector as well, but if a is just a number
> then postpad (and fft later) return n:1 vector.
>
> I really do not know what would be the best fix. In particular I am
> not sure that postpad behavior is OK.
>   

Try out the CVS version of freqz--it should fix your problems. I think I 
reported this same bug a while ago on the 2.1.x branch. The freqz 
function had quite a few changes made to it recently, but apparently 
after 2.9.4. I suppose the best fix for this would be a 2.9.5 release 
for Fedora. The reason I hadn't done that yet was that the current 
octave-forge release has the old behavior for the system function, so it 
shouldn't really be used for 2.9.5 or later. I could try patching it, 
but I thought a new release would be easier. Anyway, I'll figure 
something out soon, but it's getting late for me to do it today.

-Quentin



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