From bug-request at octave dot org Sun Jan 29 00:31:36 2006
Subject: freqz fails on FIR filters
From: Quentin Spencer <qspencer at ieee dot org>
To: bug at octave dot org
Date: Sun, 29 Jan 2006 00:27:57 -0600

This bug appears to be introduced relatively recently, and is in both 
2.1.72 and 2.9.4. With an IIR filter, everything is OK:

octave> freqz([1,1],[1,-0.1]);

Octave creates a plot as expected. On the other hand, when the filter is 
FIR, meaning there is only one argument, or the second argument is a 
scalar, the function fails:

octave> freqz([1,1],[1]);
error: quotient: nonconformant arguments (op1 is 1x512, op2 is 512x1)
error: evaluating binary operator `./' near line 144, column 10
error: evaluating assignment expression near line 144, column 5
error: called from `freqz' in file `/home/quentin/freqz.m'


The problem arises because the postpad function appends along the first 
non-singleton dimension, which creates a column vector rather than a row 
vector when the input is a scalar. This can be fixed by explicitly 
specifying the dimension, as in the patch below.


-Quentin




Index: scripts/signal/freqz.m
===================================================================
RCS file: /cvs/octave/scripts/signal/freqz.m,v
retrieving revision 1.22
diff -u -r1.22 freqz.m
--- scripts/signal/freqz.m      15 Dec 2005 01:16:27 -0000      1.22
+++ scripts/signal/freqz.m      29 Jan 2006 06:25:41 -0000
 at @ -131,12 +131,12 @@
     ## would be faster to use polyval but in practice the overhead for
     ## polyval is much higher and the little bit of time saved isn't
     ## worth the extra code.
-    hb = fft (postpad (b, n));
-    ha = fft (postpad (a, n));
+    hb = fft (postpad (b, n, 0, 2));
+    ha = fft (postpad (a, n, 0, 2));
   else
     f = Fs/2 * (0:n-1)' / n;
-    hb = fft (postpad (b, 2*n))(1:n);
-    ha = fft (postpad (a, 2*n))(1:n);
+    hb = fft (postpad (b, 2*n, 0, 2))(1:n);
+    ha = fft (postpad (a, 2*n, 0, 2))(1:n);
   endif

   h = hb ./ ha;



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