freqz produces incorrect output

[Ben Abbott]

On Jan 25, 2009, at 12:46 PM, Søren Hauberg wrote:

> søn, 25 01 2009 kl. 12:07 -0500, skrev Ben Abbott:
>> Søren, can you try these instead?
>>
>> 	freqz ([ones(1, 20), zeros(1, 768-20)], 1, 512, "half", 2);
>>
>> 	freqz ([zeros(1, 768-20), ones(1, 20)], 1, 512, "half", 2);
>
> I've updated http://hauberg.org/freqz to only show these results
>
> Søren

Thanks Søren!

The second phase curve is different from Octave's. Specifically, the  
phase slope of the prepad (2nd) example has the opposite sign.

Using octave, I notice the sign of the phase slope is dependent upon  
the degree by which the input sequence exceeds the length  of the  
frequency vector. For the present example, the sign of the phase slope  
changes sign when the length input vector is greater than 522. See the  
examples below ...

	freqz ([zeros(1, 522-20), ones(1, 20)], 1, 512, 'half', 2);

	freqz ([zeros(1, 523-20), ones(1, 20)], 1, 512, 'half', 2);

So this problem manifests when the nominal delay exceeds 512.

My understanding is that the two responses below should be the same.

	p = [zeros(1, 768-20), ones(1, 20)];
	hz = freqz (p, 1, 512, 'half', 2);
	hf = fft (p, 1024)(1:512);

... and Octave does produce identical results.

Assuming that there is no problem with either Matlab or my  
understanding, I'm inclined to question the manner by which the two  
implementations are unwrapping the phase.

If the phase is unwrapped such that only positive delays are possible,  
I octave will produce the same plot as does Matlab. For example, the  
result below, looks convincing.

	f = (0:511)/512;
	da = -pi*f*512;
	plot (f, (unwrap(angle(hz)-da)+da)*180/pi, ...
                  f, (unwrap(angle(hf)-da)+da)*180/pi)

If my inference is correct, then freqz.m is not to blame. The problem  
is superficial, and is in freqz_plot.m

I'm not familiar with the signal processing functions. If someone has  
an opinion how/where this should be resolved, I'd appreciate the input.

For the time being, I'll assume it is best to modify freqz_plot.m, and  
generate a changeset including each of these changes.

Ben