On Mon, 07 Nov 2011 10:16:39 -0800, Carnë Draug <carandraug+...@gmail.com>
wrote:
> On 7 November 2011 16:43, Alberto Cuadrado <a.cuadr...@gmail.com> wrote:
>> Hello,
>>
>> I think there is a bug in the fir1 function in signal-1.0.11 (and
>> probably
>> also in 1.1.1 because I don't see significant changes in the code). The
>> problem is that single-band lowpass filters returned by fir1 don't have
>> a DC
>> gain of 1. For example:
>>
>> octave-3.2.4:1> sum(fir1(240,0.01))
>> ans = 1.1822
>>
>> That should return 1 (with low-order filters the problem is not so
>> obvious).
>>
>> Regards
>>
>> Alberto Cuadrado
>
> I can confirm that this still occurs in octave 3.4.3 with signal 1.1.
> Do you think you can contribute a patch to fix this?
>
Are you sure it is a bug? I am assuming that you want the filter to be
normalized the same way as Matlab's fir1 function, and they say:
"By default, the filter is scaled so that the center of the first
passband has a magnitude of exactly 1 after windowing."
In other words, they are normalizing so the *frequency domain* passband
comes out to 1. The d.c. output level divided by the bandwidth will be
(roughly) 1.
Regards,
Jeff Cunningham
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