Re: [Discuss-gnuradio] Filter in rational resampler

[Andy Walls]

On Thu, 2014-07-31 at 12:01 -0400, address@hidden
wrote:

> Message: 5
> Date: Wed, 30 Jul 2014 18:42:14 +0200
> From: Daniele Nicolodi <address@hidden>
> To: GNURadio <address@hidden>
> Subject: [Discuss-gnuradio] Filter in rational resampler
> 
> Hello,
> 
> I was studying the code of the rational resampler block in
> gnuradio/gr-filter/pythoin/rational_resampler.py and I have a doubt
> about the low pass filter generated by the design_filter() function.
> 
> It seems that the generated filter does not take into account the
> decimation factor. Is that correct? I don't see how this may result in
> the correct anti-aliasing filter when it is applied by
> rational_resampler_base_xxx.
> 
> Can someone point me to a relevant explanation?
> 
> Thanks a lot. Cheers,
> Daniele

Daniele,

I just had occasion to use the rational resampler for a 25 Ksps -> 16
Ksps resampling and low-pass filtering all in one step, with a LPF that
cut off frequencies higher than 3000 Hz.  I started by using this
expression for the taps, following the filter design in
gr-filter/python/rational_resampler.py:

        filter.firdes.low_pass(16.0, 16000.0, 3250.0/16.0, 500.0/16.0, 
filter.firdes.WIN_KAISER, 5.0)

That filter only includes the interpolation factor, 16.0, and seemed to
do the wrong thing.  The FFT scope showed the rolloff started at around
~4700 Hz, about 25/16 * 3000 Hz.

This expression for the taps, which included the decimation factor of
25.0, appeared to do the right thing:

        filter.firdes.low_pass(16.0, 16000.0, 3250.0/25.0, 500.0/25.0, 
filter.firdes.WIN_KAISER, 5.0)


Can someone else take a closer look at
gr-filter/python/rational_resampler.py and confirm it is doing the wrong
thing?

Regards,
Andy