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Re: [Discuss-gnuradio] Filter in rational resampler

From: Andy Walls
Subject: Re: [Discuss-gnuradio] Filter in rational resampler
Date: Mon, 27 Oct 2014 16:40:21 -0400

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

> 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


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

        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


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