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Re: [Discuss-gnuradio] Filter in rational resampler

From: Jeff Long
Subject: Re: [Discuss-gnuradio] Filter in rational resampler
Date: Mon, 27 Oct 2014 18:36:30 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

On 10/27/2014 04:40 PM, Andy Walls wrote:
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


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

Can someone point me to a relevant explanation?

Thanks a lot. Cheers,


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


It looks like the default filter is only valid where interp > decim, and it's not really meant to have an arbitrary cutoff. As Daniele pointed out, decim is not taken into account.

I think that 'interpolation' in design_filter() should be replaced with 'max(interpolation,decimation). If taps are supplied by the caller, it needs to understand how the taps will be used.

Andy, to mimic design_filter, you'd need to do:

  low_pass(16.0, 25000.0, 3250.0, 500.0, ...)

Not that I know if that's right, but that's what design_filter() does.

- Jeff

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