Re: [Discuss-gnuradio] Filter in rational resampler

[Jeff Long]

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

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