On 10/27/2014 04:40 PM, Andy Walls wrote:
On Thu, 2014-07-31 at 12:01 -0400, discuss-gnuradio-requ...@gnu.org
wrote:
Message: 5
Date: Wed, 30 Jul 2014 18:42:14 +0200
From: Daniele Nicolodi <dani...@grinta.net>
To: GNURadio <discuss-gnuradio@gnu.org>
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
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