On Thu, 2014-07-31 at 12:01 -0400, [email protected]
wrote:
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