RTL2832U FIR filter

Stefan Sydow st at metafly.info
Sun Jul 15 13:36:06 UTC 2012


Hi Michael,

> Thanks for the script, just some minor notes: SAMP_RATE should be
> 28.8e6, not 28e6. Also you don't seem to do any overflow checking on the
> conversion. If you are trying to build narrow filters, I expect you
> might hit the 1/16 limit on the outer 8 coefficients, so the filter
> coefficients might need to be shrunk (reducing the filter gain, of
> course) before converting to integers.

Good point. Thanks 
 
> I'm afraid that constructing a filter using gnuradio firdes and then
> truncating it to 32 coefficients is likely not resulting in a good
> filter. In my oppinion (not backed by math knowledge in that area,
> though), the filter length should already be considered during
> coefficient generation.

Yes that's right. Using gnuradio firdes was a first hack with acceptable 
error as the fft shows. A better way is to use sinc() and a window function.
Had a look into my "Signale and Systeme" script.

There is also a software called winfilter if you want a 'classic' filter. 
It works well under wine.
http://www.winfilter.20m.com/

> It looks like your plot uses the convention that 10dB is a factor of 10.
> As the filter affects amplitude values and not power values, you should
> use 20dB for a factor of 10, if I understand the dB scale correctly.

I missed that too. A fix version is attached.

Regards,
  Stefan
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