Re: [Discuss-gnuradio] gqrx branch osmosdr

[Patrick Strasser]

Alexandru Csete wrote on 2012-06-05 19:06:
> On Tue, Jun 5, 2012 at 6:51 PM, Patrick Strasser
>> In comparison to the rtlsdr fork I see you added FM-W. FM-N in
>> comparison sounds more clipped, FM-W is clipped with a narrow filter,
>> but noisy with a wide filter. With a filter that has no clipping, noise
>> is clearly audible. I'm not sure if this is bound by the low dynamic
>> sampling range (8 bit) in combination with big bandwith which means a
>> lot more noise energy with wide filters. Anyway, gqrx gets greater every
>> time!
> 
> There is no audio filter yet (except de-emphasis) so you get pretty
> much 48 kHz worth of noise, including stereo pilot tone and whatever
> crap they include in a broadcast FM channel these days.

It's AM and some special BPSK. Nice ensemble of different analogue and
digital schemes, maybe we'll use it as an demonstration in one of our
lectures here at University.
Anyway, my ears have a builtin filter that cuts dramatically everything
above some 18kHz, not sure if I can hear the pilot tone. If you are not
doing dirty tricks every decimating filter should prevent aliasing, so
no effect of everything that's higher than half the sampling rate. With
48 kHz you should not get even close to the higher energy parts of AM at
arround 38kHz.
I guess where noise comes in is before FM demodulation: the bigger the
sampled frequency bandwidth, the more noise energy. With the limited
dynamic range this decreases the signal (FM) to noise (background)
ratio, which opposes good demodulation results.
Per design and regulation a FM radio signal has mono 180 kHz bandwidth
and stere 300 kHz, but I do not know of a station sending in mono here
in Austria. Maybe a kind of matched filter would be best: you can expect
that the wider fare a frequency is from center frequency, the less
energy it has. Maybe thirds would work: < -150 kHz stop, up to -50 kHz
transition, -50 to +50 kHz pass, +50 up to +150 transition, and then
again stop. Frequency deviation is specified as 75 kHz.

Regards

Patrick
-- 
Engineers motto: cheap, good, fast: choose any two
Patrick Strasser <patrick dot strasser at  tugraz dot at>
Student of Telematics, Graz Univ. of Technology, Austria