On Wed, Jun 6, 2012 at 12:29 AM, Patrick Strasser <patrick.stras...@student.tugraz.at> wrote: > 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.
Thanks for the info. I suspect reception quality will also depend on the device, gain settings and how much interference there is. Anyway, I have increased the sample rate of the WFM receiver to 240 ksps, done some tweaking of the resamplers (some would say I fixed a major screw-up) and I think it now sounds quite all right. I can even use the Funcube Dongle for WFM and get decent audio with minimal distortions. Here is a short video showing hotswap between RTL device and Funcube Dongle on the same WFM station: http://www.youtube.com/watch?v=O9pLDLvOsPk Alex _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
