[Hackrf-dev] FM 96.5MHz rx works but 70cm 441.1MHz rx does not?

[Cinaed Simson]

On 11/09/2016 08:31 PM, David Hubbard wrote:
> On Wed, Nov 9, 2016 at 3:16 PM, Cinaed Simson <cinaed.simson at gmail.com
> <mailto:cinaed.simson at gmail.com>> wrote:
> 
> 
>     In the NBFM receive block, try reducing Tau to 50u, and Max deviation to
>     4k.
> 
> 
> (to the hackrf-dev list this time)
>  
> Thanks, Cinaed! Playing around with max deviation in particular helps
> with reception quality, for the one good recording (see below).
> 
> I wasn't hearing anything but noise out the audio sink even eyeballing
> it on the FFT plot, the signal is 20dB over the noise floor. So I added
> a file sink, saved a few seconds of the 2.4Msps complex data output of
> the DC blocker (so almost directly out of the osmocom source). I then
> used a file source instead of the osmocom source -> DC blocker, and the
> result is loud and clear audio.
> 
> Even more puzzling is it only did it once. I collected several more
> recordings at 2.4Msps but they do not play back audio. Only the first
> recording produces audio. They all show a strong 20dB signal in the FFT
> plot and waterfall.

Try changing the audio sampling rate from 24kHz to 48kHz and increasing
the RF sampling rate to 5 MHz (if you can do it without dropping data) -
and change the resampler(s) accordingly.

Also, loose the waterfall on the source signal and add a FFT after the
low pass filter - so there's one on the carrier at the source and
there's one after the carrier has been multiplied by the 900k cosine and
filtered.

Does the spectrum after the low pass filter make sense? Can you see the
DC offset shift when you compare the 2 FFTs?

Can you see any frequencies shift?

You're sampling at 2.4 MHz - what is the Nyquist frequency?

How does that compare to 0.9 MHz?

Do you really need to shift signals by 0.9 MHz in order to avoid the DC
offset for a NBFM signal?

> 
> My grc file is attached. Possibly a lack of precision on frequency? I
> tried setting the tune_offset to values from 598e3 to 654e3, which has
> the normal effect on the first recording, but only static on the
> subsequent recordings.
> 
> I would like to add a "sync block," (is that the right term?) so that
> the graph only plays back at real time, to make it easier to change
> tuning parameters while playing back a recording. Any tips on how to do
> that? The attached grc plays back at real time but buffers a lot of
> audio at the audio sink so that changing things like tune_offset cannot
> be heard for about 2 sec.
> 
> 
> 
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