Re: [Discuss-gnuradio] Success (kinda)

[Alan Gray]

>I'm assuming that you're tuned to a broadcast FM station, right?

Yes
>
>If you run adc3_fft and look at the spectrum, is there a nice
juicy
>spike where you expect your IF to be?

Yes, though the noise level surrounding the spike is larger
than I would like (see attached screenshot radio_fft.png) 

>
>Have you experimented with the RF and IF gain settings for
the tuner
>module?  For lowest noise figure, you want to max out the RF gain
>before ramping up the the IF gain.

The 4702 tuner only has one gain stage as it does a direct
conversion from RF to the 36MHz IF. Essentially I change this
gain to maximise the size of the peaks in the FFT while not
amplifying the noise

>In fm_demod1 and/or hifi_fm did you you change the definition
of the
>IF frequency?

Yes, I've tried both 4MHz (where the spectrum will be
reversed) and -4MHz (where it should not be reversed) to see
if that made any difference, no luck though

>> Basically what I'm hearing sounds like a radio which is tuned
>> to about 50KHz away from the actual carried frequency. I can
>> make out beats and baselines but it sounds as if I'm missing
>> the high end frequencies. I can make out when words occur, but
>> again they're very distorted and it's hard to determine what
>> is being said.
>
>Of the two wide band FM receivers we've got, hifi_fm sounds
better
>than fm_demod1.  In any event fm_demod1 doesn't suck
completely (sort
>of like a low cost hand held radio from the old days).  It's not
>really processing a wide enough piece of the spectrum to grab
it all
>(related to the fact that there aren't nice integer
decimation factors
>that get us from the IF input sampling rate to the sound card
output
>rate).  hifi_fm interpolates, and does a couple of other
things more
>right, and as a result sounds quite good.

Ok, I guess the problem is somewhere in the hardware then as
what I'm getting sounds nowhere near like even the most
primitive radio

>
>If anyone's looking for a not too hairy project, building a
version of
>hifi_fm.cc that decodes the L-R stereo info wouldn't be too hard.
>
>> Is this consistent with the results obtained by others or do
>> you get a clearer output? If others do get a clearer signal,
>> can anyone suggest a possible fix?
>> 
>> Thanks
>> Alan
>
>If you like I can send you some audio samples of FM I've
>demodulated...
>
>Eric
>

I actually grabbed the file with 5 seconds worth of FM data in
it that was posted here a while ago and ran that through
fm_demod1 to see what it sounded like. It was a whole lot
better than what I'm getting so other audio sample are
probably not necessary. 

I've also tried to tune to frequencies a little to either side
of my desired frequency to check if there is an issue with
drift in the frequency of the output IF, and that doesn't seem
to be the case. I'm starting to suspect that there may be an
issue with the amount of noise introduced into the system.

I'm using a different ADC to the MC4020, an Adlink PCI9812,
and I'm starting to think that might be having an impact on
performace. The second attached image (no_connection_fft.png)
shows the fft I'm getting with no signal input into the ADC.
Seems to be a large amount of noise there unfortunately,
though I'd like to compare it to a similar FFT using the MC4020.

If anyone has any suggestions on what it going wrong (and how
to fix it) they would be most appreciated.

Alan