Re: [Discuss-gnuradio] Success (kinda)

[Alan Gray]

This is starting to get more than a little frustrating, there go my two 
theories as to why things aren't working as they should. I've been 
experimenting for the past two days, and I've found a lot of things that 
aren't wrong, but not the thing that is:

The RF front end is working as expected. I fed the output IF into an 
AOR5000 receiver and could listen to FM radio fine at 36MHz and change the 
station and gain through the PC

The way I've written the ADC interface doesn't seem to be the influencing 
factor, I used a function supplied with the card to write a bunch of 
samples to file then ran the file through adc_fft and it looked the same as 
what I'm getting through the GNU radio interface.

I guess this is indicating that there is a problem somewhere in the 
software side of things, however I can read in a file of sample data and 
process it with no problems, so I'm not sure where I should start looking. 
My initial thought was that possibly Australian FM is in some way different 
to US FM, however this does not seem to be the case (though I haven't been 
able to find comprehensive standards for either case, so I may be missing 
something)

I've attached a few screen shots of the output I'm getting out of fm_demod1.
music.png and music2.png show the output when music is being received
no_signal.png shows the output when there is no sound (between speech etc)

In all cases, it appears that I have a reasonable lock onto the carrier 
frequency, as there seems to be a nice spike at about 19kHz. The 80 kHz 
wide complex down converted channel I'm a little concerned about, as this 
had nowhere near that magnitude of that in the only screenshot I could find 
of this online, I don't know how current that is though. In the audio 
output fft's you can see that I seem to be picking up a lot more signal at 
the low end than at the high end, and that the fft drop dramatically at 
about 15kHz, which is probably not a good thing.

If anyone has ANY ideas about where I could look to find my issue, it would 
be most appreciated

Alan


At 06:04 PM 16/09/03 +1000, Rob Judd wrote:

> Alan,
>
> A 60dB S/N is all you can expect from a tuner of this type. I don't
> think that's the problem, that looks just like the spectrum I get on the
> EA '92 Spectrum Analyser designed by Jim Rowe.
>
> Rob
>
>
> Alan Gray wrote:
> >
> > >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
> >
> >   ------------------------------------------------------------------------
> >  [Image]  [Image]
> >
> >   ------------------------------------------------------------------------
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no_signal.png
Description: PNG image

music2.png
Description: PNG image

music.png
Description: PNG image