Re: [Discuss-gnuradio] Success (kinda)

[Eric Blossom]

On Mon, Sep 15, 2003 at 01:05:00PM +1000, Alan Gray wrote:
> All,
> 
> I've been working on putting together a circuit to use the
> microtune 4702 tuners I have and I think I've almost got it
> nailed. At the moment I'm sampling the 36MHz IF at
> 20MSample/sec which aliases the centre to 4MHz (reversed). I'm
> just trying to demodulate and listen to FM signals currently,
> however they're not sounding incredibly great.
> 
> I had a quick search through the archives here and found a
> thread on "Why FM doesn't sound good" so before I go nuts
> trying to figure out just why I'm not getting a good sounding
> audio stream I thought I'd ask just now "not good" FM sounds
> using the other hardware. 

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

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

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.

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

> 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.

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