[Discuss-gnuradio] Why FM doesn't sound good

[Matthew Ettus]

        I believe that the reason FM radio doesn't sound as good as it should
is that the demod bandwidth is too low.  In the example code, the 20MHz
sample rate is decimated by 125, to 160ksamples/sec (complex), for less
than 160kHz BW.  To get good fidelity you really need 200 to 250 kHz, or
the signal will be distorted ("soft" clipping).

        Unfortunately, since we are constrained by soundcard output rates, the
choice of decimation rate is not free.  I came up with the following
decimation plans:

1)      Dec by 75, demod, interpolate by 3, decimate by 25, for a 32kHz
output rate

2)      Decimate by 50, demod, interpolate by 3, decimate by 25, for a 48 kHz
output rate.

        The second one will let a little too much adjacent channel noise in,
and have a higher computational load.  However, 48 kHz is a better
sampling freqency for high quality FM -- since FM doesn't cut off until
after 16 kHz it needs more than 32ks/s.

        Tomorrow I'm going to try the 1st (since my computer is too slow for
the second).  I need to check whether floating point sample rates are
allowed in pSpectra, since the intermediate sample rates are
non-integral.

Matt


P.S.  I updated the VrAudioSource and VrAudioSink to (optionally) take a
char * with the name of the device.  This is useful for those like me
with multiple soundcards, USB sound dongles, etc.  It's backwards
compatible, BTW.