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

From: Ettus, Matt
Subject: RE: [Discuss-gnuradio] Why FM doesn't sound good
Date: Thu, 27 Dec 2001 15:00:49 -0800

> >     Unfortunately, since we are constrained by soundcard 
> output rates, the
> > choice of decimation rate is not free.
> I think with AC97 codecs the choice is actually free.  I believe that
> they will resample up to 48 kHz on the fly.  I haven't tried it; just
> noted it in some spec I read a while ago.

Under OSS and kernel drivers, many soundcards will allow you to set whatever
rate you want, and they report back that its been accepted, but then just
use the nearest rate anyway.  Many are designed for the 11.05, 22.1, 44.1
kHz sequence, and only approximate the 48 kHz sequence (or vice versa) , yet
report that they support the exact rates.  For most audio its ok since you
can't hear the difference.  It causes over or underruns for us.  ALSA
drivers will tell you the exact rates available, including the numerator and
denominator of the dividers in the chip.

The ideal setup is to have all codecs clocked from a common source, but no
consumer and few professional cards allow that.

> Is the adjacent channel noise because the default windowed FIR doesn't
> have steep enough skirts?  As I recall, the Q of the windowed filter
> (normalized narrowness of pass band) is linear with number of taps.
> I.e., twice as many taps ==> twice as narrow of pass band.  We ought
> to be able to get a narrower pass band (if reqd) by increasing the
> number of taps (with concommitant increase in mips).

        The adjacent channel noise is aliased in because the filter is too
wide in frequency.  If you go up to 150 samples long (which is about all my
machine can handle), its better.  When you use a 75 tap filter with a 125
decimation ratio, you don't even use 50 out of each 125 samples, and thus
you are throwing away signal and aliasing in noise. 

        The 160ksamples/sec IQ rate gives you a maximum of 160 kHz of BW.
FM signals are really at least 200kHz wide, so you are losing the peaks of
the signal.  You are also aliasing the signal's own energy back on itself.

        Lastly, the window functions have very slow transition bands, so if
you make them narrow enough to cut out adjacent channels, they will
significantly attenuate the signal's high frequency components.  They are
better suited for the case where you would be decimating down to 160
ksamples per sec complex for a signal which is 50 to 100 kHz wide, and would
have a better filter in the next stage.

        Is there any reason why they chose to use VrComplex instead of
standard complex templates?  It would be nice to have complex shorts.  It
fits better with the AD card and with MMX.


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