Al, Matt:
Thanks for your input. I am pretty ignorant about these FM standards
and basically I googled my way to a reasonable spec, drew the flow
graph and coded it. The stereo demux was definitely not working with
DEemphasis applied to the composite signal. And it was clear why. The
taper was awful and the lower sideband was approximately 10 dB higher
than the USB on the L-R, DSBSC stereo signal. I removed the deemphasis
and voila, stereo. That said, when the pilot/carrier is 20 dB out of
the noise in the 512 pt FFT, that should be strong enough that you do
not have hiss that is irritating. But the COMPOSITE signal is what is
FM modulated but I bet they do not preemphasize the composite signal.
They might do preemphasis on the L+R baseband signal as Matt suggests
but I just don't know about the others. I am trying to understand what
about that would make sense. It would be very interesting to read a
real spec. I have googled and not really found one a useful one.
I hate this RDS Top Secret MONEYWORD specification. Frank and I are
determined to fix their wagon.
I did not know about Achilleas work. I will search the archive.
Bob
al davis wrote:
On Monday 13 February 2006 20:06, Matt Ettus wrote:
I seem to remember that the preemphasis on stereo signals is
not performed on the multiplexed signal. It is actually
performed on the audio components separately, before mixing
with the stereo subcarrier. Therefore deemphasis needs to be
done after the stereo part is mixed back down to baseband.
To me, this is backwards, and is not useful, but I think that
is the standard. Achilleas sent an email to the list several
months ago on this subject. He also had a simple
implementation that did stereo.
Actually it is very useful the way it is. It would be bad to
preemphasize the composite signal.
To preemphasize the composite in effect converts the system to
phase modulation. The subcarrier would then consume most of
the bandwidth, and the baseband (mono) would be drastically
reduced, resulting in a significant reduction in apparent mono
signal to noise ratio. Stereo signal to noise ratio would be
about the same as it is now, but there would be no advantage to
mono.
In the existing system, if stereo SNR is not good enough, you
can switch to mono and get almost as good SNR as if stereo
wasn't there. "Almost" means about a 6 db degradation, which
is due to the fact that baseband modulation must be lowered by
that much.
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