Re: FM stereo
- From: "JosephKK"<quiettechblue@xxxxxxxxx>
- Date: Wed, 02 Dec 2009 23:34:07 -0800
On Tue, 24 Nov 2009 12:44:05 GMT, Jan Panteltje
<pNaonStpealmtje@xxxxxxxxx> wrote:
On a sunny day (Mon, 23 Nov 2009 16:07:30 -0800 (PST)) it happened RichDNot QAM (at least not originally) just a double balanced modulator
<r_delaney2001@xxxxxxxxx> wrote in
<1e37a98c-ad31-48d4-9c9a-6cd05fb4d569@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>:
I have a digital armband radio, for jogging.
It has a FM stereo detector, with internal
threshold. When the signal gets weak or
noisy, it switches to mono, and the noise
drops significantly.
How does FM stereo decoding work, and
why is it so much noisier than mono?
--
Rich
FM encoding uses a 38kHz carrier on which the L-R and R-R signals are QAM modulated.
To sync the carrier, a 19 kHz (f/2) pilot tone (above audible) is transmitted with the audio.
The audio is normally transmitted as L+R.
thus no 38 kHz carrier.
After decoding, the signals can be combined by simple addition into L and R.
The noise decreases when you switch to mono, because the
whole decoder is switched off, and that part of the spectrum that contains noise,
the sidebands around 38 kHz, is no longer fed into the signal matrix.
All this IIRC.
It was actually set up so that the math worked correctly for TDM
decoding at 38 kHz or FDM subcarrier and matrix decoding. Some
performance tradeoffs were observed later. It turned out that decoder
solutions that used a bit of both techniques had the best performance.
All known 40 years ago.
.
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