"Peter Larsen" wrote in message
...
Isaac Wingfield wrote:

Noise in Stereo FM is out of phase between the channels, and thus
disappears when the signalchannels are added to mono.

That's not what's happening. Stereo noise cannot be "out of phase"

FM stereo is transmitted as sum and difference, and the difference
channel has some 10 dB less dynamic range than the sum channel. So much
for your implied theoretical impossibilty.

Stereo FM is transmitted as L+R (baseband) and L-R (subcarrier) but nothing
is intrinsically "out of phase". The dynamic range isn't terribly relevant
either, except that we tend to undermodultate the L-R channel when
monophonic or highly correlated stereo, so noise is more noticeable. The
encoding is a way to symmetrically encode stereo over a single broadcast
channel.

because it occurs at entirely different frequencies from mono noise.
When the receiver detects a stereo signal (by noting the presence of the
19 KHz pilot), the detected bandwidth must be at least 53 KHz, in order
to be able to receive the (L-R) subcarrier. When the receiver does not
detect the pilot, the detected bandwidth is reduced to only 15 KHz, plus
there is de-emphasis which does not apply to the composite stereo
signal.

Pre-emphasis does indeed apply to the composite signal. The left and right
channels are pre-emphasized, then encoded. After you decode the stereo
channels, they are de-emphasized. The L-R audio in the subcarrier is
pre-emphasized.

To the differnece signal rather, just a hunch, it makes sense because it
explains the differnce in sn-ratio for sum and difference channels.

It's the nearly four-to-one bandwidth ratio and the lack of high
cut (de-emphasis), that accounts for the difference in noise. With a
"perfect" receiver, a stereo signal needs to be nearly 30 dB stronger
(29.7, AFAIR), to produce the same signal-to-noise ratio as a mono
signal.

The reason stereo is noisier is because the L-R information is shifted up
(effectively more than doubling the audio spectrum) and then shifted back
down, bringing the noise that is up there back down with it. It's very
simple, really.

You certainly seem to know more sbout the technicalities of this than I
do, I will just add that FM emphasis/deemphasis standars are slightly
different, my general understanding of these matters is however correct.

The propoerty that the noise is identical and out pf phase between the
channels is generally used as a means of automated noise suppression in
case of weak signals, on some tuners it is switchable whether it occurs.

Isaac


Kind regards

Peter Larsen