In article ,
Peter Larsen wrote:

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.

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.

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.

You certainly seem to know more sbout the technicalities of this than I
do,

Yes. I was involved in the design of the very first broadcast quality
stereo generators that actually met all the FCC specs. They were
designed and manufactured for RCA in the mid-to-late 1960s.

Your comment about the 10 dB reduction in dynamic range is not correct.
The difference channel is exactly that: the analog sum of the right
channel and the inverted left channel. No other processing is done to
limit the dynamics.

I see you are posting from Denmark. To be fair, I do not know the
technical details of stereo broadcasting in Europe; it may indeed be
different from what is done here in the states.

Isaac