In article ,
Peter Larsen wrote:

mc posted a question compliant with usenet standard: 3416554677697809809
section B, page 27, paragraph 8: Any initial question must omit at least
one piece of vital information, otherwise it can not be considered for
followups.

FM stereo receiver, vintage 1973, has a noticeable background hoise
(hiss/white noise) on FM stereo regardless of the incoming signal level. It
disappears completely upon switching to mono.

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"
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. 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.

Isaac