"Isaac Wingfield" bravely wrote to "All" (03 Jan 06 22:10:23)
--- on the heady topic of " FM hiss, vintage 1973 receiver"

IW From: Isaac Wingfield
IW Xref: core-easynews rec.audio.tech:186070
IW sci.electronics.repair:353881


IW In article ,
IW 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.

IW That's not what's happening. Stereo noise cannot be "out of phase"
IW because it occurs at entirely different frequencies from mono noise.
IW When the receiver detects a stereo signal (by noting the presence of
IW the 19 KHz pilot), the detected bandwidth must be at least 53 KHz, in
IW order to be able to receive the (L-R) subcarrier. When the receiver
IW does not detect the pilot, the detected bandwidth is reduced to only
IW 15 KHz, plus there is de-emphasis which does not apply to the
IW composite stereo signal. It's the nearly four-to-one bandwidth ratio
IW and the lack of high cut (de-emphasis), that accounts for the
IW difference in noise. With a "perfect" receiver, a stereo signal needs
IW to be nearly 30 dB stronger (29.7, AFAIR), to produce the same
IW signal-to-noise ratio as a mono signal.


The extra noise arises because the stereo difference signal (L-R) is
on an amplitude modulated subcarrier and thus more prone to
atmospheric noise same as with an AM radio.

A*s*i*m*o*v

.... Children come from God. He can't stand the noise either.