Article 2188 of alt.tv.beakmans-world: Path: matra.meer.net!flop.mcom.com!news.Stanford.EDU!nntp-hub2.barrnet.net!newsfeed.internetmci.com!news.sprintlink.net!howland.reston.ans.net!torn!news.bc.net!info.ucla.edu!zephyr!ia.mks.com!mks.com!richw From: richw@mks.com (Rich Wales) Newsgroups: alt.tv.beakmans-world Subject: Re: Another Question. Date: 8 Aug 1995 01:19:13 GMT Organization: Mortice Kern Systems Inc., Waterloo, Ontario, Canada Lines: 37 Message-ID: <406e2h$kr0@ia.mks.com> References: <3v3lmf$c48@zeus.tcp.co.uk> <6b.1310.2927@sound.com> <400gob$ng3@usenet.INS.CWRU.Edu> Reply-To: richw@mks.com (Rich Wales) NNTP-Posting-Host: giga.mks.com Basically, an FM radio station works by swinging the =frequency= of the signal back and forth in sync with the ups and downs of the audio wave. A decent FM receiver starts out like a good AM receiver. It tunes in and amplifies the one signal you want, generally using a "superhetero- dyne" process to shift the signal down to a constant (and relatively low) frequency band which the radio is designed to amplify very well. However, the "detector" in an FM receiver is designed to respond, not to fluctuations in the raw strength of the signal, but to fluctuations in the frequency of the signal. Then, of course, the audio signal is amplified before going to your speakers -- just as with AM (except the frequency range an FM receiver has to handle is wider than for AM). There is other stuff in an FM receiver for handling stereo. Stereo is done by adding an extra, ultrasonic component to the audio before it's fed into the transmitter. An FM stereo receiver first looks for a spe- cial ultrasonic tone called a "stereo pilot", which says that this is a stereo broadcast, and then it grabs the extra stereo info out of the audio signal and combines it with the main portion of the audio to give you "left" and "right". In order to be backward-compatible with non- stereo FM equipment, the extra stereo info is the =difference= between the left and right microphone signals; the stereo signal is added to the main audio to give the left channel, and subtracted from the main audio to give the right channel. (Neat, huh? :-}) The ultrasonic stuff is filtered out before a stereo program gets to the speakers, BTW, so Lester doesn't need to worry about being driven out of his rat mind by a stereo broadcast (unless he just doesn't happen to like the music being played, of course :-}). More complicated than AM, to be sure, but not nearly as complex as a TV video signal. I'd still say that an FM signal -- at least a monaural (non-stereo) signal -- is a more or less direct representation of the audio information. Rich Wales Kitchener, Ontario, Canada