Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!netsys!vector!telecom-gateway From: apple!zygot!john@decwrl.dec.com (John Higdon) Newsgroups: comp.dcom.telecom Subject: Re: Praise the Lord and Pass the RF Filters Message-ID: Date: 27 Jun 89 08:34:27 GMT Sender: news@vector.Dallas.TX.US Organization: ATI Wares Team Lines: 30 Approved: telecom-request@vector.dallas.tx.us X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@vector.dallas.tx.us X-TELECOM-Digest: volume 9, issue 216, message 2 of 6 In article , goldstein@delni.enet. dec.com (Fred R. Goldstein dtn226-7388) writes: > [description of FM station being heard in all kinds of devices] > But an FM station? Not so fast. FM transmitters maintain a constant > amplitude, so if you "detect" them with a simple rectifier, you won't > get anything but perhaps a steady DC voltage. (It'll light a > fluorescent tube, though, if you're close enough.) Telephones won't > detect FM the way they detect AM. I've noticed a faint hum on phone > lines caused by FM blanketing, but not the actual modulation. FM can easily be heard on non-FM receiving devices by one of two methods. The first is called "slope detection" and depends on the device doing the detecting to have a slight frequency dependent characterisic. As the frequency changes with modulation, so does the voltage produced by detection and voila! you have audio detection of the signal. A more common occurance involves "synchronous amplitude modulation". If an FM transmitter is not perfectly tuned, its output (amplitude) will vary with normal modulation. The FCC requires broadcasters to maintain this AM component at least 50 db below carrier level, but even at that, given sufficient signal you will easily hear the sermon. From what I've read here, these people might have more synchronous AM than the FCC allows. In any event, I have frequently had to exorcise FM-induced audio out of telephone lines. Not uncommon at all. -- John Higdon | P. O. Box 7648 | +1 408 723 1395 john@zygot.uucp | San Jose, CA 95150 | M o o !