Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!xanth!mcnc!unccvax!dya From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) Newsgroups: sci.electronics Subject: Re: Stereo vs. Mono: Q about AM Stereo Message-ID: <1492@unccvax.UUCP> Date: 7 May 89 17:05:36 GMT References: <4046@bgsuvax.UUCP> <21000021@m.cs.uiuc.edu> <1972@hp-sdd.hp.com> <1666@internal.Apple.COM> Organization: Univ. of NC at Charlotte, Charlotte, NC Lines: 102 In article <1666@internal.Apple.COM>, kanner@Apple.COM (Herbert Kanner) writes: > >>On the AM broadcast band, there is a station frequency every 10KHZ, starting > >>at 550 and going up in increments of 10 to 1600. If a station were at 760KHZ, > >>they would have space 5 below center frequency and 5 above center frequency > >>for their lower and upper sidebands. This is a total of 10 in width. > >>True, each sideband is limited to 5, so, they need to used single sideband > >>and gain 5MHZ in band width, they have the space there, it is just not Oh, good grief, people !!!!!!! Simply because the AM channel spacing is defined as 10 kHz does not mean that the permissable occupied bandwidth is 10 kHz. AM radio stations are never allocated to the same community 10 kHz apart for a variety of reasons, the least of which would be the horrendous 10 kHz whistle which would result from the two carriers beating in the receiver front end. Back when AM stations were required to do audio proofs of performance, there was a specification for amplitude and distortion at 7500 Hz. Since this regulation is almost as old as Moses, we can properly conclude that not only was a baseband of 5 kHz not desirable, but flat out illegal. (The measurement is to be taken from the main studio microphone and read on the station's modulation monitor.) How do we reconcile this problem? The FCC obviously expects you to have at least 7500 Hz audio bandwidth, but the channels are 10 kHz apart!!! Simple, don't allot adjacent channel stations to anywhere near the same service area. Very simply: The protected co-channel interference free contour is 500 uV/m. The FCC defines adjacent channel stations causing objectionable interference if the 500 uV/m contour of the adjacent channel overlaps the 500 uV/m contour of the desired channel (1:1 carrier to carrier ratio). The primary service area of an AM station is 5 mV/m. This is the signal a station licensed to a community must place over that community. So, at the city grade contour, the carrier to carrier ratio is at least 20 dB. In reality, since the 0.5 mV/m contour of the offending station attenuates rapidly with distance, and since the 0.5 mV/m contour of the offending station can't breach the 0.5 mV/m contour of the desired station, the carrier to carrier suppression is more like 40 - 50 dB at the 5 mV/m contour of the desired station. I just took WOGR off the air this morning for a few seconds and took a casual measurement: WOGR (in the parking lot) 5.2 mV/m WBSC 1550 kHz, Bennettsville, S.C. 0.031 mV/m WCKY 1530 kHz, Cincinnati, Ohio 0.012 mV/m D/U ratio, WOGR to WBSC: 44.5 dB D/U ratio, WOGR to WCKY: 52.7 dB A 10 kc audio notch is more than adequate to remove the whistle, if one were to drive down to between WOGR and WBSC to hear it (it's about 50 miles from where I sit). With WOGR off the air, the sidebands from WBSC and WCKY are inaudible. With WOGR on the air, you can't hear a 10 kHz whistle. Now, 40-50 dB of carrier suppression is not exactly high fidelity compared to your local CD player, but is certainly beyond adequate for many entertainment and information uses. As for SSB in the home, forget this. AM phase rotation with conventional signals is bad enough, but at least the receiver has a carrier which has hopefully been rotated about the same amount as the sidebands. With SSB, the reference is lost (obviously), so when the sidebands' "carrier phase" change, the receiver's carrier phase didn't track it. SSB is bloody hard enough to tune in on my Drake R4, and a precision synthesizer to the nearest 0.1 Hz or so is not the kind of thing I want to take to the beach or ball-park. We do have the next best thing, though, "Power-Side", explained in previous articles. Perhaps I've been way too harsh on the FM advocates here, but the practical implementation of AM does not have to be so bad that there is not parity within the same order of magnitude. My hackles get up when people who should know better state that "The bandwidth of an AM signal is 5 kHz." Even worse is the still slow adoption of NRSC-1 compliant receivers (even the (*&%*(% Becker which comes in new Mercedes still sounds like a telephone on AM, but at least you can side tune with it...on the other hand, the Delco-GM high end Caddy radio is excellent, as is the one which comes in the Toyota Supra). We have FM stations here with about a 10 dB dynamic range, so what good is a 70 dB signal to noise ratio to your average listener? There has got to be someone on this Net who has heard or owns a wideband AM tuner. If not, go down to your local Sparkomatic dealer (that's right, Spark-o-matic) and ask to see the DIN-mount SR430. This is a current model chassis with the "AM wide button." Tune in a decent station, and push the button. Hearing is believeing. York David Anthony DataSpan, Inc P.S. I am not endorsing Sparkomatic here.