🚀 go-pugleaf

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.