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Re: [ARSCLIST] FM reception way back when
Actually when the color and FM stereo rules were enacted it was required
to remove the chrominance signal when broadcasting monochrome and the 19
KC/s pilot tone when broadcasting monaural. Eventually the rules were
eliminated as the equipment got better. (My old rule books are not at
hand right now, otherwise I'd cite the rule numbers.) The FM situation
became rather perverse. Early FM stereo receivers had a mono-stereo
switch, but later that became automatic in parallel with the stereo
indicator light. Although it is obvious like Robert said that it would
be advantageous to both the station and listener to switch your radio to
mono when receiving a weaker station, it soon became the opposite.
Several manufacturers came out with FM tuners and radios that could be
switched to receive ONLY stereo stations. Stereo stations soon found
out that their stations DISAPPEARED from these radios when they switched
to mono for newscasts. Additionally, I know there was at least one mono
station in Philadelphia which did this--and I think that we might have
done an experiment at our station at Temple University to see if it was
possible for us to do it--but a mono station could mix in a 19 KC/s tone
at a low level into their audio that could light the stereo light, turn
on the stereo decoder, and make your station receivable on radios
switched to receive stereo-only. Having been the subject of a surprise
FCC Inspection in 1965, I don't think a station would be able to get
away with this if the FCC's TV monitoring truck rolled into town. They
would not consider this to be a "good engineering practice." (You
wouldn't believe the modulation flaw they could see on their
oscilloscope that caused the inspection.)
I should also mention that the specs for the stereo system allow for a
20 KHz audio response only for mono stations. Stereo stations must
have bandpass filters at 15 KHz to protect the 19 KHz pilot tone. In
the early years we found some mono stations blinking the stereo light
with the high transients. When I was still recording in mono on my old
Wollensak T-1616 I found the problem that many others had with the 19
KC/s pilot hetrodyning with the bias/erase oscillator frequency. I
think it was around 85 KC/s. Most manufacturers raised it to around 125
K within a year or two of the introduction of FM stereo. I had to
reduce the recording level when recording a stereo station. I had a
good wideband Harmon-Kardon tube mono FM-only tuner at that time (and it
is still downstairs somewhere.) That tuner could easily pull in NYC in
my 11th floor dorm room in Philadelphia. I used to listen to WBAI after
the 99.5 station in Wilmington Del. signed off at midnight.
When TV stereo audio was new, all of our local stations here put in
phony-stereo generators that switched in whenever mono audio was
detected. I bitched and complained but it didn't help until some of my
students went to work at the stations. One of them reported back to me
that the engineers at one of them (WLEX-TV, Ch 18 -- they deserve to be
embarrassed) would laugh while watching the stereo light go on and off
during programs where only the audience was in stereo.
Mike Biel mbiel@xxxxxxxxx
Michael Shoshani wrote:
Robert Cham wrote:
Hi Folks,
New to the list here. I've been a recording and radio engineer for
more than forty years. Speaking as the latter, I can tell you that
FM Stereo takes about ten times as much signal as Mono. When I was
with Vermont Public Radio, we would turn off the stereo pilot
whenever we broadcast news shows to increase the reach of the
stations. When I first started at WHA in Madison, in the early '70s,
we would turn the pilot off for mono music recordings.
That makes sense. I noticed when I was a kid that television stations
would turn off the chrominance signal when broadcasting black and
white movies, leaving just the luminence. Often when they came from
color commercials back to the movie they'd forget for a few seconds,
and the picture would be riddled with red/blue/green snow that
disappeared when the chrom signal was killed. Stereo FM, if the
signal is weak, gets all kinds of phasing errors that disappear if you
happen to be lucky enough to have a "mono FM" switch.
Let's not discuss "stereo wide"...if God intended for us to hear
stereo with one channel phase-inverted, He'd have put one of our ears
on upside-down. :-)
Michael Shoshani