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From: jhs@Mitre-Bedford
Newsgroups: net.ham-radio
Subject: Cc:
Message-ID: <11607@brl-tgr.ARPA>
Date: Mon, 15-Jul-85 18:16:51 EDT
Article-I.D.: brl-tgr.11607
Posted: Mon Jul 15 18:16:51 1985
Date-Received: Thu, 18-Jul-85 08:37:15 EDT
Sender: news@brl-tgr.ARPA
Lines: 38

Subject: Re: "PL" and tweedles
In-reply-to: Your message of Thursday, 11 Jul 1985 20:25-EDT.
             <204@redwood.UUCP>

Mr. Warnock's solution to the squelch tail problem would probably work if
the modulation method were AM rather than FM.

However, a well-designed FM receiver pays little attention to the amplitude
of the signal being processed, insofar as the audio output level is concerned.
Specifically, the discriminator output is ideally the time derivative of the
instantaneous phase of the IF signal output, and changing the gain of the IF
chain has no theoretical effect on this parameter.

As a matter of fact, it IS possible to make squelch circuits which turn off
considerably faster than the typical ones you hear.  It is also possible
to insert a time delay network, e.g. using "bucket brigade" chips, in the
audio path, so that the squelch circuit can in effect turn off before the
signal does!  Quite a few "ham" repeaters do precisely this to avoid the
"double squelch tail" effect of a repeater.  (They haven't figured out how
to get rid of the one generated by YOUR receiver, though!)

Back when I was in the Applied Research Labs at Motorola in ought sixty five
or so, they had worked out a squelch that gave only the barest little "tick"
when the signal went off, and they thought it was the greatest thing since
sliced bread, or whatever.  Then the marketing guys went out and found out
that the customers, mostly police departments and trucking fleets and such,
didn't WANT the squelch tail removed.  Maybe they thought it was sort of
"macho-radio" to hear that deafening burst of noise at the end of each
transmission, or maybe they just thought it was a nice, handy, functional
feature to have a distinctive end-of-transmission indicator, but in any case,
they wanted their old squelch tail back.  So the idea died right then and
there.  The guy who invented it threw in the towel and went into patent law
instead of trying to invent things anymore.  He also applied what he had
learned about signal detection theory, so he told me, to his dealings in the
commodities market, and got rich with his little squelch detector turned into
a "buy! buy! buy!" detector!  S'truth, s'help me!

						-John S., W3IKG