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From: brian@sdcsvax.UUCP (Brian Kantor)
Newsgroups: net.ham-radio
Subject: Re: "PL" and tweedles
Message-ID: <957@sdcsvax.UUCP>
Date: Thu, 4-Jul-85 15:42:24 EDT
Article-I.D.: sdcsvax.957
Posted: Thu Jul  4 15:42:24 1985
Date-Received: Sun, 7-Jul-85 05:57:59 EDT
References: <86@biomed.UUCP>
Reply-To: brian@sdcsvax.ARPA (Brian Kantor)
Organization: UCSD wombat breeding society
Lines: 74

In article <86@biomed.UUCP> lagasse@biomed.UUCP (Robert C. Lagasse) writes:
>the question: When the PL switch is off, the receiver audio is muted
>by the normal squelch circuit which allows you to hear receiver hiss for
>about half a second after the repeater shuts down, then all is quiet.  When
>PL is on, receiver audio is heard, but right after they finish speaking, the
>reciever is muted and no squelch tail is heard.  Are they or are they NOT
>shutting off the transmitted PL tone BEFORE thaey allow the repeater
>transmitter to shut down?  This is what it sounds like.

True sub-audible PL as done by Motorola uses a technique called
``reverse burst'', in which the transmitter emits PL for 120 ms after
you release the microphone, but 180 degrees out of phase with that which
was sent during the main part of the transmission.  Its frequently
louder too.  The idea is to cancel the vibration in the mechanical
decoder reed at the receiving station, so that the tone squelch will
shut before the received carrier goes away, thus eliminating the squelch
tail.

(Note to ham radio newcomers: the ``squelch tail'' is the burst of white
noise heard as an FM receiver squelch delays closing for a short period
of time.  It is NOT (as I have heard ignorant people say on local
repeaters) the delayed drop-off time of the repeater.  It is perfectly
possible (and common) to have a squelch tail when there is no repeater
involved at all.

More recent Pl systems use a quick burst of digital code sent at the
beginning and end of each transmission.  This DPL can address lots of
different stations both by group and individually, so its more flexible.
By combining it with dual-rate squelches (close fast on strong signals,
more slowly on weak signals that may be fluttering), even if the
trailing code burst is lost, the majority of squelch tails will be
eliminated.

>Next question:  Why does store bought 2m ham gear usually try to sell itself
>with a PL tone encoder ONLY? 

Most rice-rockets can have a decoder added to them, but since the great
majority of them have really shitty low-frequency audio response (small
speakers, little coupling caps, etc), most repeater operators choose not
to repeat PL tones through their repeaters.  It is possible to do so, by
either of two methods - a tone coupler will repeat PL tones through
(under the squelch, so that reverse-burst will continue to work), or by
having an encoder on the repeater transmitter.  So although its possible
to have a decoder, since most people won't have one, and the tones will
really distort audio in their receivers, repeaters generally don't emit
tone.

Tone bursts in the audible range, DPL, touch-tones, etc as selective
station calling devices have been used in ham radio off and on for
years, and I have not heard of their legality as such being questioned,
but they are unpopular for a number of reasons:  They are annoying as
hell to listen to, and the general air of cliquishness they generate
usually pisses off enough people that they record and play back the
tones to annoy the members of the clique.  (My, aren't repeaters FUN?)
Better to move to another band and put up a private system.

>Last question:  on commercial FM gear, is the PL tone transmitted by a
>mobile set filtered out before the signal is retransmitted by the repeater???
See above.
>N 1 A L G   Bobby L @ MGH

PL=Private Line, QC=Quiet Channel, CG=Channel Guard, and they're all
trademarks of Motorola, RCA, GE.  Touch-tone belongs to some part of
what is left of the Bell System.


	Brian Kantor	WB6CYT, repeater baron extraordinaire (ret.)

	decvax\ 	brian@ucsd.arpa
	akgua  >---  sdcsvax  --- brian
	ucbvax/		Kantor@Nosc 

"You know, I really like my RCA walkie - its the only radio I've ever
owned that withstands pounding nails with it and keeps on ticking...."