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From: hoffman@pitt.UUCP (Bob Hoffman)
Newsgroups: net.ham-radio
Subject: Re: "PL"
Message-ID: <1090@pitt.UUCP>
Date: Fri, 5-Jul-85 20:18:44 EDT
Article-I.D.: pitt.1090
Posted: Fri Jul  5 20:18:44 1985
Date-Received: Thu, 11-Jul-85 06:42:51 EDT
References: <86@biomed.UUCP>
Reply-To: hoffman@pitt.UUCP (Bob hoffman)
Organization: Univ. of Pittsburgh Computer Science
Lines: 44

In article <86@biomed.UUCP> lagasse@biomed.UUCP (Robert C. Lagasse) writes:
>
>... 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.

This is often done with what's called "tone reversal".  When the Push-to-Talk
line on the transceiver is released, the tone encoder holds it on for about
another 100 ms.  During this time, the PL tone is sent out, 180 degrees out of
phase.  This serves to turn off a PL decoder as quickly as possible.  In the
days of reed-based decoders, it damped the vibrations of the reed.  The 100 ms
time was just long enough to shut down the decoder without it locking onto
the 180-degree signal.   RCA TACTEC (TAC100) radios do this, for example.

>
>While I am on the subject of private encoding/decoding:
>   Is it legal to use this scheme on amateur radio?  What if I have a tone
>decoder circuit in my mobile set that will only open up the receiver audio
>if it receives two or three DTMF pairs in the proper sequence?  A typical
>scenario would go like this:  Another ham tries to contact me by on or off
>off a repeater by saying my call, "this is", then his call, then using her
>or his DTMF pad transmits a sequence of tones.  Do you think this would p*ss
>off people listening to the repeater, thinking that I am trying to access
>some repeater function (such as autopatch) when I don't EVEN BELONG to their
>repeater group or club?
>

I always thought having a decoder on my rig would be a good idea.  I don't
think it would be illegal -- you're not broadcasting, you're signaling a
specific individual.  And no matter what you do, you're bound to make someone
mad at you!  :-)

>
>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???
>

Yes.  In some rare cases, a *different* PL tone is used on the output than
on the input.  That, of course, requires complete filtering on the receiver.
-- 
Bob Hoffman, N3CVL       {allegra, bellcore, cadre, idis, psuvax1}!pitt!hoffman
Pitt Computer Science    hoffman%pitt@csnet-relay