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Path: utzoo!watmath!clyde!bonnie!akgua!whuxlm!harpo!decvax!genrad!panda!talcott!wjh12!biomed!lagasse
From: lagasse@biomed.UUCP (Robert C. Lagasse)
Newsgroups: net.ham-radio
Subject: "PL"
Message-ID: <86@biomed.UUCP>
Date: Wed, 3-Jul-85 11:23:18 EDT
Article-I.D.: biomed.86
Posted: Wed Jul  3 11:23:18 1985
Date-Received: Fri, 5-Jul-85 06:20:06 EDT
Organization: Mass. Gen. Hosp. - Dept. Biomed. Engineering
Lines: 43


   Just tried out a shiny new ambulance with all of its electronic goodies
(work part time as an EMT in my town) . Anyway, it has a new Motorola radio
with the standard PL on/off slide switch on the microphone holder. Here
comes 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.

Next question:  Why does store bought 2m ham gear usually try to sell itself
with a PL tone encoder ONLY?  It seems that all of the commercial radios
like in the previous question (even the walkie-talkies) have BOTH an encoder
and a decoder.  Is it because PL tones are really used in ham radio as an
electronic "key" or access device to allow only those lucky few to access a
closed repeater?  I can see two reasons to use PL tones in commercial gear:
1) It allows several local services to use the same frequency without being
disturbed by one another's received audio.
2) On low-band systems (usually repeaterless) it eliminates received "skip"
from everyone's radio listening.
  I suppose PL encoding and decoding could be used on 2m within a group of
people so they would not have to listen to all of the repeater gossip if
they were expecting a call from a certain ham operator.

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 piss
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?

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???

...............I'm just full of questions today.

N 1 A L G   Bobby L @ MGH