Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site biomed.UUCP 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