lagasse@biomed.UUCP (Robert C. Lagasse) (07/03/85)
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
ron@brl-tgr.ARPA (Ron Natalie <ron>) (07/05/85)
> 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. While their are some repeaters around here that have PL to keep out the "riff-raff," the only repeater that I generally use installed PL to stop interfering signals from unintentionally bringing up the repeater. This can either be due to another repeater on the same frequency, or as in this case, a cab company with a bunch of poorly maintained AEROTRON commecial radios. We allowed the repeater to be brought up using any one of the following three methods: 1. PL. 2. Any Touch Tone 3. Three kerchunks within one second followed by the carrier staying on. -Ron
hoffman@pitt.UUCP (Bob Hoffman) (07/06/85)
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
mikey@trsvax (07/08/85)
PL is used not just for security or access to a "private" repeater and/or functions, but also to prevent interference. On 10M repeaters, each area of the country has subtones that it is supposed to use for repeater access. This prevents skip from one area of the country from activating all the repeaters in another part of the country. It also can be used to prevent interference where repeater coverage covers more that one metro area, for example, New York to Boston. There are probably some sites that if PL was not used, keying up would probably bring up many machines in the area. As to decoding the PL on a receiver, Kenwood did something similar except they use ASCII transmission of your call sign in the TR2600 handi-talkie. As to the legality of PL, there really isn't a problem. It's not cryptic data and it conserves bandspace. mikey at trsvax KA5MJQ