jaymatt1978 said:
I still hear (no pun intended) ambulances "dial" a hospital number over 155.3400 all the time. However some ambulance radio plans I have seen have only have 155.3400 in it once. So that means ALL ambulance have at least ONE radio with a DTMF mic in it. I think what happens is the four digit DTMF tones select the right PL tone. Every hospital has its own DTMF that corresponds with a PL tone. It has been like this for years. It's just like some ham repeaters have a DTMF tone to disable PL, these radios have a DTMF tone to ENABLE a certain PL.
The DTMF code opens up the hospital's radio to carrier squelch, and really have nothing to do with the local radio. In fact the HEAR radios I used to use were purchased (ages ago) without DTMF ability and were programmed with a couple dozen channels with different PLs on 155.340, for most of the hospitals we were even remotely likely to visit. Since then the squad purchased DTMF microphones, which the radio didn't interact with except to pass through audio (voice and tone) onto the radio frequency. Another squad seemed to have butchered a touch-tone phone and wired its keypad and tone generator to the radio (surely it was a good job using parts manufactured for that purpose, and it worked well, but it looked a little odd).
When not in carrier squelch mode, it appears that many hospitals remain in tone-squelch which would permit a transmitter with the proper PL to bypass the DTMF hassle. I believe the PL functionality predates the DOH's current DTMF (minimum) requirement. Further, I believe that this is something hospitals have opted to do, and are by no means bound to do or maintain.
So the DTMF doesn't "enable a certain PL," it disables the hospital's PL squelch (or total squelch). And in that regard it is just like the amateur repeaters whose receive tone squelch is deactivated by a DTMF code.
Another anecdote: at least one local EMS agency always transmits 103.5 on 155.340, apparently because that's what MedCentral seems to squelch to except when opened up by RWJ's DTMF code. As a result, MedCentral and Somerset MC are presumably listening when they dial in to some other hospital.
Jim