TRS & Open Mic

[chrismol1]

School me on open mics on a fdma trs. Does the dispatcher interrupt? Does the transmitting radio realize its at fault? Does it reset on a timeout, the dispatcher seems to interrupt

 

[mmckenna]

School me on open mics on a fdma trs. Does the dispatcher interrupt? Does the transmitting radio realize its at fault? Does it reset on a timeout, the dispatcher seems to interrupt

Depending on the console integration to the radio system, the dispatcher can interrupt.
The transmitting radio may not hear the dispatcher though, since it is transmitting and not able to receive.

It's a —really— good reason to have a time out timer set on the radio. If the radio gets keyed up without the user knowing, it will eventually time out. That's a good time for the dispatcher to announce "check your mics", or if they know the radio user by the radio ID, specifically call them out.

 

[kb4mdz]

Used to hear open/stuck mic on a security police channel on a former but unnamed military base with big runway in Seacoast NH; (I worked in a radio shop across the highway from them). Simplex base, and when it happened Dispatch would call out 'Check your mics, we've got an open mic'

Obviously the TX power of the base could overpower the flea power of the portables, and capture receivers in the units NOT transmitting, but Dispatch could never understand why the offending radio couldn't hear them, yelling back. Eventually, shop stopped trying to explain it, except when the uniforms got their knickers in a twist and really really complained, that things weren't working right. No, that's the way it works. BTW, we still don't have full-duplex audio coming out of an RF simplex VHF handheld.

 

[MTS2000des]

Console priority allows a dispatch console user to overtake outbound audio regardless on FDMA/TDMA/analog even. As mmckenna stated, all depends on how a dispatch site is setup when it comes to integrating with the radio system(s) the console is connected to. At my center, console priority is a standard feature we have had since the days of Smartnet and CentraCom Gold Elites.

 

[tyler3328]

Our dispatch centers have priority so they can interrupt. All of our APXs are also programmed with an absent voice timer as well. If there is no prominent voice then after 5 seconds the radio will automatically unkey

 

[speedway_navigator]

One group I serviced would say 'someone is sitting on a key' for a stuck mic. Since most users were in pit lane they were trained to raise their hand if they heard the stuck mic, then look around to see if someone didn't have his/her hand up.

One year at the start of the Portland race the starter leaned on the guard rail and hit his PTT. The officials in pit lane figured out who it was and started pointing and waving at him. Nick was talking to his assistant saying 'look at all those guys jumping up and down and waving, what's wrong with them?'

 

[GTR8000]

When the consoles transmit, they act no differently than they would on a "4 wire" system. The subscribers are transmitting on the repeater input and are rebroadcast over the output, whereas the consoles are keying up the repeaters directly on the output, so that audio preempts anything coming in through the input. Really no different from a conventional repeater, but with the extra logic that trunking provides.

The transmitting FDMA subscriber would not hear the console, because as @mmckenna points out, it's still actively transmitting. TDMA subscribers, on the other hand, will hear the console because they are capable of receiving while in transmit mode. The transmitter is pulsing on/off every 50ms, and monitoring the other slot of the traffic channel while doing so. That is why TDMA can provide enhanced features such as emergency preemption and in this case, the ability for the consoles to get a transmission through to a transmitting subscriber. No, they don't actually simultaneously transmit and receive, but 50ms cycling is damned close to it.

Yes, a time-out timer is a must, and the End Tx on Voice Absence feature of the APX series provides even more protection, as the radio will stop transmitting if the vocoder doesn't detect human voice within X amount of seconds of keying up (default is 5 seconds).

 

[chrismol1]

Thank You. Very informative. As the case in question of an EMS portable with a stuck mic, transmitting for 20+ seconds of background laughter before the dispatcher interrupted, "closing" the repeater output and resetting the channel. I'd assume the subscriber may still be transmitting until timeout and the dispatcher hopefully alerting nearby units of the open mic as they did transmit "portable xx you have an open mic". I have listened to conventional P25 open mics up until the point you could hear the cell phone ring and the subsequent "oh sh$t"

 

[GTR8000]

Correction to my previous post: the TX/RX cycling for P25 TDMA is 30ms, not 50ms.

 

[speedway_navigator]

Back in the 80s at my first shop we had the service contract for a school bus system. It was CSQ and there was a co-channel user up in LA that had a PL tone. A new person was working dispatch one day and got tired of hearing the LA traffic break squelch. She noticed when she pressed the wide plastic bar at the base of the mic the speakers went silent, so she put a large book on it, problem solved.