"Idle" voice channel

[Jay911]

I am monitoring a P25 trunking system which is pure "standard P25" according to the data I'm seeing. On some sites, some of the voice channels seem to be constantly transmitting silence (if you can get what I'm trying to say) when they're not carrying voice traffic. This is to say, monitored with a non-P25 receiver, you can hear the P25 digital "noise" at a constant rate. Monitored with a P25-capable receiver, the scanner stops and identifies the channel as containing a P25 modulated signal, and displays the NAC, but remains quiet.

Is there any particular reason this system might be set up this way? Is it an attempt to "mark" the frequency so that it is not appropriated by other services? (This system is immediately adjacent, geographically speaking, to an EDACS network using the same frequencies.)

The control channel doesn't indicate any traffic on the voice frequency when this "idle" transmission is going on.

 

[mancow]

Bizarre

 

[DaveH]

I've heard/seen this a few times. On RENIR system, it appeared to be testing channels
during system build-out. I could tell by the digital "sound" on a non-P25 receiver that
it was not a CC, nor random-sounding voice, but a regular pattern.

In another case, all VCs on one site of the Gatineau PS/FS system where transmitting
continuously; can't confirm it was audio "silence" being transmitted. I think that case
may have been a system malfunction situation, eventually went away.

Dave