Question about console interfacing to trunking systems

[d119]

I've been doing some work on a SmartNet II single-site system lately. It's 10 channels, non-simulcast, non-voting. It's console is Centracom Gold Elite.

There are TBIM's and TIMI links that come from the dispatch center to the central controller (a 6809 SmartNet II+ Central Site Controller). The stations are MSF 5000's.

What I'm curious about, is when a unit calls dispatch and dispatch responds, you can hear a short burst of what sounds like 2175Hz which is then ALMOST completely muted, and when dispatch unkeys, you can hear a little "cheep" of 2175Hz....

Almost sounds like the 2600Hz idle tone you'd hear on Ma Bell's old telephone exchanges.

Why am I hearing this 2175Hz tone? What generates it?

I kinda like it - would be nice if it could be implemented on my conventional ham system... I have a T5600 connected to an MSF 5000 via a DigiTAC.

Thanks for any help.

[Bruce1807]

From memory the TBIMS generate a tone on keyup
Also if your centracom is a remote CEB there is a tone on the audio lines between the CORI and the ROCI

[d119]

The CEB is in the same building as the consoles, but there are TIMI links with modems between the CEB and the CSC.

Still wondering where the tone comes from.