[TrunkCom] digital and subaudible

[Brian J Cathcart]

On Fri, 1 Nov 2002 09:54:48 -0800 (PST) JERRY NONE
<thepolishdude@yahoo.com> writes:
> Please clue me in on how this works.
> Please explain.

Each voice channel in a Motorola trunked system transmits a sub-audible
data stream.  You can actually hear it if you connect your scanner to a
speaker with good low-frequency response (it's a low "rumble" sound). 
Rob Berezowski explained it in another post:

=

The subaudible data on the voice channel carries the talkgroup id (or
individual radio id) for the current voice call in progress. After
receiving a call grant on the control channel, the radios switch to the
voice channel and immediately check the subaudible data to ensure they
are
on the correct voice channel. This happens before the user hears any
audio,
again to ensure the radio doesn't accidently pick up a wrong
conversation.

=

The PRO-92 and PRO-2067 were designed to "listen" to this sub-audible
data on the voice channels.  It would decode the talkgroup information
from the data stream and use it for tracking, and would not need the
control channel. For small systems (4-5 channels) it worked ok, but
larger ones, it sucked.  But even on small systems you would get bogus
talkgroups displayed.  It also could not properly decode even-numbered
hex talkgroups, used rarely on some systems (like St. Petersburg, FL). 
The PRO-2067 and later versions of the PRO-92, however, allowed you to
select either the control channel or sub-audible method.  But again the
sub-audible method did not work nearly as well as the Uniden version.

--

The Scanner Dude
Brian J. Cathcart - KE4PMJ
South Florida Frequency & Trunking Guide - 6th Edition (On CD-ROM too!)