Mark wrote:
I have a question about logging recorders that are used at police
headquarters for example to record communications. I understand that
they record the audio received by the receiver from the mobile unit
and they also record the transmissions made by the dispatcher on the
local mic. How exactly is the logger connected to the radio system.
Where are these two audio sources combined? There is no single
connection in a typical radio transiver to get both the transmit
audio
and the receive audio.

The reason I want this detail is to understand what happens when the
dispatcher "talks over" one of the mobile units or a mobile units
"talks over" the dispatcher. Is there any way that both audio
signals
can get recorded together? I understand in a typical 2 way radio
transceiver, when it is transmitting, it cannot receive. But in a
more complex system typically used for police communications that may
involve multiple remote satellite voting receivers, are there any
conditions where both the active Rx and the dispatchers mic audio
might both get recorded on the logger together?

Does anyone know the details of how a logging recorder is typically
connected to a complex communications system? Is the dispatcher mic
sometimes always recorded even when the transmit button is not
pressed?

thanks
Mark
WB2WHC

Mark, most dispatch centers have radio systems consisting of muliple
receivers that are scattered throughout a geographical area, referred
to as "voting receivers".. These copy the "inbound" traffic from the
field units, and route the traffic to the communications center where
the dispatcher is located .. These receivers are connected to a
comparator unit that selects the best quality audio from the best
receiver and routes it to the dispatcher. The comparator looks
simultaneously at all of the receivers on a specific channel in order
to "vote" which is the best quality to hand off to the dispatcher..

When the dispatcher transmitts, their PTT is sent to the base
transmitter.. At our agency, when the dispatcher transmits, the inbound
traffic (if any)can still be copied by the dispatcher. The allows a
police or fire unit to be able to be heard, and provide additional
information to the dispatcher, even if the dispatcher is transmitting..


Such a system is usually referred to as full duplex. For our police
radio systems, they are like many UHF amateur repeaters in that the
field unit transmitts on a frequency 5 MHz higher than the base station
transmitter does.. Full duplex allows the field unit to interrupt the
dispatcher if they have something important to pass to them..

Hope this helps a little ..

Patricia Gibbons - wa6ube
Communications Services Manager
City of San Jose ITD/Radio Program.