Codan NGT 2010 RF Technical & Service Manual page 772

General information
NOTE
Application of a 300 Hz control tone from any point removes all audio from the system
in preparation for data signalling. Data is used to communicate to and from all
microprocessors within the network and includes commands such as select Transmit
mode (PTT), channel change, system status and so on.
In the absence of the control tone, the system provides an audio path from the
transmitter/receiver through the 8571, via the land line (2-wire) or VHF/UHF radio link
(4-wire), to one or more master control points.
When sending data, the control tone is present for the duration of the signalling, and on
completion of the data transmission, reverts to the Receive mode. Data is transmitted in
simplex mode at 550 baud using FSK.
The control tone is notched out of all external audio paths.
Receive audio path
In Receive mode, audio from the transceiver is applied to the Rx slave 8571 via an
interface cable. The audio passes through a 300 Hz notch filter IC3 and IC4 to remove
any 300 Hz that may be present in the receive audio that could possibly corrupt the
performance of the system. The signal output from the filter is connected to a gain-
controlled amplifier/compressor IC7 to maintain a constant audio output level.
At the output of IC7, the audio is fed via the signalling modem IC6 to the line driver
IC102 and is then connected to either a 2-wire private line via the hybrid and first
isolating transformer, or to a second isolating transformer for the 4-wire system.
At the other end of the 2-wire or 4-wire link, the received audio is applied to the 8571 via
the appropriate isolation transformer to a hybrid and then to the first amplifier IC101.
From the output of this amplifier, the 300 Hz control tone is fed via a 300 Hz BPF IC106
(to remove the receive audio) to a threshold detector IC104a/IC107. The receive audio is
applied to a DC gain-controlled amplifier IC103a, then to a DC-controlled equaliser
IC103b/IC104b. From here it passes via a 3 kHz LPF IC105a through a switch, selected
to a 300 Hz notch filter, to remove the 300 Hz control tone, and finally via an interface
cable to the split-site interface controller receive audio path.
NGT Transceiver System Technical Service Manual
The information in this section should be read in conjunction with the
block diagram 03-01108.
8571—Brief description
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