M8SL2-00-002-812 Circuit Operation D2.3
© Copyright Tait Electronics Limited August 2004. All rights reserved.
2.1 Circuit Overview
Figure 2.1 High Level Block Diagram
The T803 is connected to a leased line from control room equipment by means of an
RJ45 connector. 600R transformers are used to couple the audio into and out of the T803.
On the secondary of these line matching transformers are analogue transmission
switches which configure the line interface as either 2-wire or 4-wire compatible. Audio
from line is passed through a programmable electronic potentiometer (e-pot) level con-
trol circuit before entering a CODEC where it is digitised. Audio to line is output from
the same line CODEC, via a second programmable e-pot level control circuit.
The line CODEC is connected to a digital signal processor (DSP) which is also connected
to a similar radio CODEC (the CTCSS CODEC is not used in this application). The radio
CODEC receives audio from the T800 receiver and sends audio to the T800 transmitter
to which it is connected via the special backplane PCB. Receiver audio is low frequency
filtered to extend the radio CODEC’s effective frequency range.
The DSP processes the digitised audio streams from the CODECs in accord with instruc-
tions sent to it by the T803’s microcontroller. The DSP can:
• detect high level guard tone, low level guard tone and function tones coming
from line, notch filter line in and receiver audio (used to eliminate low level
guard tone from line out and transmitter audio);
• generate Selcall DTMF/function tone alarms and pip tones to Line-Out and
transmitter audio;
• generate voting tones to Line-Out;
• generate morse code to transmitter audio;
• cross connect line and radio audio paths;
• adjusts the Line-In and Line-Out levels;
Digital
Signal
Processor
Radio Codec
Micro
Controller
Test Mic
CTCSS Defeat
Power Alarm I/Ps
Channel Select
Tx KeyTx Key
Rx Mute
Aux I/Ps
Aux O/Ps
PGM800Win Interface
RAM
A/D
RSSI
Voltage Sense
Speaker
2W/4W
Select
Line Codec
LINE
Front Panel
LEDs & Switches
EEPROM
RX Audio
TX Audio
Progammable
e-pot level
control
Progammable
e-pot level
control
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