Terminal Audio System - Motorola MTM800 User Manual

THEORY OF OPERATION (TRANSCEIVER)

Terminal Audio System

External Mic
Internal Mic
Tx Audio
Handset
Audio
Speaker +
Speaker -
SP Audio
RX Audio
The Audio system consists of the following components:
• RX and TX path multiplexer
• RX and TX programmable potentiometer
• CODEC
• DSP
• Audio PA
TX Path
The speech audio is fed to the internal microphone input (INT_MIC), the external microphone input
(EXT_MIC) or the TX audio input (TX_AUDIO). The internal and external microphone inputs have a
DC bias prepared for connecting a microphone.
The TX audio input has a DC blocking capacitor. Multiplexer U0201 selects one of the possible
inputs and feeds the signal to amplifier U206-4. The gain of this amplifier is switchable via gate
U0202.
The programmable potentiometer U0203 attenuates the signal according to the terminal mode of
operation. Finally, the CODEC FL0200 converts the analog signal into digital format and transfers it
to the DSP via the serial audio port (SAP).
The DSP performs the functions of audio filtering, ACELP speech compression, digital modulation,
and transfers the data to the RF section. When the terminal is operating in the telephone
interconnect mode, the DSP performs the required tasks such as echo and noise reduction.
U0201
MUX
U0206-2
MUX
U0205
Audio PA
Figure 4.1-7 Audio Block Diagram
U0206-4
U0202 U0203
U0204
U0206-1
4.1 - 21
FL0200
A/D
DSP
A/D