SmarTrunk Systems ST-9116C, Service Manual, Page: 10 / 67

ST-9116C SM R 1.1 SMARTRUNK SYSTEMS, INC. 10
by the audio processor (U35).
Decoded information or received
tone is reported to the main
processor (U37).
4.2.7. DIGITAL AUDIO PROCESSING
Digital received information is
processed by the receiver DSP (U8)
then delivered to the audio
processor (U35), which includes a
high-speed modem.
Data decoded by the modem is
transferred to the main processor
(U37)
which de-encrypts the
information, checks the integrity,
correct the wrong bits and extract
the audio information form the
received frame.
The portion of the data received,
error free, is moved to the RX
vocoder (U21).
The vocoder re-builds the audio
from the compressed data then
applies filters it, adjust the volume
and fed it back to the processor
(U37) where the analog signal is
sampled with AdPCM format,
filtered and processed to be
delivered to the PWM filter then to
the audio amplifier (U36). AdPCM
audio frames are also sent to the IP
port when the repeater is working
on wide area environments.
4.3. TRANSMITTER SYSTEM
4.3.1. ANALOG TX SIGNAL FROM
EXTERNAL MIC
The Mic signal coming from the
front connector is conditioned on
Mic Amplifier (U43), which is an
active filter to make band limitation
and level adjustments.
The result of the conditioned and in
band-filtered signal is fed to the TX
Audio Processor (U20), which
provides
audio compander,
emphasize, limitation, encryption,
etc.
The high audio band of the signal
from 300 to 3500 Hz are directly fed
to the transmitter (U2), which
performs a direct modulation of a
generated carrier.
Low frequency signaling audio are
also fed directly to VC TXCO (Y1)
main clock generator and U2 with a
corresponding balance controlled
by software.
Figure 4: Analog Modulation path
Figure 3: Digital Receiver Audio Path Block Diagram
Main Processor
[U37]
Audio
Processor
RX Modem
[U35]
RX Radio
U8
AUDIO
PA
[U36]
PWM
FILTER
Vocoder
[U21]
AMPLIFIER
[U7]
AUDIO
PROCESSOR
[U12]
TRANSMITTER
[U2]
VC TXCO
[Y1]
Mic
Connector