SmarTrunk ST-1000D, Service Manual

Page: 9 / 35

ST-1000D SM R 1.1B SMARTRUNK SYSTEMS, INC. 9
firmware, removing noise then
qualifying the signal level. If the
signal level is higher than the
expected for the current
programmed squelch threshold,
then the processor analyzes the
expected signaling programmed on
the current channel table.
In case of not any signaling is
required, or the signal has been
detected, the processor opens the
virtual audio path, unmuting the
audio amplifier (U9).
4.2.4. Virtual VOLUME CONTROL
The potentiometer position (RV1) is
measured by one analog to digital
converter on the main processor
(U8). The firmware immediately
adjusts the processor path gain to
get the appropriate overall circuitry
volume control.
Minimum volume, emergency and
private audio level are controlled
only by software overriding the
information received from the DAC
on U8.
4.2.5. Sub audio signaling
Received sub audio signaling as
CTCSS and DCS are received,
processed, and detected by the
receiver (U1). Once decoded, the
sub audio signaling is reported to
the processor where the firmware
compares it with the value
programmed into the current
channel programming.
If the received signaling matches
the channel programming, then
the main processor (U8) opens the
audio patch.
4.2.6. ANALOG AUDIBLE SIGNALING
DTMF signals, MDC1200 and tone
signals are processed and decoded
by the main processor (U8).
Decoded information or received
tone is analyzed by firmware on
the main processor (U8).
4.2.7. DIGITAL AUDIO PROCESSING
Digital received information is
processed by the receiver DSP (U1)
then delivered to the main
processor (U8), which includes a
very sensitive FSK modem.
Data decoded by the modem
firmware is transferred to the main
code, which de-encrypts the
information, checks the integrity,
corrects the wrong bits and extract
the audio information form the
received frame.
The portion of the data received,
error free, is moved to the vocoder
firmware module internally into
the main processor (U8).
The vocoder firmware re-builds the
audio from the compressed data
then applies filter it, adjust the
volume and fed it to the audio
power amplifier (U10) through one
RECEIVER
[U1]
AMPLIFIER
[U10]
MAIN
PROCESSOR
[U8]
VOCODER
U6]