SmarTrunk Systems ST-3118D Service Manual - page 7
ST-3118D SM R 1.1 SMARTRUNK SYSTEMS, INC.
9
analyzes the expected signaling
programmed
on
the
current
channel table.
In case of not any additional
signaling must be decoded, or the
signal has been detected, the main
processor (U13) asks to the audio
processor (U12) to open the audio
path, unmuting the audio amplifier
(U8).
4.2.4. Virtual VOLUME CONTROL
The potentiometer position (R15 –
front board) is measured by one
analog to digital converter on the
audio processor (U12), and then the
information is reported to the main
processor.
The
firmware
immediately adjusts the audio
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 U12.
4.2.5. Sub audio signaling
Received sub audio signaling, as
CTCSS and DCS, are received and
processed by the receiver (U2) then
detected on the audio processor
(U12). Once decoded, the sub audio
signaling is reported to the
processor where the firmware
compares it with the programmed
into
the
current
channel
programming.
If the received signaling matches
the channel programming, then the
main processor (U13) requests the
audio processor (U12) to open the
audio patch.
4.2.6. ANALOG AUDIBLE SIGNALING
DTMF signals, MDC1200 and tone
signals are processed and decoded
by the audio processor (U12).
Decoded information or received
tone is reported to the main
processor (U13).
4.2.7. DIGITAL AUDIO PROCESSING
Digital received information is
processed by the receiver DSP (U2)
then delivered to the audio
processor (U12), which includes a
high-speed modem.
Data decoded by the modem is
transferred to the main processor
(U13)
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 vocoder
(U14).
The vocoder re-builds the audio
from the compressed data then
applies filters, adjust the volume
RECEIVER
[U2]
AUDIO
PROCESSOR
[U12]
AMPLIFIER
[U8]
MAIN
PROCESSOR
[U13]
VOCODER
U14]
Figure 3: Digital Receiver Audio Path Block Diagram