2-10. RECEIVE AUDIO SIGNAL PATH. Continued
The switching module detects the presence of the 150-Hz squelch tone. If present, the BIT SYNC/TONE
SQUELCH line is set to logic 1. The switching module routes the FM DEMOD signal through the processing
circuitry. For single channel (SC), plain text (PT) operation, the signal continues as RCV PT AUDIO. It is routed
through the audio power supply and the audio control module. The audio/data I/O module sums the audio signal
(RCV AUDIO/SIDETONE) with the 600-Hz alarm tone, if present, and amplifies it. It is routed through a high-pass
filter, low-pass filter, and the VOL control. It is amplified using a voltage-to-current converter and routed to the
AUD/FILL and AUD/DATA connectors at pin B.
In the FH and PT mode, the FM DEMOD signal is a digital data stream. It is routed to the ECCM module as BIT
SYNC DATA to be deinterleaved. (See paragraph 2-11.) The signal is sent through a digital-to-analog converter
to recover the original audio signal. The audio signal is returned to the switching module as RCV FH PT AUDIO and
continues along the RCV PT AUDIO path.
In the cipher text (CT) mode, the FM DEMOD signal is again a digital data stream. It is routed to the COMSEC
connector (J5) as RCV CT. The COMSEC device decrypts the signal and recovers the audio signal. The
recovered audio (RCV CT AUDIO DECODED) is routed to the audio control module and back into the audio
In the FH and CT mode, the ECCM module deinterleaves the data stream. The data stream (FH DATA) is routed
to the COMSEC device where the audio signal is recovered.
2-11. TRANSMIT AUDIO SIGNAL PATH.
The transmit audio signal proceeds through the same modules as the receive audio signal only in reverse order.
See figure FO-4. The audio transmit (AT) signal is input at J3 or J4 pin D. A PTT is required for the transmit mode.
The AT signals from J3 and J4 are summed by the audio/data I/O module. The audio signal is routed to an
automatic gain control (AGC) amplifier. If the WHSP switch is on, the gain of the AGC amplifier is increased. The
XMT AUDIO signal is routed through the audio control module to the audio power supply. The audio power supply
combines the XMT AUDIO signal with the 150-Hz squelch tone. From there it is routed through the switching
module to the exciter/power amplifier.
A PTT at J3 is routed through the audio/data I/O module to the audio control module. A PTT at J4 is routed directly
to the audio control module. Both are combined to generate the AUDIO MDL PTT-N. It is buffered by the audio
power supply and sent to the control module and the switching module.
When operating in the FH mode, the XMT AUDIO signal is routed from the audio power supply to the ECCM
module prior to the addition of the 150-Hz squelch tone. The ECCM module converts the XMT AUDIO signal into a
digital data stream (FH DATA). The FH DATA signal goes to the switching module where it is amplified and routed
to the exciter/power amplifier module.
When operating in cipher text, the XMT AUDIO signal is routed from the audio control module to the COMSEC
connector (J5). The CT XMT signal that is returned is a digital data stream.
2-12. RECEIVE DATA SIGNAL PATH.
The RT can receive audio data and digital data. Audio data can be processed using either AD1 or AD2. When the
RT is set to use AD1, the analog data signal follows the audio receive path. When AD2 is used, the analog data is
converted into 16 kb/s digital data by the transmitter. The receiver converts it back into analog data. Low speed
digital data (75 b/s to 4.8 kb/s) is also transmitted as 16 kb/s digital data. Majority logic error correction is
provided for AD2 and low speed digital data. The data rate adapter performs the data rate conversion and error