4-2. TRANSMIT AUDIO/DATA SIGNAL PATH. Continued
Plain-Text Data. Continued
The fourth block is data. The data has been exclusive ORd with the 15-bit code word. FSK audio from the
intercom enters the dra as XMT AUDIO on J2A and J2B, It is hard limited and applied to the FSK detector circuit
on the FSK detector module. When FSK is detected, the FSK audio is demodulated from FSK to digital by the FSK
demodulator. The low-speed data is converted to 16 kb/s by the 16 kb/s rate converter. An interleaver redistrib-
utes the data in time. Next the data is exclusive ORd with the code word. This is to ensure the signal has a 50%
duty cycle. The exclusive ORd signal is sent to the FSK detector module. From there it follows the same
path as the phasing pattern.
Voice audio from the intercom enters the dra as XMT AUDIO at J2-A and J2-B. It is buffered and amplified on the
data processor module. A PT/CT switch on the data processor routes the audio out to the KY-58 as XMT AUDIO
on J4-A and J4-B. After encryption in the KY-58, the signal returns to the bit sync module as CT100 at J3F. The
signal is routed through a CT/PT switch and out to the rt as X-MODE XMT at J1-F.
In cipher-text operation, each data transmission has three main blocks. Compared with plain-text data, the
phasing pattern is omitted because the KY-58 supplies a 16 kb/s clock.
The first block consists of 30 repetitions of a cipher-text code word. The code word is generated on the data
processor module. AS with plain-text data, the code word is used by the receiving equipment. It tells the receiver
that the incoming signal is data, not digitized voice, Also, the receiver uses it to synchronize the deinterleaver
with the transmitter interleave,
The second block is a single inverted code word. This tells the receiving equipment that what follows is data.
The third block consists of interleaved data. Unlike plain-text data, this data is not exclusive ORd with the code
word. The data will be encrypted by the KY58 which will ensure 50% duty cycle,
FSK audio from the intercom enters the dra as XMT AUDIO on J2-A and J2B. It is hard limited and applied to the
FSK detector circuit on the FSK detector module. When FSK is detected, the FSK audio is demodulated from FSK
to digital by the FSK demodulator. The low-speed data is converted to 16 kb/s by the 16 kb/s rate converter. An
interleaver redistributes the data in time. The data is then level shifted and output to the KY58 as DIG DATA IN at
J4-D. The KY-58 returns the encrypted data to the bit sync module as CT100 at J3-F. The data passes through a
PT/CT switch on the bit sync module and is sent out to the rt as X-MODE XMT at J1-F.
4-3. RECEIVE AUDIO/DATA SIGNAL PATH
Figure FO-12 shows the
l plain-text data
l cipher-text data
audio/data signal paths. There are four types of receive operation:
Voice audio from the rt enters the dra as RCV AUDIO at J1-R. It passes through PT/CT switches on the bit sync
module and data processor module. It passes through a VOICE/FSK switch and push-pull driver on the data
processor module and is routed out to the intercom as RCV AUDIO at J2-K and J2-L.
Digital data from the rt is applied to the dra at J1-b. It is routed through an integrate and dump network on the bit
sync module to reshape the signal. The resultant signal is routed through the bit sync and digital squelch net-
work. The bit sync portion derives a synchronous clock from the received signal, The digital squelch portion
detects when the bit sync has acquired synchronization with the data.
It then supplies control signal DIG
SQUELCH (see para 4-5) that alerts the dra to the presence of a locked-in digital signal,