Table of Contents
Advertisement
Chapter
1
Description
MSF
5000
Instruction Manual
Secure
Board Functional Description
Audio Encryption
Operation
Audio
to be encrypted and transmitted can originate from the
TX
AUDIO,
LOCAL
AUDIO,
or
INBOUND MRTI inputs
to the Secure board.
The
selected
audio
source
is amplified, pre-emphasize, low-pass filtered, and routed to the Continuously
Varying
Slope Delta
(CVSD)
devices
to be
digitized. When jumper JU4003
is in its
alternate position,
INBOUND MRTI audio is always summed with
the selected
encryption audio
source. Refer to
Appendix G
-
Jumper Settings for additional
information.
Depending
on the system and the type of encryption
hybrids
used,
a secure
coded
message
may
be preceded
by
a
BOM bit
stream
and/or followed by
an
EOM
bit
stream.
The
EOM
and
BOM bit
streams consist
of
a
variable length alternating
12
kHz
bit pattern
(i.e. 101010...)
which
is generated
by
the Secure board software.
Receive Audio Circuitry
Receive audio circuitry is divided into two sections providing two different
modes
of
operation:
©
Transparent operation
0
Receive audio decryption
Figure
1-19
shows
a
block diagram of
the receive
audio circuitry.
Transparent Operation
The input
to the receive audio
circuitry is
from the wireline (P803-27).
The input
signal (QUAD AUDIO)
consists of the demodulated signal from the receiver.
This
signal
has been buffered
by
the
SSCB
and routed to the Secure board equalizer
filter.
For special applications,
the
equalizer filter can
be
bypassed
by placing jumper
JU4004 in its alternate setting. Refer
to
Appendix G
-
Jumper Settings for
additional information.
The filtered signal
is
digitized by
a
comparator to translate the signal into discrete
logic level
data.
An
Rx Code
Detect test is performed on the data
stream.
Whenever
a
valid
12
kBit/Sec
code,
BOM,
or
EOM
is
detected, the
SSCB
front
panel
Rx
CD LED
lights.
The on-chip circuitry of
the secure
ASIC
can route the reclocked and buffered
received data to the transmit splatter filter,
and/or
to the
wireline, depending on
the station type.
In
a
conventional
base station, the
received data is routed to
the
secure
ASIC,
where the data is pulled up, attenuated, and filtered
by
a
low-pass
filter.
This
filter band-limits
the
signal to
6
kHz
for the wireline.
When
the
wireline
is
in
close
proximity
to other conductors, eliminating the higher frequencies
with
this filter reduces the potential for crosstalk.
1-42
68P81092E02
3/14/94
Chapter
1
Description
MSF
5000
Instruction Manual
Secure
Board Functional Description
Audio Encryption
Operation
Audio
to be encrypted and transmitted can originate from the
TX
AUDIO,
LOCAL
AUDIO,
or
INBOUND MRTI inputs
to the Secure board.
The
selected
audio
source
is amplified, pre-emphasize, low-pass filtered, and routed to the Continuously
Varying
Slope Delta
(CVSD)
devices
to be
digitized. When jumper JU4003
is in its
alternate position,
INBOUND MRTI audio is always summed with
the selected
encryption audio
source. Refer to
Appendix G
-
Jumper Settings for additional
information.
Depending
on the system and the type of encryption
hybrids
used,
a secure
coded
message
may
be preceded
by
a
BOM bit
stream
and/or followed by
an
EOM
bit
stream.
The
EOM
and
BOM bit
streams consist
of
a
variable length alternating
12
kHz
bit pattern
(i.e. 101010...)
which
is generated
by
the Secure board software.
Receive Audio Circuitry
Receive audio circuitry is divided into two sections providing two different
modes
of
operation:
©
Transparent operation
0
Receive audio decryption
Figure
1-19
shows
a
block diagram of
the receive
audio circuitry.
Transparent Operation
The input
to the receive audio
circuitry is
from the wireline (P803-27).
The input
signal (QUAD AUDIO)
consists of the demodulated signal from the receiver.
This
signal
has been buffered
by
the
SSCB
and routed to the Secure board equalizer
filter.
For special applications,
the
equalizer filter can
be
bypassed
by placing jumper
JU4004 in its alternate setting. Refer
to
Appendix G
-
Jumper Settings for
additional information.
The filtered signal
is
digitized by
a
comparator to translate the signal into discrete
logic level
data.
An
Rx Code
Detect test is performed on the data
stream.
Whenever
a
valid
12
kBit/Sec
code,
BOM,
or
EOM
is
detected, the
SSCB
front
panel
Rx
CD LED
lights.
The on-chip circuitry of
the secure
ASIC
can route the reclocked and buffered
received data to the transmit splatter filter,
and/or
to the
wireline, depending on
the station type.
In
a
conventional
base station, the
received data is routed to
the
secure
ASIC,
where the data is pulled up, attenuated, and filtered
by
a
low-pass
filter.
This
filter band-limits
the
signal to
6
kHz
for the wireline.
When
the
wireline
is
in
close
proximity
to other conductors, eliminating the higher frequencies
with
this filter reduces the potential for crosstalk.
1-42
68P81092E02
3/14/94
Advertisement
Table of Contents
