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12.1
DESCRIPTION
The Audio Synthesizer
board
generates and
pro-
cesses
baseband
signals,
providing
the composite
modulating
signal
for RF-signal
generation.
The A10
board
consists
of an Audio Synthesizer
that
generates
frequencies
in the range of 5 to 19999.9
Hz,
a
digital
private line (DPL) generator, and a fixed 1-kHz
source.
The AlO board
provides
external
microphone
and
external audio
inputs,
and sums all
modulation
sources
to form
the
composite
modulation
source.
A
block diagram
of the Audio Synthesizer
board
is
shown at the end of this section
in Figure
12-1,
a sche-
matic in
Figure
12-2,
and the
printed
wiring board
assembly
and parts
list in
Figure
12-3.
12.2 THEORY
OF OPERATION
12.2.1 AUDIO
SYNTHESIZER
12.2.1.1 General
Audio
frequencies
from
5 to 19999.9
Hz, in incre-
ments of
0.1
Hz,
are synthesized
using
a
phase-accu-
mulative
technique. Consider the 360
degrees in
a cycle
to be divided
into
2
20
pieces.
A 20-bit
digital
accumu-
lator,
incrementing at some fixed
rate,
could
then
at
any
instant
represent
a
fixed point
in the 360-degree
cycle.
That
is,
if
the accumulator were
half full, it
would
represent the
180-degree point,
and
if
totally full,
would
represent
the 360-degree
point.
The number
of
times per second that the accumulator goes through a
complete cycle
determines
the output
frequency.
If
the
increment
rate
is
fixed, the time
required
to
accumu-
late
22°
bits
can be varied
by
changing
the number
of
bits
added at each time
increment.
12.2.1.2 Clock
Generator
The 104.8576-kHz clock generator (U6 and
U7) pro-
vides the increment
rate
for the Audio
Synthesizer.
If
only one
bit
were added each
time,
the time to
com-
plete one cycle would be
10
seconds. Processor-loaded
control latches U40-U44 determine the
number
of
bits
to
be
added
at
each time increment
and, thus,
the final
output
frequency.
A 20-bit adder (U26-U30)
adds
the
control word to
the
current word in the
20-bit
accu-
mulator latch
(Ul4-Ul8).
At
the next time
increment,
the adder output is
latched,
becoming the
next
input
to
the adder.
12-1
SECTION 12.
AUDIO SYNTHESIZER BOARD (A 1 0)
12.2.1.3 Digital-To-Analog Conversion
The linear digital
output of the 20-bit
latch
accu-
mulator
is
converted
into
a sinusoidal
digital
output
by
the decode EPROM
(Ull).
Following
the decode
EPROM
is
a
digital-to-analog
converter (U3) which
converts the
sinusoidal information
into
a quantized
sinewave
with a period equal to the cycle time of the
20-bit accumulator latch.
12.2.1.4
Bandpass
Filter
Bandpass filter Ul and
U2,
with a
passband
of 5 to
10kHz, filters the quantized waveform to a sinewave
having less
than 1 percent distortion.
The level
of the
sinewave
is
processor-controllable by a
programmable
attenuator (U23C and U23D) having
dB
settings
of 0,
10,
and 30.
12.2.2
DPL GENERATOR
12.2.2.1
General
The
23-bit digital
private
line
(DPL) word
is
gener-
ated by
the
processor from the 3-digit code. The 23-bit
word is then transferred to a serial
shift
register and
clocked out at a rate
of 133 Hz.
Connecting the output
of the
shift
register
back
to
its
input
causes the
23-bit
word
to
be
continuously
repeated.
12.2.2
.2 Shift
Register
There
are two
modes
to the DPL
generator.
During
the
load
mode,
the shift-register control (U34 and U36)
gates
a control latch to the input
of
the
shift
register
(U20 and
U3).
Twenty-three
data
bits
and clock
pulses
are then
provided
by
the processor to
load
the DPL
word.
At the completion of the
load
mode, the
shift-
register
control
switches
back
t
o
the
output mode,
which
cycles the
DPL word through the
shift
register
at
a
133-Hz
clock
rate.
The
133-Hz
clock comes from
one of the
sinusoidal
digital
lines
on the Audio
Syn-
thesizer (U4 PIN
15).
12.2.2.3
Bandpass
Filter
Bandpass
filter
U8,
with
a 0.2 to 135-Hz bandpass,
follows the
shift-register
out
put
to remove the
higher
frequency components
of the digital
signal.
For the
DPL
off-code
(133-Hz
tone),
the
processor
switches
the
INT
MOD
line
to t
he
Audio
Synthesizer's output,
outputting a 133-Hz sinewave.
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