A
sample-and-hold circuit
is
associated
with the timing module.
The
sample
and
hold provides
a
reference for the circuit
in EXP release shape mode (it
essentially
tells
the circuit
where the
release
started). Each time ICIOA turns its output
transistor ON, it also
turns on the switching
transistor
in IC3
(pins
13, 14,
and
15).
This pulls
pin
15 to
ground,
and
pulls
the gate
of Q5
to ground, turning
Q5
ON
and
capturing a sample of a
control voltage
on C27. This is buffered
by 1C
1 1
A
and
fed
back to
the timing module through
a
resistor network and the
RELEASE
SHAPE
switch
SI.
A second section of the
ATTACK TIME control
R79b
is
provided to slow the
release time at very slow attack times,
lest
the
release become faster
than the
attack and severe inaccuracies
occur in the compression ratio.
R79b is buffered
by
ICI7A,
the
output of
which
is fed into the resistor
network associated with SI.
Other details
of
the operation
of
the circuit and how
it
interacts with
the
timing
module are
beyond
the scope
of
this manual.
Threshold-Setting
Circuitry and RATIO Control: As was
explained above, the
compression threshold
is modified
by the setting
of R79
in order to
lower the
threshold
as
the attack
time is
lengthened.
The threshold is modified
by three other factors.
The first factor is the gain
of
the COMPRESSION RATIO control
R92.
The basic
compressor circuit
has a
high
compression ratio (approaching infinity-to-one). To
achieve lower
compression ratios,
the
VCA gain-control voltage is fed back
into the
threshold circuit through
R9I and R92 in such a way as to
increase the threshold
as
more gain reduction
occurs.
The
amount
of feedback is
adjusted by
R92
from
none (infinity-to-one)
to enough to produce a 2:1 ratio at compression threshold.
It
can thus
be seen
that the
compressor output level increases as gain
reduction
increases, lowering the compression ratio as desired.
(Because of
details of the
circuit configuration,
all
ratios increase
as gain reduction
increases beyond
threshold, yielding
a
"soft knee" curve.)
The second factor
is the position of
the COMPRESSION
RATIO
control R92.
The
network
R88, R89,
R90,
R9I, R92 provides
a
bias current for
1C
IB
which in turn
determines the basic
compression threshold. This
bias
current
is
decreased
as R92
is
turned counterclockwise
towards lower ratios, thus lowering the threshold
and
increasing the gain reduction
to avoid potential VCA overload as its
output level
increases.
The circuit
has
been designed
so
that
at
25dB (maximum achievable)
gain
reduction, the
VCA output is approximately constant regardless
of
the
position
of
R92.
The change
in VCA gain-control voltage fed into the circuit
through
R9I
approximately compensates
for the change in current flowing through
R90
and
R88
as
the loading
on these resistors is changed
by adjustment
of
R92.
The
third
factor is the OUTPUT TRIM control
R64.
This adjusts the gain of the
VCA
and
simultaneously adjusts the compression
threshold to
keep the
amount of
gain reduction
approximately constant.
R64's wiper
is
coupled to the
VCA
gain port through
R65,
and to the gain-control
input of log/antilog multiplier
1C I
(and associated components). The input to
the
multiplier is the current output
of
the
RATIO control
and associated
network
discussed above. The multiplier varies
its gain
(and
thus the
threshold voltage) by
20dB
as R64 is adjusted throughout
its range. The gain of
the VCA is
simultaneously adjusted by 20dB,
so
that
the VCA's output varies by
20dB but the
amount
of gain reduction stays approximately constant.
36
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