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The
gain-control
voltage
is
applied
to
the
(+)
input of
ICI6B.
The
output
of
ICI6B
drives a peak detector network CI6,
CI7,
CRIO, CRII, CRI2, CRI3. When
ICI6B's
(+)
input goes
more
negative than
its
(-)
input (indicating that
the output
of
the
meter
driver circuit
needs
to
be
updated), ICI6B's output goes negative
and charges
both
CI6
and
CI7
to
approximately
equal voltages
through
CRIO
and
CRI3.
ICI6B's
output
stays
negative
and
continues charging
until
CI6
is
charged
to
the negative
peak
value
of
the
gain control
voltage.
ICI6A, configured
as
a voltage-follower,
buffers
CI7
and
provides a negative
feedback
voltage
for
ICI6B.
As
long as
CI6
is
charged
to
a
more
negative voltage than
is
present on the
(+)
input
of
ICI6B,
the output
of
ICI6B
will
be
positive,
turning
off
both
CRIO
and
CRI3
and
permitting
CI6
to
discharge towards ground through
R44
and R45.
As
CI6
discharges, the voltage
differential
between
CI6
and
CI7
ultimately
becomes
sufficient
to turn
CRII
and
CRI2
ON,
discharging
CI7
as well. Until
these diodes
are
turned
on,
the
peak
voltage
on
CI7
is
held constant. This
permits
the
meter
to
mechanically
rise
to
the
full
peak
value
and
to
display
the
peaks
accurately.
4)
De-Esser
Control Circuitry
The
VCA
output (which
includes
the
effect of
ICI3B,
the
de-esser
gain-control
element)
is
applied
to
the
de-esser control
module.
This
contains circuitry
which
detects the
presence
of sibilance
and which
outputs a current
when
sibilance levels
exceed
a
level
set
by R55,
the de-esser
SENSITIVITY
control.
The
output current
of
the
de-esser control
module
is
applied
to
CRI5
and
CRI7,
which
charge
CI8
and
09
to
approximately
equal
positive voltages.
The
release
time
of the circuit
is
determined by
R49
which
discharges
CI9.
When CI9
has
been
discharged
sufficiently
to
overcome
the
turn-on
voltage
of
CRI6,
CI8
will
also
discharge.
Since
the
voltage
on
CI8
determines
the
amount
of
de-esser
gain
reduction,
this
recovery
delay
function
provided
by
CRI7
substantially
aids
in
smoothing
the de-esser control
voltage and reduces
dynamic
distortion.
IC9A
is
a non-inverting
voltage-to-current
converter.
When
the
de-esser
control
voltage
(across
Cl
9)
is
at
ground,
15
volts
appears across
R47
because feedback
holds IC9A's
(+)
and
(-)
inputs at
the
same
voltage. This
causes current
to
flow
in
R47, which
flows
into
the emitter of
IC8A.
Essentially
the
same amount
of
current
flows out
of
IC8A's collector
to
the control-current port
of
ICI3B,
thus
determining
the quiescent gain
of
ICI3B.
When
the
de-esser gain control voltage
becomes more
positive,
the voltage across
R47
is
decreased, thus reducing
the control
current
into
ICI3B and
reducing the
gain
of
the
VCA.
When
the
de-esser
control
voltage
becomes more
positive
than
0.1
volt
(corresponding
to
0.06dB
gain reduction),
it
turns
on
IC20B and
lights
the
NORM
de-esser
LED. When
the
control
voltage
becomes more
positive
than
10
volts
(corresponding
to
9.5dB
gain reduction),
it
turns
on
IC20A
and
lights
the
HEAVY
de-esser
LED.
5)
VCA
LEVEL
Meter
The
peak-detecting
VCA
LEVEL
meter
monitors
the output
of
the
VCA
at
IC9B.
This
meter
is
driven
by
a
peak
detector with peak-holding
capability,
and
is
capable
of
reading the
peak
level
of
a
10
microsecond
pulse
with an accuracy
of
0.5dB
38
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