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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