A current
developed from
the de-esser control loop is injected into the
control
current port
of ICI3B.
The gain of
ICI3B is
directly proportional
to its
control
current. The de-esser
function
results
from
varying
this
control
current. The
de-
esser
function is functionally in series
with
the output of the compressor/limiter
and does
not affect its operation, since the feedback signal to activate the
compressor
limiter is taken from
the output
of
1C
14,
which
is unaffected by
the
gain of ICI3B.
Second-harmonic
distortion is
introduced
by
differential
offsets in
either ICI3A or
ICI3B. This distortion
is cancelled by
applying a nulling
voltage directly
to the
input of
ICI3B by
means of
resistor network R24, R25, R26.
If the VCA is
not
perfectly
balanced, "thumps" due
to
control
current feedthrough
can appear at the
output. These are
equivalent
to
multiplying the control current
by DC. If
a
correct DC offset is
applied
to
the VCA input, then
this
equivalent
DC
multiplication can be
nulled to zero and the "thumps" eliminated.
Such an
adjustable DC
offset is
provided
by
RI7, RI9.
C8, RI8
are frequency-compensation components
to prevent the VCA
from
oscillating supersonically.
"Thumps"
can
also appear due
to
de-esser control current
feedthrough at ICI3B's
output. Nulling compressor feedthrough
by
means
of RI9
may
make de-esser
feedthrough
worse. Therefore,
a
separate
de-esser
feedthrough
null
circuit
consisting
of IC8B-E, R27, R28 is
included.
The de-esser
control
current
flows from the
collector
of IC8A.
A current identical
to
the de-esser control current flows from
the collector
of IC8B,
since
IC8A
and
IC8B are
matched
transistors. The current output
of IC8B
is inverted in current
mirror
IC8C-E,
and injected
into the junction of R27 and
R28,
where
it develops a
voltage.
The
voltage is
amplified in both inverting
and non-inverting modes
by
IC9B. When the
wiper
of
R28
is at
50%
rotation, then
IC9B's
inverting
and
non-
inverting
gains are equal, and
no effect is produced
at IC9B's
output due to
cancellation.
Moving
the
wiper of
R28
to either
side of
this
null point
permits
introduction
of
sufficient control
voltage into IC9B's output
to
cancel
any
feedthrough in
ICI3B,
regardless of the
feedthrough's polarity.
Exponential
Converter:
The
basic
current-controlled gain in the compressor/limiter
is inversely proportional
to
the control current.
We wish to transform
this
into a
gain which is proportional
to a
control
voltage in dB. This is done in the
exponential current converter
consisting of
1CI2B and
associated components.
ICI2B,
IC7A, IC7B,
IC7C and
associated
components
form
a
log/antilog multiplier.
This
multiplier multiplies the
current flowing in
R63
by
the
exponential of the
voltage on
the base of IC7B. The
current gain of the multiplier increases as the
voltage on
the
base
of IC7B becomes
more
negative.
Because
the
voltage
on
the base
of IC7B
is
in
log (i.e.,
dB-linear) form,
various
control
voltages
can
be
summed into this
base,
and
they
will add
in a
dB-linear
manner. These control
voltages
include the
main gain-control output of the
timing
module
(through R62),
a
gain
trim
(through
R6I), and the user-adjustable OUTPUT
TRIM (through
R65)
which
adjusts not
only
the gain of the
VCA,
but
the
compression threshold as
well, as will
be
described
below.
The
current output
of
the log/antilog
multiplier appears on the collector of IC7A.
It is the
wrong polarity and level
to correctly drive the control-current
port of
1C
1 3 A. It
is therefore applied
to
a
current inverter ICI2A,
Ql, R57,
R58,
C2I.
This circuit has
a
gain
of 6
. 66x, and
operates
as
follows:
33
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