The
performance of the VCA used in the
424A
is
optimized with regard to
noise and distortion over
a
25dB gain reduction range. When
the OUTPUT TRIM
control
is
on
the
"0"
detent,
it guarantees that the VCA will
be
operated
in
the optimum
part
of
its range regardless of the amount of
gain
reduction as
indicated on the GAIN REDUCTION meter.
Turning down the OUTPUT TRIM control simultaneously lowers the VCA gain
and
the compression threshold
so
that the
gain
reduction remains approximately
constant. At
the
same
time, the amount
of available gain
reduction
is
reduced.
For example, if the OUTPUT TRIM control
is set
to "-5dB", then the
maximum
available gain reduction is 20dB, instead
of 25dB.
Certain
very
"spiky"
program
material will tend to overshoot more than
average
program
material, thus partially foiling the automatic adjustment
of
compression threshold as ATTACK and/or RATIO controls are operated
(as
described above). These excessive overshoots (which are clearly indicated on the
true
peak-reading VCA LEVEL meter) can clip the VCA
unless
the OUTPUT
TRIM control is
turned down.
If you wish
to obtain a more natural sound than that produced by very fast
attack times
in
applications
where
the absolute peak levels are important (such
as
overmodulation protection of
a
broadcast
studio-transmitter link or disk
cutter protection), this improved
naturalness
can
often be achieved by operating
with slower attack times and clipping
off the higher-amplitude overshoots in
the VCA. This
can
be done by advancing
the OUTPUT TRIM control until the
desired
amount of clipping is observed.
You will find that turning the control u£
usually does not result in
output
level increases proportional to
those calibrated on the dial, and also
results in
increased gain reduction. This
is
because the
424A
contains circuitry which
automatically
speeds
up
the attack
time and reduces overshoots
as
the VCA
approaches its clipping point, thus
"fighting"
any tendency towards clipping by
automatically increasing the gain reduction if clipping
is
imminent.
To
reiterate: IF THE OUTPUT TRIM
CONTROL IS NOT NEEDED TO EITHER
PREVENT
OR
CREATE CLIPPING,
IT SHOULD BE LEFT ON
"0".
The
OUTPUT ATTENuator
control
is
located
on
the
rear
panel. It adjusts
the
gain of
the
balanced
line
amplifier
in the 424A, and is used to match
the
output level
of
the
424A to subsequent equipment
which
lacks
a
convenient
input sensitivity adjustment.
The peak output level required from the
424A
is ordinarily
determined
by
the
overload point
of
the driven equipment (amplifier
clipping, tape
saturation,
etc.). In almost
all
cases,
the
424A
OUTPUT ATTENuator therefore
need
be
adjusted
only once in
a
given system.
The effect of adjusting the OUTPUT ATTENuator is not indicated on the VCA
LEVEL meter
as
the
OUTPUT ATTENuator control is located
after
all
compressor/limiter and de-esser circuitry.
The IDLE GAIN control
is
a
bit
tricky.
It
enables the user
to
preset the "idle" gain
of
the compressor/limiter
at
some
level between
no gain reduction and full gain reduction
when either:
a)
A low-level passage
or
pause occurs in the program, activating
the
gating
feature
and causing the
GATED
lamp to come on, or
14
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