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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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