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by voice can occur
to
whatever degree
desired
by
adjust-
ing
how
hard the
voice
drives the
418
(and
thus,
the re-
sulting
degree
of
gain reduction obtained)
.
OPERATING INSTRUCTIONS
Connect
the
418
to
a source
of
AC
power
correspond-
ing to the
voltage
for
which
the
power
transformer
is
strapped
(see
INSTALLATION:
ELECTRICAL
above).
Turn
the
AC POWER
switch
ON.
The neon
pilot
lamp
should glow.
All
metering
positions
should read zero,
except
for
GAIN
REDUCTION
(100%
with
no
signal),
and
+15 and -15 monitor
the positive
and
negative
regulated
power
supplies respectively,
and
verify that
these are
working
properly.
The
L
IN and
R
IN
positions
monitor the
input levels
after
the
input
"active transformer"
differential amplifier,
but before the input attenuator.
This position
is
princi-
pally
useful
in
verifying
whether
signal
exists
on
the in-
put
line,
and
if
so
the
approximate
signal level.
"O"
VU
on
the meter corresponds
to
approximately +4
dBm
(
into
600 ohms
)
equivalent
input level.
The
L
OUT
and
R
CUT
positions
monitor the output
before the output attenuator.
The
G/R (GAIN REDUCTION)
position indicates
the
approximate
amount
of
broadband
gain reduction
in
dB.
The amount
of
gain reduction
is
determined by
the
input level
being presented
to
the
limiter; this
can be
affected by the drive
level to
the 418,
and
also
by
the
418's
INPUT
ATTEN
control.
As
this
control
is
turned
clockwise,
the
amount
of gain
reduction
will
increase.
If
the operator attempts
to
exceed
approximately
15
dB
gain reduction, the
red
OVERLOAD
lamp
will light,
and
distortion
and
severe
high-frequency
loss
may
be
per-
ceived
.
The
RELEASE TIME
control
is
not a release
time
con-
trol
in
the
classic
sense,
because automatic
circuits
in-
side the 418 are constantly
analyzing
the
program
mate-
rial
and
continuously varying the release time
to
mini-
mize
audible
limiting
"action".
The
function
of
the
RE-
LEASE TIME
control
is
to adjust
the
speed
of
this
entire
process.
As the
release
time
is
speeded
up
(control
to-
wards
"fast"),
the 418
will
increase the
average
level
("density")
of
the
program
to
a greater
and
greater
ex-
tent.
As
the
RELEASE TIME
control
is
moved
into
the
last
quarter of
its
range towards
"fast",
there
is
greater
and
greater
danger
that
the
limiting process will
produce
objectionable
results.
In
general,
it
is
desirable to use
less
than 6 dB gain reduction
when
operating with such
fast
release times.
As
the release
time
is
slowed
down,
more and more
gain reduction can
be
used without signi-
ficant
audible
side-effects.
Slower
release times are
particularly useful
for
"gain riding" functions, such as
mixing through
the
limiter,
where no change
in
the qual
-
ity
of
the sound
is
desired, but rather protection
from
excessive
level
which would
tend
to
overload the follow-
ing
recording
medium
is
wanted.
The
automatic
release
time adjustments
assure that
heavy
transients
(like
kick
drum)
do
not
knock
"holes"
in
the
audio, while simulta-
neously
instruments
like
French horn
or
strings
do
not
"pump".
Slower
release times are also useful
for
prepar-
ing masters
for
cassette
duplication,
where
acceptable
signal-to-noise
ratios
must be obtained
by
a certain
amount
of
compression
of the master.
Most
recording
media
(with
the
exception
of
15
and
30
ips
tape)
are significantly
more
subject to
overload
at
high frequencies than
at
low frequencies,
due
to the
application
of
record pre-emphasis.
Cassettes are par-
ticularly
problematical.
The high-frequency
limiter
in
the 418
can be switched
in
to
automatically
control the
high-frequency
content
of the
audio
so as
to
avoid audi-
ble
high-frequency overload and
accompanying
distor-
tion.
The
four
"time constants"
do
not refer to
attack
or
release times
(which are
not adjustable),
but rather
to
the frequencies
at
which
the
control
threshold
is
3
dB
below
the
broadband
(low frequency) threshold.
The
following table gives these frequencies, as well
as
sug-
gested
uses
for
each
time
constant:
25
uS
6.37
kHz
15 ips
tape
37.5 uS
4.24
kHz
7-1/2
ips
tape (modern
oxide);
RIAA
disc
(allowing 6
dB
headroom)
50
uS
3.18
kHz
3-3/4
ips
tape;
7-1/2
ips
broadcast tape
cartridge
75 uS
2.12
kHz
1-7/8
ips
tape;
cassette;
optical
film;
RIAA
disc (al-
lowing
no high-frequency
headroom)
It
should be noted
that
more
high
frequency content
may
always
be recorded on a given
medium
by reducing
the low frequency
level, thus
allowing
more headroom
for
high-frequency pre-emphasis.
In
some
cases,
this
may
be
a desirable
expedient
to
achieve
a
brighter
sound.
In
other cases, the
loss
of
signal-to-noise
ratio
may
be
intolerable.
If
the former
expedient
is
adopted,
the
high-frequency
limiter
may
be
s'Bt
on
a higher
fre-
quency
time constant than normal,
thus
reducing
the
amount
of
high-frequency
limiting.
Because
of the
com-
plex nature
of
high-frequency overload
distortion, the
ear should
always be
the
final
arbiter of
the proper
set-
ting of the
TIME
CONSTANT
control
Further details of 418
operation are largely
dependent
upon
the
sound
and
effect desired
from the
device.
Ex-
perimentation
in
actual
use situations
is
the
best
way
to
develop
a
knowledge
of the 418's usefulness as
well
as
its
limitations
MAINTENANCE
Preventive
Maintenance:
The
418
is
an
entirely solid-state
device.
The
only
preventive
maintenance
required
is
keeping
the
unit
clean.
Dust on the
circuit
board
can
absorb moisture.
3
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