INTRODUCTION
The principal
use of your new 418
Limiter
is to condi-
tion arbitrary
complexaudio
signals (not
just single
tracks, although
the 418
may be
used
with
these, too)
in such a way that they
can be recorded
on tape or opti-
cal
film without
overload,
excessive noise,
or distortion
due to excessive high
frequencies.
In order
to do this
while introducing
minimal artifacts
into the
signal, the
418
incorporates a highly
refined
program-controlled re-
lease time circuit,
as well
as
a
separate
high-frequency
limiter which can control
the high frequencies
present in
the program on an instantaneous basis without distortion
and without disturbing the basic program loudness.
The release time
and high-frequency
limiter
are
both
adjustable
by the
user in order to obtain the
desired
amount
of high-frequency
control, level
control, com-
pression, and density augmentation
.
The controls have
been arranged to minimize
the probability of
audible
side-effects of
an undesirable nature
regardless of their
settings
.
Some
uses
of the 418
include:
(1)
mixing
through
while doing multitrack
reductions in
order to
save
time
while
doing radio
commercials, demo sessions,
and the
like;
(2)
conditioning
recorded program material
so
that
it
can
be
copied onto
cassette or low-speed
tape
without
excessive hiss and/or high-frequency
distortion;
(3)
trans-
fering from magnetic
to optical film;
(4)
transferring from
disc to 7.5
ips tape cartridge in
broadcast stations;
(5)
re-
cording
production work through in
broadcast
production
studios;
and
(6)
transferring
from tape
to disc (with certain
limitations
—
see
OPERATING INSTRUCTIONS). Other
applications will
doubtless
suggest themselves
to the
creative engineer
.
The 418
is not rigorously
RFI/EMI
shielded,
and
its
output contains
substantial amounts of
fast overshoots.
It is
therefore
specifically not recommended for
driving
broadcast transmitters.
Orban
Associates
Division manu-
factures
a
line of devices,
the
OPTIMOD-FM
and the
OPTIMOD-AM,
specifically designed for
this
purpose.
INSTALLATION:
MECHANICAL
The
418
Limiter
is designed to mount
in
a standard
19"
(48.26 cm) rack,
and
requires
3.5"
(8.89
cm) of
vertical
space. All
operating controls
are accessible from the
front.
The fuse,
AC
line
cord, and
audio connections
are made from
the rear.
The audio
connects to
a Jones
140-type
barrier
strip (^5
screw),
and connections
can
be made
by means
of spade
lugs or
a
fanning
strip,
in
those
cases where
quick
connects/disconnects
are required.
In
a
rack mount
installation, the 418 will pick
up its
chassis ground from the rack.
Be sure to measure the
re-
sistance
from
chassis to rack
after installation
and correct
any high-resistance
situations
before
proceeding.
It may
be necessary
to scrape the paint from the rack
and/or the
rear of the
panel in order to effect
an adequate ground.
It
is advisable to make
sure
that
the rack is grounded to
some earth
ground
simultaneously. Grounding of
racks
and other
equipment to
power line conduit grounds
as a
sole means of
grounding often
creates troublesome prob-
lems.
In rack
-mounting the
418,
very
strong
AC magnetic
fields should
be avoided because these
can introduce hum
into the input
transformers. In addition, the
418
should
not be mounted
directly over
equipment producing
large
amounts of heat,
like
vacuumtube power
amplifiers.
Ambient temperature should
not
exceed 45°C
(11
3°F
)
when the 418 is powered.
If the 418
is rack-mounted,
the
jumper
on the
rear
barrier strip connecting
terminal
7
(signal
ground)
to
terminal
8
(chassis
ground) should
be
removed
to
avoid
ground
loops. If
the
418 is
used in
a portable
situ-
ation, this
jumper should
be
retained in
order to
assure
that the
chassis
is grounded.
When the
jumper
is
retained,
it is
advisable to terminate
the line
cord with
a three-
prong to two-prong
AC
adapter in
order to
avoid intro-
ducing ground
loops
through the
AC
power
line
grounding
system.
The center
(ground)
prong on the
418
line
cord
(green
wire)
is connected directly
to the
chassis.
INSTALLATION:
ELECTRICAL
Input;
The input of
the
41
8A
is
a
100K
ohm
balanced bridging
input.
It is synthesized by
means of
an electronic
differ-
ential
amplifier;
no transformer is used.
Absolute
input
clipping
occurs
at
+21
dBm; higher
levels
require
use of
an
external
balanced
pad. With the
INPUT
ATTEN
full
clock-
wise,
-10
dBm will
produce 10
dB
gain
reduction.
It
is important that
both
(+)
and
(-)
inputs
be
driven
by
a
source impedance
of 600 ohms
or less
in order
to
assure
best "common
mode rejection"
(i.e.,
ground
loop
hum
rejection).
If the device
driving
the
41
8A
has
a
balanced output, the
two output
leads should
be driven
directly
into the
(+)
and
(-)
inputs
of the 41
8A.
If
the
device
driving
the 41
8A has
an
unbalanced
output,
it
snould
also be connected to
the
41
8A
with
a two conductor
shielded cable.
The
black
wire should be
connected
bet-
ween the driving
device's signal
ground and the
418A's
(-)
input.
The second wire should
be connected
between
the
driving device's
output and the
418A's
(+)
input.
This arrangement
takes maximum
advantage of the
hum-
reducing
ability of
the
418
A's
balanced input.
In either
case,
the shield
of the interconnecting
cable should be
connected
to chassis ground at
one end only.
No special
RF suppression techniques have been
used
in
the
418A. If
RF
interference
is experienced
in
high
RF
fields,
we suggest bypassing audio
inputs, outputs,
and
the
power line to
chassis ground through 0.001
mfd ceramic
capacitors
with short
as possible leads.
Be sure that
the
voltage
rating of the
power line bypass
capacitor is at least
1 .6
kV,
and that
bypassing occurs after
the
fuse to avoid
a
potential
fire should the
capacitor
short
.
Because the
chassis
metalwork
has not been designed
to
be rigorously
RF-tight,
it is
unlikely that the
418A
could be
operated
immediately
adjacent to
a
transmitter.
In a broadcast
production
studio, sufficient
RF
suppression usually
exists
so that
successful
operation can be easily obtained.
Output:
The
output
of the 418
is unbalanced, and
follows
the
OUTPUT ATTEN
control, which
is configured
as a
standard
potentiometer.
Maximum output resistance
occurs with
this control
at
12
o'clock; in this case, the
output
resis-
tance is
approximately 400 ohms.
Maximum
available
level is +1
3
.4 dBm
.
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