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1
The
Hilbert-Transform Clipper
circuit
permits
harmonic
distortion to
occur
only
on
program
material
above 4kHz,
substantially
reducing
audible
clipping
distortion
on voice,
particularly
from
optical
sound
tracks.
Yet
the
8100A
's
excellent
performance
on
music
is
retained.
The
left
and
right
Hilbert-Transform Clipper cards
reside
in
slots
#0
and
#1
respectively,
which
are not used
in
the
8100A.
--
Loudness
control circuitry has
been
added.
The
loudness
controller resides
in
slot
//2,
which
is
not used
in
the
8100A.
Comparison
With
OPTIMOD-TV
Model 8180A:
If
you
are familiar
with the
8180A,
this
section
may
be
of interest.
Otherwise,
skip to
the next
section.
Generally, the
8182A
is
very
similar
to
the
8180A
except
for the
added Loudness
Controller,
Hilbert-Transform
Clippers,
and
the
various
minor changes and
modifications necessary
to
implement
these
new
features.
Certain
minor changes
were
made
in
response
to
new
knowledge about
TV
stereo, especially
with regard
to
companding
requirements.
—
The
Loudness
Controller
circuit
has
been added
in
card
slot
#2.
—
New
Hilbert-Transform
Clippers
have been added
in
card
slots
#0
and
#1
to
replace
the
clipping
circuitry
formerly
located
on
cards
#8
and
#9.
Further
details are
provided
at
the beginning of
Appendix
A
(System
Description).
Split
Configuration:
An
alternate,
dual-chassis
system
configuration
permits
the
Dual-Band Compressor
to
be
operated
separately
from
the
remainder
of
the
circuitry. This
permits placing the Dual-Band
Compressor
at
the studio
side of the
STL
(telephone
line,
dual
microwave,
or
FM
subcarrier on a video
STL)
to
protect
the
STL from
overmodulation.
This configuration
consists of
an
OPTIMOD-TV
Model 8182A
in
conjunction with
an
Accessory
Chassis (with
power
supply)
and
two jumper
cards.
Cards
#2, #3, #4, and
#
5
are
removed
from
the
818
2A
(Main)
Chassis
and
installed
in
the
Accessory
Chassis.
Card
slots
#3
and
#4
in
the
Main
Chassis are then
fitted
with the
jumper
cards.
The
Accessory
Chassis
is
known
as
the
Model 8182A/ST.
If
the
gain
between
the
output
of
the
Accessory
Chassis
and
the
input
of
the
Main
Chassis cannot be maintained
within
a
0.75dB window,
we recommend
using
the
full
single-chassis
OPTIMOD-TV
at
the
transmitter,
and
using
an
Orban
Compressor/Limiter
(such
as
the
424A)
at
the studio side of the
STL
to
protect
against
STL
overload.
The
studio
compressor/limiter
is
adjusted
so
that
gain
reduction
is
not
produced
unless excessive levels are
output
from
the
switcher
or
console.
Simplified
System
Description:
OPTIMOD-TV
consists
of
seven
basic
blocks:
1.
Input Conditioning
Filter:
This
consists
of
an
allpass
phase
scrambler
to
make
peaks more symmetrical
(thus
reducing
clipping
distortion
and permitting
higher
loudness),
and
a
30Hz
18dB/octave
highpass
filter
to
prevent subsonic
information
from
disturbing
the
operation
of
the
audio
processing
or exciters
AFC's.
Even
if
an
AFC
doesn't
unlock,
it
can attempt
to
"track"
subsonic
information, producing
IM
distortion.
The 30Hz
highpass
filter
can
be
defeated
(although
we
have
purposely
made
it
slightly
inconvenient
to
do
so);
the phase
scrambler
is
an
essential
part of the
system and
is
non-defeatable.
1-3
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