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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
119.
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
8182A ( 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
slietly
inconvenient
to do so);
the phase
scrambler is an essential part of the system and is non-defeatable.
1-3
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