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PART
2:
Various Applications
This Part of the Manual
provides essential
information on how OPTIMOD-TV
fits
in with the rest of the equipment at your station.
Studio/Transmitter Links: There
are five types of studio/transmitter links ( STL's)
in
common
use
internationally
in
TV
audio
service,
whether
single-
or
multi-
channel. These are:
1) Analog land- lines ( telephone lines);
2) 50-15,000 Hz microwave STL's ( single or dual);
3) Composite baseband microwave STL's ( stereo service);
4) Composite video/audio microwave SIL ' s; and
5) PCM ( Pulse- Code Modulation) links.
All
except ( 3) carry the audio either directly or in some encoded form other than
the
stereo baseband to be
found in TV
multiplex stereo service ( once approved).
These links are ordinarily fed audio in non- encoded form, and their receiver output
is
audio.
In
the case of ( 4), the audio in placed on an FM subcarrier above the
video baseband.
The
composite STL ( 3) carries the multiplexed stereo baseband, and is therefore
fed
from the output of a stereo generator. The receiver output of the composite
STL
is the stereo baseband signal, which is applied directly to wideband input of
the TV transmitter's aural exciter.
In general, highest quality is obtained by use of a composite microwave STL ( 3) or
video/audio
microwave STL ( 4)
provided that
a line- of- sight
transmission path of
less than 10 miles or so exists between studio and transmitter. If not, RF signal-
to-noise ratio, multipath distortion, and diffraction effects can cause serious quality
problems.
The
50-15,000Hz
microwave
system ( 2)
provides
more
noise
immunity
than
composite systems. However, problems include gain- and phase- matching of the left
and
right
channels
in
stereo
service,
preemphasis-induced
overloads,
and
a
requirement that the audio applied to the microwave transmitters be processed to
prevent their overmodulation.
Land- line ( 1)
quality
is
extremely
variable,
ranging
from
excellent
to
atrocious.
The
decision
on
whether
to
employ
land- lines
depends
a great
deal
on
the
line
quality
locally
available.
However,
even
the
best
land- lines
tend
to
slightly
veil
audio quality
due
to line equalizer characteristics,
phase shifts,
and repeaters of
indifferent quality.
PCM
links ( 5) are generally unavailable in the USA as of this writing, although
they
are
widely
used
in
Europe.
They
achieve
good
noise
performance
and
consistency at the expense of a very sharp high- frequency cutoff, rapid changes in
group
delay
around
cutoff ( unless
elaborate
phase
equalization
is
used),
and
quantization distortion. At the moment, there is considerable disagreement over how
elaborate the coding must be to render quantization distortion inaudible to critical
listeners,
and no PCM system should be accepted without
critical
listening tests.
OPTIMOD-TV is available in either single- or dual- chassis configurations. The dual-
chassis
splits
the
system
at
a point
between
the
output
of
the
Dual- Band
Compressor and the input of the high- frequency Limiter.
2-1
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