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The dual- chassis ( split) configuration is ordinarily used with STL ' s of types ( 1), ( 2),
(4),
and ( 5)
of
modest
performance
characteristics.
By
performing
initial
compression
before
the
STL
input,
the
dual- chassis
version
can
prevent
STL
overload
and
can
aid
in
achieving
superior
STL
signal-to-noise
ratio. ( See
the
Introduction for further information.)
Locating the high- frequency limiter
and peak limiting sections at
the
transmitter
can minimize the potential for peak level increases caused by subsequent passage of
the peak- limited signal through transformers,
filters,
and other
devices with non-
linear
group delay. Modulation is thus more tightly controlled and higher average
levels can be obtained, optimizing receiver signal-to-noise ratios and minimizing the
audible
effects
of
such
problems
as
intercarrier
buzz
and
incidental
phase
modulation in CATV and MATV systems.
The
single- chassis
configuration
is
suited
for
any
microwave
STL
whose
group
delay
is essentially constant
from
50-15,000Hz, whose - 3dB
low
frequency cutoff
point
is
below
5Hz,
and
whose
response
above
15,000Hz
rolls
off
in
an
approximately Gaussian manner. In many ways, the requirements are similar to the
requirements
for
high- quality
video
transmission
with
the
exceptions
that
much
narrower bandwidth is required and more stringent limits are placed on noise and
non-linear distortion.
While many audio frequency microwave STL ' s do not have these characteristics as
delivered from their manufacturers, modifications to achieve them are often trivial,
involving increasing the size of coupling capacitors and eliminating input and output
transformers,
replacing
them
with
modern
active
input
and
output
stages.
The
single- chassis
configuration
is
also
suited
for
installations
where
studio
and
transmitter
are
at
the
same
site
or
are
connected
by
short,
high- quality
lines.
Because it is less expensive than the dual- chassis version, the single- chassis version
is also suited for use with any STL having extremely wide dynamic range ( 80dB or
better) such that unprocessed audio can be passed to the compressor without danger
of
noise
build-up
when
the
compressor's
gain
increases
towards
its
maximum.
It
is
important
to note
that
the compressor
section
alone does not control peak
levels
accurately,
and does not compensate for
overloads caused by
preemphasis.
(Peak limiting and high frequency limiting are performed later in the system.) It is
therefore
necessary
to
allow
headroom
in
the
STL
to
accomodate
compressor
overshoots. If the STL is preemphasized at
50 or 75us ( as is the case with many
50-15,000Hz microwave systems), further headroom must be allowed to accomodate
the
peak
level
increases
caused
by
the
preemphasis.
Precise
SIL
setup
recommendations are provided in Part 4 (Setup) of this Manual.
If
STL preemphasis can
be readily
modified, use
of 25us preemphasis will
match
headroom
to
the
typical
spectral
distribution
of
contemporary
program
material,
thus achieving optimum STL signal-to-noise ratio.
Exciter= OPTIMOD-TV
will
interface with
all TV
aural
exciters, whether
direct-
FM
or
phase
modulation. However,
it
is important
to realize that
modern solid-
state direct-FM aural exciters provide both vastly improved reliability and audible
improvements in sound quality when compared to older designs ( particularly phase
modulators), and such older exciters should be retired if at all possible. In addition,
phase- modulator
exciters
will
almost
certainly
be
unsuited
to
stereo
TV
broadcasting.
The
discussion
above
regarding
microwave
STL
performance
requirements ( to
correctly
handle
the
peak- limited
OPTIMOD-TV
output
without
increasing
peak
levels uncontrollably) applies also to aural exciters. While the output of OPTIMOD-
TV
is
supplied
strapped
for " flat" ( requiring
use
of
the
exciter's
internal
2-2
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