Orban 8182A Operating Manual page 29

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
STL
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.
Exciters:
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