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preemphasis network), far more accurate results are obtained if the exciter's input
transformer,
preemphasis
network,
and
any
bandwidth- limiting
filters
are
entirely
bypassed, and the preemphasized output of OPTIMOD-TV is introduced as close to
the
modulator stage as practical. Some exciters have an auxiliary or " test" input
available for this purpose.
Instructions
on
how
to
restrap
the
OPTIMOD-TV
output
for
preemphasized
operation are provided in Part 3 (Installation) of this Manual.
Additional Aural Subearriers: OPTIMOD-TV
operates well with subcarriers because
OPTIMOD-TV
provides
excellent
baseband
spectrum
control,
thus
protecting
that
part
of the baseband occupied by the subcarrier. No special subcarrier precautions
need be taken; the subcarrier should be implemented according to the instructions
of the exciter manufacturer.
Older
aural exciters may not have separate subcarrier inputs. In most cases, one
can be
added simply by passively summing into the modulated oscillator through a
resistor and small capacitor. Note, however, that such older exciters often suffer
from
narrow
RF
bandwidths
which
may
cause
subcarrier " birdies"
due
to
intermodulation.
Stereo: All OPTIMOD-TV mainframes are fully equipped for stereo. An explanation
of
the
relationship
between
OPTIMOD-TV,
the
Orban
8182A/SG
BTSC
Stereo
Generator,
and
other
vendors'
stereo
generators
is
provided
in
Part
1 of
this
Manual.
It
is
important
to
note
that
OPTIMOD-TV
has
only
one
control
circuit
which
adjusts the gain reduction of both stereo channels based on the higher of the two.
It
is
therefore unsuited for dual- mono operation ( such as bilingual
programming).
[In
the BTSC system used in the United States, bilingual programming, if any, is
provided
on
the Separate Audio Program ( SAP) subcarrier. Orban manufactures
a
SAP
subcarrier
generator
with
integrated
audio
processing ( Model
8182A/SAP)
to
provide this subcarrier.]
2-3
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