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7.
Stereo
Generator Interface Port And Line
Amplifier/Deemphasis:
A special
rear-panel connector is provided to interface the 8182A to the Orban 8182A/SG
BTSC Stereo Generator. This connector permits the output of the 8182As Dual-
Band
Compressor ( in
L/R
form)
to
be
looped
through
a first
set
of
lowpass
filters
in the 8182A/SG to
protect subsequent 8182A
processing
from
15,734Hz
contamination on the program line. If the 8182A is to be operated without the
8182A/SG ( during
bench
tests
or
with
stereo
generators
other
than
the
Orban
8182A/SG,
for
example),
this
loopthrough
connection
can
be
bypassed
by
switching
the
8182A's
rear-panel
STEREO
GENERATOR
IN/OUT
switch
OUT,
thus
restoring
signal
path
continuity.
When
the
8182A
is
operated
with
the
8182A/SG, this switch must be IN.
After
the
signal
is
returned
to
the
8182A
for
preemphasis,
HF
limiting,
and
peak limiting ( still in L/R form), it is applied to a matrix which produces either
L+R/L-R or L/R outputs depending on jumper strapping.
The
matrix drives
the line
amplifiers,
which are balanced
and
transformerless.
These
amplifiers drive
the 8182A's external audio output
terminals,
which are
interfaced
to
the
outside
world
through
non- overshooting
RFI
filters
effective
from approximately 500kHz to 1GHz. The line amplifiers also drive the interface
connector to the Orban 8182A/SG.
The
signal
which is applied to the output amplifiers is preemphasized. Jumpers
determine whether the line amplifiers pass the signal in preemphasized form, or
if deemphasis is applied ( providing a " flat" output).
In
general,
best
system
peak
control
is
obtained
by
defeating
exciter
preemphasis and applying the preemphasized signal from OPTIMOD-TV to the flat
exciter.
However,
in
some
systems
it
is
extremely
inconvenient
to
defeat
the
exciter 's
preemphasis,
and
the
exciter
must
be
supplied
with
a " flat" ( i.e.,
deemphasized) signal from OPTIMOD-TV.
The
main part of the Orban 8182A/SG ( following its first set of lowpass filters)
is
driven the by the 8182A line amplifiers and requires preemphasized L+R/L-R
inputs. This is achieved by placing several sets of jumpers on the 8182A's Card
#7 ( Output
Amplifier/Deemphasis)
according
to
instructions provided
in Part 3
(Installation) of the 8182A/SG Operating Manual.
VU
Meter:
The
front- panel
VU
meter
can
monitor
the
audio
level
at
several
different
points
in
the
circuitry
as
selected
by
the
METER
switch. ( See
Block
Diagram in Appendix J for metering points.)
BTSC
Stereo And The 8182A:
The
8182A
uses FM
stereo- style
processing.
This
means that the compressor gains track each other ( to preserve stereo imaging), and
that
the
peak
output
levels
of
the
Left
and
Right
channels
are
independently
controlled so that neither exceeds 50% L+R modulation. It ran he shown that this
is conservative for modulation control in the BTSC system: When program material
containing
a large amount of stereo content ( i.e., L- R energy)
is processed, L+R
modulation will be less than 100% and loudness may be slightly reduced on mono
receivers. However, significant overmodulation of the overall composite signal will
almost always be avoided.
(There
is
a slight uncertainty because of the presence of the dbx noise reduction
encoder in the L- R channel following all other audio processing. The action of the
N/R
encoder
can
increase
peak
modulation.
However,
it
has
been
specifically
designed to prevent significant overmodulation.)
1-7
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