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1
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
8182A's
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
"fiat"
(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
81B2A/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
can
be
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
ail
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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