Line-Up Facilities; Left/Right Input; Composite; Pilot Injection - Orban 8218 Operating Manual

Orban Model 8218
1-10
INTRODUCTION
If the
STL
microwave uses pre-emphasis,
its
input
pre-emphasis network
will
probably
introduce overshoots
that will
increase
peak modulation without any
increases
in
average
modulation.
We
strongly
recommend
that
you
defeat the
microwave STL's
transmitter
pre-emphasis and receiver de-emphasis.
Perform pre-emphasis
in
the
audio processing
system
driving the link
(like
the
Orban
8200).
This reduces
potential
overshoot.
Line-Up
Facilities
Left/Right Input Level
The
Left/Right
meter
on
the
8218 shows
left/right
input levels as a
10-segment bar graph (5%
to
1
10%),
absolute instantaneous peak-indicating.
100%
is
the threshold
of
the
Overshoot
Control Limiter (when
activated).
Composite
Output Level
Composite
modulation
is
indicated
in
percent
modulation,
absolute instantaneous
peak
indicating.
You
ordinarily adjust the 8218's output attenuators to
make 100%
on
the
meter
correspond
to
±
75kHz
carrier deviation.
Note
that
if
subcarriers
(SCAs)
are applied
to the
821
8's subcarrier input,
the
meter
will
ordinarily
read higher than
100%. For
example,
in
the U.S.A.,
if
two
subcarriers are used, the
meter
will usually
read peaks of 110%,
corresponding
to
±
82.5kHz
deviation.
The
composite
output
meter
indicates the
sum
of
the
encoded
stereo
baseband and
the stereo
pilot
tone.
The
composite meter
may
read
lower than the Left/Right meters
for three reasons.
First,
if
a
100%
audio
peak does not coincide with
the
peak
instantaneous value of
the
pilot
tone
waveform,
the
composite meter
will
read
below 100%.
Nevertheless,
it
is
accurately
showing
the instantaneous
peak
modulation. Second, a small
effect called "pilot interleav-
ing" will
reduce
the
composite modulation by
2.8%
as the
ratio
of
L-R
to
L+R
increases
from
zero
to unity,
even though
the
peak
levels
of
the
left
and
right
audio have not changed.
Third, the
sensitivity
of
the
composite meter
is
reduced by
4%
to
accommodate
operating
margin
when
the
Overshoot Control Limiter
is
activated.
Pilot Injection
Metering
You
can
switch the
lower meter
to read
pilot
injection
in
percent
modulation.
Most
government
authorities require pilot injection to
be
between
8%
and 10%.
9%
is
the
value
customarily used.
About
the 8218's J.17
De-emphasis
When
the
Orban 8200
is
set
for
"pre-emphasized" output
(at 50(is
or
75ps)
the
8200D/SRC
digital
I/O card
in
the
Orban 8200
scales the gain of
its
optionally applied J.17
pre-emphasis
so
that
the
J. 17
pre-emphasis gain
at
15kHz
is
OdB.
When
the
8200
is
set
for "flat"
output
(that
is,
the output of
the
audio processing receives
50(xs
or
75(i.s
de-emphasis before
the
audio
is
applied to the
8200D/SRC
digital
I/O
card),
then the
8200D/SRC
card
scales the
gain of
its
optionally-applied J.17 pre-emphasis so
that