Composite Baseband; Subcarriers - Orban 8218 Operating Manual

8218
Digital
Stereo
Encoder
INTRODUCTION
on
the
821
8' s
front panel.
Input level
range select
is
via
DIP
switches located
on
the
821
8'
rear panel.
The
82
1
8 accepts
any
digital
AES/EBU
input
sample
rate
between
32kHz
and
48kHz
by use
of a
built-in
sample-rate
converter.
The
8218 can be
switched between
digital
and analog
inputs
by the front
panel Input
select
switch or
by remote
interface.
Both analog and
digital
outputs are active continuously.
Composite Baseband
Outputs
The
stereo
encoder has
two unbalanced baseband outputs
(Composite
1
and Composite
2)
on two
BNC
connectors
on
the rear panel.
Each output can be strapped for
OQ
or
750
source
impedance
(jumper-selectable),
and can
drive
up
to
8.8V
peak-to-peak into
750
in parallel
with
up
to
0.047pF (100ft/30m of
RG-59/U
cable) before
any noticeable performance
degradation occurs.
(The 8218
is
shipped
from the factory with
OO
source impedance.)
Level
control
of each output
is
via
a separate screwdriver control accessible
from
the
front
panel.
A
ground
lift
switch
is
available
on
the rear panel.
This can prevent
ground
loops
between
the
8218 and
the transmitter.
The Composite meter on
the
82
1
8
shows
composite output
levels
as a
1
0-segment
bar graph
(5%
to
1
10%).
The
sensitivity
of
this
meter changes
when
the
Overshoot Control Limiter
is
activated.
If
the
Overshoot Control Limiter
is
defeated,
applying a
sinewave
equally
to
both
left
and
right
inputs
such
that the
two
input
meters read
100%
will
cause
the
Composite
meter
to
read
100%,
provided
that
the
pilot
tone
is
ON
and
set to
9%
injection.
Activating
the
Overshoot Control Limiter reduces the
sensitivity
of
the
Composite meter by 4%,
allowing a
margin such that the
Overshoot
Control Limiter meter
will
normally read
100%
on program
peaks.
The
Overshoot Control Limiter has a slight residual
overshoot
(less
than
0.5dB) because, unlike a simple clipper,
it
controls
its
output
spectrum very
tightly.
The
non-linear filtering
necessary
to
accomplish
this
causes a small
amount
of
uncertainty
in
the peak output
level.
While
the
Overshoot
Control Limiter
uses
lookahead techniques
to
minimize
this
uncertainty,
these techniques
cannot
entirely
eliminate
it
without introducing
some
audible gain
pumping, which
we
considered unacceptable.
We
therefore
designed
the circuit for
maximum
audible transparency
at
the cost
of
a
small
amount
of overshoot.
You
ordinarily adjust the
8218's output
level
so
that
100%
corresponds
to
±75kHz
carrier
deviation.
Note
that
if
you
apply subcarriers
(SCAs)
to the rear
panel subcarrier
input,
the
meter
will
read higher than 100%. For example,
in the
U.S.A.,
if
two
subcarriers are used,
the
meter
will usually
read peaks of
1
10%,
corresponding
to
±
82.5kHz
deviation.
Subcarriers
The 8218
has
two
unbalanced
lOkQ
subcarrier
(SCA)
inputs
with Rear-panel
BNC
connec-
tors to
accept
any
subcarrier
at
or
above 23kHz. The
subcarrier will
be
mixed
into
each
composite
output,
and
its
level will
be
affected
by
the
composite
level control for that output.