Video Microwave; Analog Land; Dual Microwave - Orban 8218 Operating Manual

8218
Digital
Stereo
Encoder
INTRODUCTION
decoded
the
NICAM
signal
and converted
it
to
PCM
in
AES/EBU
form. (See
Chapter 2
for
a
complete
discussion of the
8218s extensive
pre-emphasis switching capabilities.)
Video microwave
STLs
with
PCM
adapter
The
video
STLs
in
use typically
operate
above
20GHz,
with
consumer
PCM
adapters
(from
Sony or dbx, for
example)
to
encode
left
and
right
audio into
a video-like signal.
The
quality
of signal
received
at
the transmitter
through this
type of
STL
is
high.
However,
the
high
carrier
frequencies
make
these
links subject to rain fading.
Other potential
problems include
very sharp high-frequency
cut-off,
rapid
changes
in
group delay
around
cut-off,
and
quantization
distortion.
These problems
can only be alleviated
by use of
digital
I/O.
To
the
best
of our knowledge, of these various units
only the
Sony
PCM-601
has a suitable
digital
I/O,
which
is
SPDIF. (The
8218's
AES/EBU
input will
often lock
to
a
SPDIF
signal
without
difficulty,
although
we
cannot guarantee
this
for
all
cases.)
The Sony
F-l
-series units
(including the
above mentioned
PCM-601)
ordinarily
operate
at
a sampling frequency of
44.056kHz.
In
our experience,
most
(but not
all)
of these units will
lock to
a
44.
1
kHz
signal.
This
will
vary
from one individual unit
to the next. If
you plan
to
use a
PCM-601
with
an
Orban
8200,
you must
use the
8200's
8200D/SRC advanced
digital
I/O card
and
set
the
8200
to
provide a
44.1kHz
digital
output.
You
must then
verify that the
particular pair
of
PCM-601s you
are
planning to
use
will
lock reliably to
44.1kHz
over
temperature.
The Sony
and dbx encoders are
no
longer
manufactured, but
may
be found
on
the
used
market.
Analog
land
line
(PTT/post
office line)
Analog land
line
quality
is
extremely variable.
Even the best land lines will
have
slight
frequency response irregularities
and non-constant
group
delay,
which
will
cause over-
shoots at their outputs.
This
will
increase the
peak-to-average ratio
and
will
thus require
use
of the
821
8's
Overshoot Control Limiter. In
competitive
environments these lines are
never
suitable
because of
this
overshoot, (see
Optimal Control of
Peak
Modulation Levels
on page
1-7).
Dual
microwave
STLs
Dual microwave
STLs can provide satisfactory results
if
carefully designed.
Otherwise they
can
introduce non-constant
group delay
in
the
audio spectrum,
distorting
peak
levels
and
requiring use of the
8218's Overshoot Control Limiter.
Some
dual
microwave
links
may be modified
to
meet the specification for
frequency
response
and phase
linearity
stated
in
Optimal
Control of
Peak
Modulation Levels on page
1
-7.
Many
such
links
have been designed
to
be
easily
configured
at
the factory for
composite
operation,
where
the entire
FM
stereo
baseband
is
passed, including the pilot
tone
and
stereo
subchannel.
The
requirements for
maintaining stereo
separation in
composite operation are
similar to the
requirements for
high
waveform
fidelity
with
low overshoot. Therefore,
most
links
have the potential for excellent
waveform
fidelity
if
they are
configured for
composite
operation (even
if
a composite
FM
stereo signal
is
not actually
being applied
to the
link).