Orban OPTIMOD-FM 8700, Operating Manual

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INTRODUCTION ORBAN MODEL 8700i
In this example, the network could be fixed by adding a second resistor
between ground and the capacitor, which would introduce a zero.
Low-pass filters (including anti-aliasing filters in digital links), high-pass filters, trans-
formers, distribution amplifiers, and long transmission lines can all cause the above
criteria to be violated, and must be tested and qualified. It is clear that the above
criteria for optimal control of peak modulation levels are most easily met when the
audio processor directly feeds the stereo encoder. In the 8700i, no circuit elements
that might distort the shape of the waveform are interposed between the audio
processor and the stereo encoder. We therefore recommend using the 8700i with its
built-in stereo encoder whenever practical.
Best Location for OPTIMOD-FM
The best location for OPTIMOD-FM is as close as possible to the transmitter, so that
its stereo encoder output can be connected to the transmitter through a circuit path
that introduces the least possible change in the shape of OPTIMOD-FM’s carefully
peak-limited composite waveform—a short length of coaxial cable. If this is impossi-
ble, the next best arrangement is to feed the 8700i’s AES3 digital output through an
all-digital, uncompressed path to the transmitter's exciter, although this will pre-
clude using the 8700i’s composite limiter.
Use the 8700i’s left and right analog audio outputs in situations where the stereo
encoder and exciter are under the jurisdiction of an independent transmission au-
thority and where the programming agency’s jurisdiction ends at the interface be-
tween the audio facility and the link connecting the audio facility to the transmitter.
(The link might be telephone / post lines, analog microwave radio, or various types
of digital paths.) This situation is not ideal because artifacts that cannot be con-
trolled by the audio processor can be introduced by the link to the transmitter, by
transmitter peak limiters, or by the external stereo encoder.
If the transmitter is not accessible:
All audio processing must be done at the studio and you must tolerate any damage
that occurs later. If you can obtain a broadband (0-75 kHz) phase-linear link to the
transmitter and the transmitter authority will accept the delivery of a baseband en-
coded signal, use the 8700i’s internal stereo encoder at the studio location to feed
the STL. Then feed the output of the STL receiver directly into the transmitter’s ex-
citer with no intervening processing.
If an uncompressed left/right digital link is available to the transmitter, this is also an
excellent means of transmission, although it will not pass the effects of the 8700i’s
composite processor (if you are using it). However, if the digital link employs lossy
compression, it will degrade peak control. To prevent overshoots caused by spectral
truncation in the link, set the 8700i’s output sample rate to 44.1 kHz or higher.
If only an audio link is available, use the 8700i’s left and right audio outputs and
feed the audio, without preemphasis, directly into the link. If possible, request that
any transmitter protection limiters be adjusted for minimum possible action—
OPTIMOD-FM does most of that work. Transmitter protection limiters should re-
spond only to signals caused by faults or by spurious peaks introduced by imperfec-
tions in the link. To ensure maximum quality, all equipment in the signal path after