Orban OPTIMOD-FM 8600, Operating Manual

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1-14
INTRODUCTION ORBAN MODEL 8600
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 8600’s AES3 digital output through an
all-digital, uncompressed path to the transmitter's exciter, although this will pre-
clude using the 8600’s composite limiter.
Use the 8600’s left and right analog audio outputs in situations where the stereo en-
coder and exciter are under the jurisdiction of an independent transmission authori-
ty and where the programming agency’s jurisdiction ends at the interface between
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 controlled
by the audio processor can be introduced by the link to the transmitter, by transmit-
ter 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 8600’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 8600’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 8600’s output sample rate to 44.1 kHz or higher.
If only an audio link is available, use the 8600’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 respond only to
signals caused by faults or by spurious peaks introduced by imperfections in the link.
To ensure maximum quality, all equipment in the signal path after the studio should
be carefully aligned and qualified to meet the appropriate standards for bandwidth,
distortion, group delay and gain stability and such equipment should be re-qualified
at reasonable intervals. (See
Optimal Control of Peak Modulation Levels on page 1-
13).
If the transmitter is accessible:
You can achieve the most accurate control of modulation peaks by locating
OPTIMOD-FM at the transmitter site and then using its stereo encoder to drive the
transmitter. You can usually also obtain good results by locating OPTIMOD-FM at the
studio and connecting the baseband output of its stereo encoder to the transmitter
through a composite baseband STL (see page 1-18). However, many analog compo-