Orban OPTIMOD-FM 8600Si, Operating Manual

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OPTIMOD-FM DIGITAL INTRODUCTION
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overshoot with processed audio. Many such links have been designed to be easily
configured at the factory for composite operation, where an entire FM stereo base-
band is passed. The requirements for maintaining stereo separation in composite
operation are similar to the requirements for high waveform fidelity with low over-
shoot. 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).
Nevertheless, in a dual-microwave system, the 8600Si is usually located at the main
FM transmitter and is driven by the microwave receivers. One of Orban’s studio level
control systems, such as the 1101e, protects the microwave transmitters at the studio
from overload. These units also perform the gain riding function ordinarily executed
by the AGC section of the 8600Si’s processing and optimize the signal-to-noise ratio
obtainable from the dual-microwave link.
If the STL microwave uses pre-emphasis, its input pre-emphasis filter will probably
introduce overshoots that will increase peak modulation without any increases in
average modulation. If the studio level control system is capable of producing a pre-
emphasized output, we strongly recommend that the microwave STL’s pre-emphasis
be defeated and pre-emphasis performed in the studio level control system. This
frees the system from potential overshoot. (The Orban 8200ST can be readily con-
figured to produce a pre-emphasized output.)
Further, it is common for a microwave STL to bounce because of a large infrasonic
peak in its frequency response caused by an under-damped automatic frequency
control (AFC) phase-locked loop. This bounce can increase the STL’s peak carrier de-
viation by as much as 2dB, reducing average modulation. Many commercial STLs
have this problem.
Some consultants presently offer modifications to minimize or eliminate this prob-
lem. If your exciter or STL has this problem, you may contact Orban Customer Service
for the latest information on such services.
Analog Landline (PTT/Post Office Line)
Analog landline quality is extremely variable, ranging from excellent to poor.
Whether landlines should be used or not depends upon the quality of the lines lo-
cally available, and upon the availability of other alternatives. Due to line equalizer
characteristics and phase shifts, even the best landlines tend to veil audio quality
slightly. They will certainly be the weakest link in a FM broadcast chain.
Slight frequency response irregularities and non-constant group delay characteristics
will alter the peak-to-average ratio, and will thus reduce the effectiveness of any
peak limiting performed prior to their inputs.
STL and Exciter Overshoot
Earlier in this section, we discussed at length what is required to prevent STLs from
overshooting. There are similar requirements for FM exciters. Nevertheless, in some
installations some overshoot is inevitable. If this is a problem in your installation, the
8600Si’s remote control feature offers the means to reduce the peak level of the