Orban Optimod-FM 8300 Operating Manual page 39

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OPTIMOD-FM DIGITAL INTRODUCTION
nity than composite microwave STLs. However, problems include gain- and phase-
matching of the left and right channels, overloads induced by pre-emphasis, and re-
quirements that the audio applied to the microwave transmitters be processed to
prevent over-modulation of the microwave system.
Lack of transparency in the path will cause overshoot. Unless carefully designed,
dual microwave STLs can introduce non-constant group delay in the audio spectrum,
distorting peak levels when used to pass processed audio. Nevertheless, in a system
using a microwave STL, the 8300 is sometimes located at the studio and any over-
shoots induced by the link are tolerated or removed by the transmitter's protection
limiter (if any). The 8300 can only be located at the transmitter if the signal-to-noise
ratio of the STL is good enough to pass unprocessed audio. The signal-to-noise ratio
of the STL can be used optimally if an Orban Optimod-PC 1101, Optimod 6300,
8200ST Compressor / Limiter / HF Limiter / Clipper or an 4000 Transmission Limiter
protects the link from overload. Of these, the 1101 and 6300 are currently manufac-
tured as of this writing and are the preferred choices because their AGCs are identi-
cal to the AGC in the 8300.
If the 8300 is located at the transmitter and fed unprocessed audio from a micro-
wave STL, it may be useful to use a companding-type noise reduction system (like
dbx Type 2 or Dolby SR) around the link. This will minimize any audible noise
buildup caused by compression within the 8300.
Some microwave links can be modified such that the deviation from linear phase is
less than +10 from 20 Hz to 15 kHz and frequency response is less than 3 dB down
at 0.15Hz and less than 0.1 dB down at 20 kHz. This specification results in less than
1% overshoot with processed audio. Many such links have been designed to be eas-
ily configured at the factory for composite operation, where an entire FM stereo
baseband is passed. The requirements for maintaining stereo separation in compos-
ite 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).
Nevertheless, in a dual-microwave system, the 8300 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 8200ST, protects the microwave transmitters at the stu-
dio from overload. These units also perform the gain riding function ordinarily exe-
cuted by the AGC section of the 8300'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