OPTIMOD-FM INTRODUCTION
1-15
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, distort-
ing peak levels when used to pass processed audio. Nevertheless, in a system using a mi-
crowave STL, the 8400 is sometimes located at the studio and any overshoots induced by
the link are tolerated or removed by the transmitter’s protection limiter (if any). The 8400
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 op-
timally if an Orban 8200ST Compressor/Limiter/HF Limiter/Clipper or an Orban Trans-
mission Limiter protects the link from overload.
If the 8400 is located at the transmitter and fed unprocessed audio from a microwave
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 8400.
Some microwave links can be modified such that the deviation from linear phase is less
than +
10° 20-15 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 easily configured at the fac-
tory for composite operation, where an entire FM stereo baseband is passed. The re-
quirements 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 opera-
tion (even if a composite FM stereo signal is not actually being applied to the link).
Nevertheless, in a dual-microwave system, the 8400 is usually located at the main FM
transmitter and is driven by the microwave receivers. The microwave transmitters at the
studio are protected from overload by one of Orban’s Studio Level Control Systems, such
as the 8200ST. These units also perform the gain riding function ordinarily executed by
the AGC section of the 8400’s processing, and optimize the signal-to-noise ratio obtain-
able from the dual-microwave link.
If the STL microwave uses pre-emphasis, its input pre-emphasis filter will probably in-
troduce 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 easily configured 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 deviation by as much
as 2 dB, reducing average modulation. Many commercial STLs have this problem.
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