Orban OPTIMOD 6200, Operating Manual

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OPTIMOD INTRODUCTION
1-9
As discussed above, asynchronous sampling problems will cause overshoots if any ana-
log path (even a perfectly transparent one) passes the 6200’s processed output to the
transmitter. Lack of transparency in the analog path will cause even more overshoot.
Unless carefully designed, microwave STLs can introduce non-constant group delay in
the audio spectrum, distorting peak levels when used to pass processed audio. Neverthe-
less, in a system using a microwave STL the 6200 is sometimes located at the studio and
any overshoots induced by the link are tolerated or removed by the transmitter’s protec-
tion limiter (if any). The 6200 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 8200ST Compressor/Limiter/HF Lim-
iter/Clipper or an Orban Transmission Limiter protects the link from overload.
If the 6200 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 6200.
Some microwave links may be modified such that the deviation from linear phase is less
than +10
°
20-20kHz, and frequency response is less than 3dB down at 0.15Hz and less
than 0.1dB down at 20kHz 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).
Further, it is not unusual 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 2dB, reducing average modulation. Many commercial STLs have this prob-
lem.
Some consultants presently offer modifications to minimize or eliminate this problem. 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 locally available,
and upon the availability of other alternatives. Even the best landlines tend to slightly
veil audio quality, due to line equalizer characteristics and phase shifts. They will cer-
tainly be the weakest link in a DAB 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 limit-
ing performed prior to their inputs.