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OPTIMOD 6300 DIGITAL
digital input and output accommodate sample rates of 32 kHz, 44.1 kHz, 48, 88.2,
and 96 kHz.
Microwave STLs
In general, an analog microwave STL provides high audio quality as long as there is a
line-of-sight transmission path from studio to transmitter of less than 10 miles (16
km). If not, RF signal-to-noise ratio, multipath distortion, and diffraction effects can
cause serious quality problems. However, the noise and non-linear distortion charac-
teristics of such links are likely to be notably poorer than 16-bit digital even if
propagation conditions are ideal.
Lack of transparency in the analog path can cause overshoot. Unless carefully de-
signed, microwave STLs can introduce non-constant group delay in the audio spec-
trum, distorting peak levels when used to pass processed audio. Nevertheless, in a
system using a microwave STL OPTIMOD 6300 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). OPTIMOD 6300 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,
8200ST Compressor / Limiter / HF Limiter / Clipper, a 4000 Transmission Limiter, or
another 6300 protects the link from overload. Of these, the 1101 and 6300 are cur-
rently manufactured as of this writing and are the preferred choices because their
AGCs are identical to the AGC in the 6300.
If OPTIMOD 6300 is located at the transmitter and receives 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 OPTIMOD 6300.
Some microwave links may be modified such that the deviation from linear phase is
less than +10 20-20 kHz, and frequency response is less than 3dB down at 0.15Hz
and less than 0.1dB down at 20 kHz. This specification results in less than 1% over-
shoot with processed audio. Many such links have been designed to be easily con-
figured at the factory for composite operation, where an entire FM stereo baseband
is passed. The requirements for maintaining stereo separation in composite opera-
tion 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 ac-
tually being applied to the link).
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.
Analog landline (PTT/post office line)
Analog landline quality is extremely variable, ranging from excellent to poor. (For-
tunately, they are largely obsolete, having been replaced by digital links.) Whether
landlines should be used or not depends upon the quality of the lines locally avail-
able, and upon the availability of other alternatives. Due to line equalizer character-
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INTRODUCTION
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Table of Contents
Troubleshooting
