Orban OPTIMOD 5750 Operating Manual page 20

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Orban 5750 Technical Manual Installation
If an uncompressed left/right digital link is available to the transmitter, this is also an excellent means of
transmission, although it will not pass the effects of the 5750's composite processor (if you are using it). However, if
the digital link employs lossy compression, it will degrade peak control. To prevent overshoots caused by spectral
truncation in the link, set the 5750's output sample rate to 44.1 kHz or higher.
If only an audio link is available, use the 5750's left and right audio outputs and feed the audio, without pre-emphasis,
directly into the link. If possible, request that any transmitter protection limiters be adjusted for minimum possible
action— THE OPTIMOD-5750 does most of that work. Transmitter protection limiters should respond only to signals
caused by faults or by spurious peaks introduced by imperfections in the link. To ensure maximum quality, all
equipment in the signal path after the studio should be carefully aligned and qualified to meet the appropriate
standards for bandwidth, distortion, group delay and gain stability and such equipment should be re-qualified at
reasonable intervals.
If the transmitter is accessible: You can achieve the most accurate control of modulation peaks by locating THE
OPTIMOD-5750 at the transmitter site and then using its stereo encoder to drive the transmitter. You can usually
also obtain good results by locating THE OPTIMOD-5750 at the studio and connecting the baseband output of its
stereo encoder to the transmitter through a composite baseband STL.
However, many analog composite baseband STLs do not control peaks perfectly and locating THE OPTIMOD-5750 at
the transmitter site (where it can control peaks just prior to the transmitter's RF exciter) is thus likely to maximize
loudness. The ideal link is an uncompressed digital composite STL because these have virtually flawless waveform
fidelity and allow full use of the 5750's composite limiter.
Because THE OPTIMOD-5750 controls peaks, it is irrelevant whether the audio link feeding THE OPTIMOD-5750's
input terminals is phase-linear. However, the link should have low noise, the flattest possible frequency response
from 30-15,000Hz, and low nonlinear distortion.
We strongly recommend that you use the 5750's internal stereo encoder to feed the output of the encoder directly.
You will achieve a louder sound on the air, with better control of peak modulation, than if you use current external
stereo encoders.
The shorter the baseband cable from THE OPTIMOD-5750 to exciter, the less likely that ground loops or other noise
problems will occur in the installation. If you require a long cable run, you can use a Jensen JT-123-BMCF
transformer1 to break any ground loops. This transformer will usually cure even the most stubborn hum or noise
caused by the composite connection between THE OPTIMOD-5750 and the exciter.
If a separate stereo encoder must be used, feed the encoder directly from the 5750's left and right analog outputs.
If possible, bypass the pre-emphasis network and the input low-pass filters in the encoder so that they cannot
introduce spurious peaks.
Because of their special design, THE OPTIMOD-5750's pre-emphasis network and low-pass filters perform the same
functions while retaining tight peak control. Connect the composite output of the 5750 to the baseband input of the
exciter through less than 100 feet (30 meters) of coaxial cable.
100 feet of coaxial cable (assuming 30-pF / foot capacitance) will reduce measured separation at 15 kHz (worst case)
to approximately 60dB. This separation is comfortably above the separation (approximately 20dB) that starts to
cause perceptible changes in the stereo image.