Orban OPTIMOD-FM 8700, Operating Manual

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OPTIMOD-FM DIGITAL INTRODUCTION
1-15
the studio should be carefully aligned and qualified to meet the appropriate stand-
ards for bandwidth, distortion, group delay and gain stability and such equipment
should be re-qualified at reasonable intervals. (See
Optimal Control of Peak Modula-
tion Levels on page 1-13).
If the transmitter is accessible:
You can achieve the most accurate control of modulation peaks by locating
OPTIMOD-FM at the transmitter site and then using its stereo encoder to drive the
transmitter. You can usually also obtain good results by locating OPTIMOD-FM at
the studio and connecting the baseband output of its stereo encoder to the trans-
mitter through a composite baseband STL (see page 1-18). However, many analog
composite baseband STLs do not control peaks perfectly because of bounce (see
page 1-19) and locating OPTIMOD-FM 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 8700i’s composite limiter.
Because OPTIMOD-FM controls peaks, it is irrelevant whether the audio link feeding
OPTIMOD-FM’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 8700i’s internal stereo encoder to feed the
output of the encoder directly. You will achieve a louder sound on the air, with bet-
ter control of peak modulation, than if you use most external stereo encoders. An
exception is Orban’s 8218 stereo encoder, which does not add overshoot, and, in
fact, contains its own overshoot limiter. However, because it accepts audio in
left/right form, the 8218 will not let you exploit the 8700i’s composite limiter.
The shorter the baseband cable from OPTIMOD-FM 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 transformer
1
to break any ground
loops. This transformer will usually cure even the most stubborn hum or noise
caused by the composite connection between OPTIMOD-FM and the exciter. For
connection instructions, please refer to the Orban CIT25 Composite Isolation Trans-
former schematic diagram, which is shown on page 12 of the CIT25 manual. This is
available from:
ftp.orban.com/CIT25/CIT25 Instructions.pdf (Please note that Orban
no longer manufactures the CIT25, which used the Jensen transformer.)
If a separate stereo encoder must be used, feed the encoder directly from the 8700i’s
left and right analog outputs. If possible, bypass the preemphasis network and the
input low-pass filters in the encoder so that they cannot introduce spurious peaks.
Because of their special design, OPTIMOD-FM’s preemphasis network and low-pass
filters perform the same functions while retaining tight peak control.
Connect the composite output of the 8700i to the baseband input of the exciter
through less than 100 feet (30 meters) of coaxial cable. 100 feet of coaxial cable (as-
suming 30-pF / foot capacitance) will reduce measured separation at 15 kHz (worst
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