Orban OPTIMOD-FM 8600, Operating Manual

Page: 43 / 397

OPTIMOD-FM DIGITAL INTRODUCTION
1-15
site 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 8600’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 non-linear
distortion.
We strongly recommend that you use the 8600’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 8600’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 ordinarily 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 8600’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 8600 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
case) to approximately 60dB (see Figure 2-3 on page 2-10). This separation is com-
1
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-123-bmcf.pdf