If The Transmitter Is Accessible - Orban OPTIMOD-FM 8500S Operating Manual

1-15
OPTIMOD-FM DIGITAL INTRODUCTION
the STL. Then feed the output of the STL receiver directly into the transmitter's ex-
citer with no intervening processing.
If an uncompressed AES3 digital link is available to the transmitter, this is also an ex-
cellent means of transmission, although it will not pass the effects of the 8500S's
composite processor (if you are using it). However, if the digital link employs lossy
compression, it will disturb peak levels.
If only an audio link is available, use the 8500S'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 —
OPTIMOD-FM does most of that work. Transmitter protection limiters should re-
spond only to signals caused by faults or by spurious peaks introduced by imperfec-
tions 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 stan-
dards 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 transmitter
through a composite baseband STL (see page 1-18). However, many composite base-
band STLs do not control peaks perfectly because of bounce (see page 1-20), and lo-
cating 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.
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 8500S's internal stereo encoder to feed
the output of the encoder directly to the baseband input of the exciter through less
than 100 feet (30 meters) of coaxial cable. 100 feet of coaxial cable (assuming
30pF/foot capacitance) will reduce measured separation at 15 kHz (worst case) to
approximately 55 dB. (See Figure 2-3 on page 2-9.) This separation is comfortably