Orban Optimod-FM 8300 Operating Manual page 36

Digital audio processor
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1-12
INTRODUCTION
ORBAN MODEL 8300
studio and connecting the baseband output of its stereo encoder to the transmitter
through a composite baseband STL (see page 1-14). However, many composite base-
band STLs do not control peaks perfectly because of bounce (see page 1-15), 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 8300'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 ap-
proximately 60 dB. This separation is comfortably above the separation (approxi-
mately 20 dB) that starts to cause perceptible changes in the stereo image.
1
You will achieve a louder sound on the air, with better 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 8300'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 Orban's CIT25 Composite Isolation Transformer 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.
Its instruction manual contains complete information on its installation and applica-
tion.
If a separate stereo encoder must be used, feed the encoder directly from the 8300'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.
1
Julie M. Adkins and Robert D. Sorkin: "Effect of Channel Separation on Earphone-
Presented Tones, Noise, and Stereophonic Material," J. Audio Engineering Society,
vol. 33 pp. 234-239, 1985.
Subjects listened to 500-Hz tones, broadband noise, and stereophonic program ma-
terial through earphones and adjusted the channel separation, via a manual control,
until the degradation of the spatial effect became detectable. Mean channel separa-
tions ranged from 10 to 15.9 dB for the musical selections employed and from 13.7
to 16.8 dB for the noise and tonal stimuli. The results are discussed in terms of exist-
ing data on detectable stereo separation and on the discrimination of interaural
time differences. [Abstract ©Audio Engineering Society, Inc.]

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