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INTRODUCTION ORBAN Model 8400
ter 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 dis-
tortion.
We strongly recommend that you use the 8400’s internal stereo encoder to feed the out-
put 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 separa-
tion at 15 kHz (worst case) to approximately 60 dB. This separation is comfortably above
the separation (approximately 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. However,
because it accepts audio in left/right form, the 8218 will not let you exploit the 8400’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 application.
If a separate stereo encoder must be used, feed the encoder directly from the 8400’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, OPTIMOD-FM’s pre-emphasis network and low-pass filters perform the
same functions while retaining tight peak control.
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 material through
earphones and adjusted the channel separation, via a manual control, until the degradation of the
spatial effect became detectable. Mean channel separations ranged from 10 to 15.9 dB for the mu-
sical selections employed and from 13.7 to 16.8 dB for the noise and tonal stimuli. The results are
discussed in terms of existing data on detectable stereo separation and on the discrimination of
interaural time differences. [Abstract ©Audio Engineering Society, Inc.]
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