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OPERATION ORBAN Model 8400
Section 1 of this manual has an extensive discussion of overshoot in trans-
mission paths. See page 1-13 and following pages.
However, many paths have overshoot, and this forces you to reduce the average modula-
tion to avoid overdeviating the transmitter. This would reduce the multiplex power by the
same amount, forcing the multiplex power below the ITU requirement.
To compensate for this, match the
Multiplex Power Threshold to the peak overshoot of the
transmission system following the 8400. For example, if RF peak deviation exceeds the
peak deviation produced by the 8400’s sinewave oscillator (set for 100% modulation) by
3 dB, set the
Multiplex Power Threshold to “+3.”
8400 Processing and the Multiplex Power Threshold Control: The multiplex power
controller reduces multiplex power by increasing the gain reduction in the multiband
compressor, thereby reducing the drive into the clippers. Typically, the attack time of the
controller is about 2 seconds, and its release time is about 30 seconds.
With no power control, some of the louder 8400 presets can exceed the ITU standard by
approximately 6 dB. This means that the multiplex power controller must increase the
gain reduction in the multiband compressor by up to 6 dB, and this will vary according to
the dynamics and spectral content of the input program material.
Unfortunately, the multiplex power specification does not specify psycho-
acoustic weighting to reflect the sensitivity of the ear as a function of fre-
quency. It therefore does little to predict the psychoacoustic loudness of co-
channel interference (its stated goal), and heavy bass in the program material
can force perceptually unnecessary reduction of the average modulation. To
minimize loudness loss with the power controller active, we therefore rec-
ommend avoiding excessive bass boost in the 8400 processing. You may
also wish to set the threshold of the Band 1 compressor lower, minimizing
any dynamic bass boost caused by the processing.
Further, the power controller sees a pre-emphasized signal. Therefore, boost-
ing frequencies above 6 or 7 kHz can also produce unnecessary loudness re-
duction because these frequencies are significantly rolled off by the re-
ceiver’s de-emphasis. This reduces their ability to contribute to the loudness
heard by the listener.
Because of the lack of psychoacoustic weighting in the ITU specification, excessive ac-
tion of the power controller is likely to be audible as loudness changes that are inexplica-
ble and annoying to the listener, who is expecting a smooth, consistent presentation. We
therefore recommend turning down the
Less/More setting on a given preset until the mul-
tiplex power controller (as indicated by the “P” [power] meter on the Meters screen) is
doing very little work. If the meter indicates more than one-quarter scale, you are likely
to notice long-term loudness inconsistency. Similarly, if you create a custom preset for
use in connection with the multiplex power controller, we recommend backing off the
MB Clipping and MB Drive controls until the “P” meter reads one-quarter scale or less.
You must allow some time to make this observation because of the power
controller’s slow two-second-attack time and even slower 30-second release
time. Allow at least a minute.
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