Orban OPTIMOD-FM 8700, Operating Manual

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3-86
OPERATION ORBAN MODEL 8700i
built-in stereo encoder to drive the transmitter directly.
Section 1 of this manual has an extensive discussion of overshoot in transmission
paths. See page 1-15 and following pages.
Many paths have overshoot and this forces you to reduce the average modulation to
avoid overmodulating 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 M
ULTIPLEX
P
OWER
T
HRESHOLD
control to the peak
overshoot of the transmission system following the 8700i. For example, if RF peak
deviation exceeds the peak deviation produced by the 8700i’s sinewave oscillator
(set for 100% modulation) by 3 dB, set the
M
ULTIPLEX
P
OWER
T
HRESHOLD
to “+3.”
Audio Processing and the Multiplex Power Threshold Control
The multiplex power controller reduces multiplex power by applying gain reduction
after the Optimod’s FM peak limiting system, which reduces the tendency of the
MPX power controller to produce unnatural-sounding gain reduction because the
standard forces MPX power to be measured after preemphasis and without psycho-
acoustic weighting.
With no power control, some of the louder 8700i presets can exceed the ITU stand-
ard by as much as 16 dB. This means that the controller must reduce gain by as much
as 16 dB depending on the dynamics and spectral content of the input program ma-
terial. To prevent unnatural loudness variations, your Optimod applies a static loss
(preset-dependent and set by the
M
ULTIPLEX
P
OWER
O
FFSET
control) before the FM
peaks limiters when the multiplex power controller is activated. This complements
the dynamic gain reduction produced by the multiplex power controller. See the
notes on the
M
ULTIPLEX
P
OWER
O
FFSET
control on page 3-48.
The MPX offset is applied before the peak limiters. Turning it up (for example, from
–12 to –9 dB) increases both the amount of peak limiting and the amount of wide-
band gain reduction performed by the MPX Power Controller
The multiplex power controller does not use the output of the 8700i’s stereo encod-
er as its reference. Instead, it computes the multiplex power directly from the left
and right audio signals, the setting of the
P
ILOT
L
EVEL
control, and the setting of the
C
OMPOSITE
L
IMIT
D
RIVE
control. Hence, the multiplex power controller does not take
into account the effect of any composite limiting on the multiplex power. This is not
a problem because a BS412-compliant broadcast does not cause enough composite
limiting to affect the multiplex power measurably. The purpose for this change was
to allow the multiplex power controller to work even when diversity delay is applied
to the stereo encoder.
The multiplex power controller is operational with all of the Two-Band and Five-
Band processing structures.
It is not active in Test mode and will not prevent the
8700i’s test oscillator from producing illegal modulation. It is the responsibility of
the operator to make sure that the test oscillator does not violate the ITU require-
ments. (To ensure this, never modulate the carrier with a single L+R tone that pro-
duces total carrier modulation, including pilot tone, of more than 24%.)