Orban OPTIMOD 5750 Operating Manual page 125

6-8
Technical Data
If your country enforces the standard, you should set the control to complement the amount of peak overshoot in
the transmission system following the 5750. Setting the control at "0" will correctly control the multiplex power when
there is no overshoot after the 5750. This will typically be true when you are using your Optimod's built-in stereo
encoder to drive the transmitter directly.
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 P T
ULTIPLEX OWER HRESHOLD control to the peak overshoot of the transmission
system following the 5750. For example, if RF peak deviation exceeds the peak deviation produced by the 5750's
sinewave oscillator (set for 100% modulation) by 3 dB, set the M P T
to "+3."
ULTIPLEX OWER HRESHOLD
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 psychoacoustic
weighting.
With no power control, some of the louder 5750 presets can exceed the ITU standard 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 material. To prevent unnatural loudness variations, your Optimod applies a static loss (preset-
dependent and set by the M P O
control) before the FM peaks limiters when the multiplex
ULTIPLEX OWER FFSET
power controller is activated. This complements the dynamic gain reduction produced by the multiplex power
controller.
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 wideband gain reduction performed by the MPX Power Controller
The multiplex power controller does not use the output of the 5750's stereo encoder as its reference. Instead, it
computes the multiplex power directly from the left and right audio signals, the setting of the P L
ILOT EVEL control,
and the setting of the C L D
OMPOSITE IMIT 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 5750'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 requirements.
(To ensure this, never modulate the carrier with a single L+R tone that produces total carrier modulation, including
pilot tone, of more than 24%.)
About the Multiplex Power Controller's Time Constants
Although the BS412 specification calls for a 60-second integration time, the integration time of the Optimod's MPX
power controller is about 10 seconds. The problem with making the integration time longer is that the BS412
standard states that the integrated MPX power in any arbitrary 60-second time period cannot exceed the average