6-8 Technical Data
If your country enforces the andard, you should set the control to complement the amount of peak overshoot in
the transmission syem following the 5750. Seng the control at “0” will correly control the mulplex power when
there is no overshoot aer the 5750. This will typically be true when you are using your Opmod’s built-in ereo
encoder to drive the transmier direly.
Many paths have overshoot and this forces you to reduce the average modulaon to avoid overmodulang the
transmier. This would reduce the mulplex power by the same amount, forcing the mulplex power below the ITU
requirement.
To compensate for this, mat the MULTIPLEX POWER THRESHOLD control to the peak overshoot of the transmission
syem following the 5750. For example, if RF peak deviaon exceeds the peak deviaon produced by the 5750’s
sinewave oscillator (set for 100% modulaon) by 3 dB, set the MULTIPLEX POWER THRESHOLD to “+3.”
Audio Processing and the Multiplex Power Threshold Control
The mulplex power controller reduces mulplex power by applying gain reducon aer the Opmod’s FM peak
liming syem, whi reduces the tendency of the MPX power controller to produce unnatural-sounding gain
reducon because the andard forces MPX power to be measured aer preemphasis and without psyoacousc
weighng.
With no power control, some of the louder 5750 presets can exceed the ITU andard by as mu as 16 dB. This
means that the controller mu reduce gain by as mu as 16 dB depending on the dynamics and speral content of
the input program material. To prevent unnatural loudness variaons, your Opmod applies a ac loss (preset-
dependent and set by the MULTIPLEX POWER OFFSET control) before the FM peaks limiters when the mulplex
power controller is acvated. This complements the dynamic gain reducon produced by the mulplex power
controller.
The MPX oset is applied before the peak limiters. Turning it up (for example, from –12 to –9 dB) increases both the
amount of peak liming and the amount of wideband gain reducon performed by the MPX Power Controller
The mulplex power controller does not use the output of the 5750’s ereo encoder as its reference. Inead, it
computes the mulplex power direly from the le and right audio signals, the seng of the PILOT LEVEL control,
and the seng of the COMPOSITE LIMIT DRIVE control. Hence, the mulplex power controller does not take into
account the ee of any composite liming on the mulplex power. This is not a problem because a BS412-compliant
broadca does not cause enough composite liming to ae the mulplex power measurably. The purpose for this
ange was to allow the mulplex power controller to work even when diversity delay is applied to the ereo
encoder.
The mulplex power controller is operaonal with all of the Two-Band and Five-Band processing ruures. It is not
acve in Te mode and will not prevent the 5750’s te oscillator from producing illegal modulaon. It is the
responsibility of the operator to make sure that the te oscillator does not violate the ITU requirements.
(To ensure this, never modulate the carrier with a single L+R tone that produces total carrier modulaon, including
pilot tone, of more than 24%.)
About the Multiplex Power Controller’s Time Constants
Although the BS412 specicaon calls for a 60-second integraon me, the integraon me of the Opmod’s MPX
power controller is about 10 seconds. The problem with making the integraon me longer is that the BS412
andard ates that the integrated MPX power in any arbitrary 60-second me period cannot exceed the average
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