Audio Processing And The Multiplex Power Threshold Control; Multiplex Power Control In Stand-Alone Stereo Encoder Mode - Orban OPTIMOD-FM 5500 Operation Manual

3-54
OPERATION

Audio Processing and the Multiplex Power Threshold Control

Multiplex Power Control in Stand-Alone Stereo Encoder Mode

standard.
The control is located in the Setup > S
applies to all operation of the processor in a given installation.
If your country enforces the standard, you should set the control to complement the
amount of peak overshoot in the transmission system following your Optimod. Set-
ting the control at "0" will correctly control the multiplex power when there is no
overshoot after the 5500. This will typically be true when you are using your Opti-
mod's 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 ITU412-9 control to the peak overshoot of the
transmission system following the 5500. For example, if RF peak deviation exceeds
the peak deviation produced by the 5500's sinewave oscillator (set for 100% modu-
lation) by 3 dB, set the MULTIPLEX POWER THRESHOLD to "+3."
The multiplex power controller reduces multiplex power by applying gain reduction
before the Optimod's FM peak limiting, thereby reducing the drive into the clippers.
With no power control, some of the louder 5500 presets can exceed the ITU stan-
dard by as much as 9 dB. This means that the clipper drive must be reduced by as
much as 9 dB, and this will vary according to the dynamics and spectral content of
the input program material. To prevent unnatural loudness variations, your Optimod
applies a static loss (set by the M
power controller is activated. This complements the dynamic gain reduction pro-
duced by the multiplex power controller. See the notes on the M
control on page 3-35.
The multiplex power controller is active with all of the Two-Band and Five-Band
processing structures. It does not operate in Test mode and will not prevent the
5500'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 to-
tal carrier modulation, including pilot tone, of more than 24%.)
When you use the 5500 in stand-alone stereo encoder mode, we recommend setting
up your transmission system to produce approximately 3 dB of average MPX power
control. This avoids unnatural loudness variations.
To achieve this goal when an Optimod-FM drives the 5500 stereo encoder, reduce
the drive into the Optimod's peak limiters by turning down the M
E screen because the regulation
TEREO NCODER
control) when the multiplex
P O
PX OWER FFSET
ORBAN MODEL 5500
P O
PX OWER FFSET
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ULTIBAND LIPPING