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OPTIMOD-FM DIGITAL
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 8500S. For example, if RF peak deviation exceeds
the peak deviation produced by the 8500S's sinewave oscillator (set for 100% modu-
lation) by 3 dB, set the MULTIPLEX POWER THRESHOLD to "+3."
Audio Processing and the Multiplex Power Threshold Control
Starting with version1.1 software, 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 psychoacoustically weighting.
With no power control, some of the louder 8500S presets can exceed the ITU stan-
dard by as much as 12 dB. This means that the controller must reduce gain by as
much as 12 dB depending on the dynamics and spectral content of the input pro-
gram material. To prevent unnatural loudness variations, your Optimod applies a
static loss (preset-dependent and set by the M
the FM peaks limiters when the multiplex power controller is activated. This com-
plements the dynamic gain reduction produced by the multiplex power controller.
See the notes on the M
In version 1.1 and higher software, the MPX offset is still 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 8500S's stereo en-
coder as its reference. Instead, it computes the multiplex power directly from the
left and right audio signals, the setting of the P
the C
L
OMPOSITE
IMIT
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 com-
posite limiting to affect the multiplex power measurably. The purpose for this
change was to allow the multiplex power controller to work even 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
8500S'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%.)
About the Multiplex Power Controller's Time Constants
Although the BS412 specification calls for a 60-second integration time, the integra-
tion 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 av-
P
O
ULTIPLEX
OWER
FFSET
control. Hence, the multiplex power controller does not
D
RIVE
P
O
ULTIPLEX
OWER
FFSET
control on page 3-43.
control, and the setting of
L
ILOT
EVEL
OPERATION
control) before
3-75
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Table of Contents
Troubleshooting
