Orban OPTIMOD-FM 8600, Operating Manual

Page: 263 / 397

OPTIMOD-FM DIGITAL OPERATION
3-83
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 8600. For example, if RF peak
deviation exceeds the peak deviation produced by the 8600’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
Starting with version1.2 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 psychoacoustic weighting.
With no power control, some of the louder 8600 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 3-44.
In version 1.2 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 8600’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
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 if 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
8600’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%.)
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