Orban OPTIMOD-FM 8600Si, Руководство по эксплуатации

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OPTIMOD-FM DIGITAL OPERATION
3-83
modulation) 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 8600Si 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
ULTIPLEX
P
OWER
O
FFSET control) before
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
ULTIPLEX
P
OWER
O
FFSET control on page 3-44.
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 8600Si’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
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 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
8600Si’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-
erage power of the sinewave that produced
±
19 kHz carrier deviation. In other
words, whenever you start measuring , you must not exceed the total integrated
power limit over the following 60 seconds.
This makes it impractical to "bank" power over the full 60-second window. For ex-
ample, at first glance one might think that a classical music station could exploit a