Orban OPTIMOD-FM 8700, Руководство по эксплуатации

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3-84
OPERATION ORBAN MODEL 8700i
sinewave that modulates the FM carrier to

19 kHz (25.3% modulation). Many Eu-
ropean countries are now enforcing this recommendation. (See ITU-R 412 Compli-
ance on page 3-15 for more information.)
The BS.1770 Safety Limiter for the analog radio processing chain is located immedi-
ately before the MPX power controller. Normally, both are used simultaneously, but
when the target loudness is –23 LUFS, the BS.1770 Safety Limiter typically produces
enough gain reduction to cause the MPX Power Controller to produce no gain re-
duction. See step 13 on page 2-25 for instructions on setting up the BS.1770 Safety
Limiter.
MPX Power Meter
The MPX P
OWER
meter indicates MPX power according to the ITU-R BS.412 standard.
All samples are weighted equally in a 60-second sliding window.
BS.412 requires limiting the integrated power of the composite signal so that it does
not exceed the power in a sinewave that deviates the FM carrier by ±19 kHz
(25.333% modulation with reference to ±75 kHz deviation). The 8700i’s MPX POWER
meter is therefore calibrated so that it indicates 0 dB when the composite output of
the 8700i is a sinewave at 25.333% modulation, which is –11.92615 dB with refer-
ence to a sinewave at 100% modulation.
The meter is calibrated with reference to the 8700i’s 100% peak modulation level.
This calibration is only correct if the transmitter and/or studio-transmitter link do not
add overshoots to program material processed by the 8700i. Such overshoots neces-
sitate turning down the 8700i’s output level control after it has been calibrated with
tone using an FM modulation meter and the 8700i’s built-in line-up tone oscillator.
If the output level is turned down after a tone calibration, the MPX POWER LEVEL
meter will read high compared to the actual on-air MPX power. The error will be
equal to the amount that the 8700i’s output level control was turned down.
See Optimal Control of Peak Modulation Levels starting on page 1-13 for a discus-
sion of overshoots and how they force the average modulation to be reduced to
prevent peak overmodulation of the FM carrier.
Because the 8700i does not digitize subcarriers applied to its subcarrier inputs, the
8700i’s
MPX P
OWER
meter (which operates in the DSP domain) cannot indicate the
power added by such subcarriers. These usually have constant power, so it is easy to
compensate for them. For example, if an FM subcarrier is injected at 4% modulation,
it adds power that can be calculated with an R.M.S. summation of the subcarrier and
the rest of the composite signal.
Assuming that the subcarrier and composite signal are uncorrelated and that the
composite signal is limited so that its power is equivalent to a sinewave at 25.3%
modulation, we calculate their R.M.S. sum as follows:
dB 81921 . 11 ) 25647 . 0 ( log 20
25647 . 0 0016 . 0 06418 . 0 04 . 0 0.25333
10
2 2
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