OPTIMOD-FM DIGITAL OPERATION
3-81
MPX Power Meter
Version 1.2 of 8600 software added a MPX
POWER meter that 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 8600’s MPX POWER
meter is therefore calibrated so that it indicates 0 dB when the composite output of
the 8600 is a sinewave at 25.333% modulation, which is –11.92615 dB with reference
to a sinewave at 100% modulation.
The meter is calibrated with reference to the 8600’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 8600. Such overshoots necessi-
tate turning down the 8600’s output level control after it has been calibrated with
tone using an FM modulation meter and the 8600’s built-in line-up tone oscillator. If
the output level is turned down after a tone calibration, the MPX POWER LEVEL me-
ter will read high compared to the actual on-air MPX power. The error will be equal
Figure 3-4: Multiplex power over 15 minute observation interval
with Multiplex power controller active, measured at the Optimod’s composite output
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