Orban OPTIMOD-FM 8600Si, Operating Manual

Page: 247 / 356

OPTIMOD-FM DIGITAL OPERATION
3-81
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 8600Si’s MPX
POWER meter is therefore calibrated so that it indicates 0 dB when the composite
output of the 8600Si 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 8600Si’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 8600Si. Such overshoots neces-
sitate turning down the 8600Si’s output level control after it has been calibrated
with tone using an FM modulation meter and the 8600Si’s built-in line-up tone oscil-
lator. 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 8600Si’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 8600Si does not digitize subcarriers applied to its subcarrier inputs, the
8600Si’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
2 2
− =
= + = +
Recalling that the MPX POWER LEVEL meter is calibrated so that it indicates 0 dB
when the composite output of the 8600Si is a sinewave at –11.92615 dB below 100%
modulation, we conclude that our subcarrier at 4% injection will add 0.10694 dB to
the multiplex power. Another calculation (not shown) indicates that 10% injection
will add 0.62889 dB to the MPX power.
Multiplex Power Threshold
Your Optimod provides a means to limit the integrated multiplex power to the ITU
standard by a closed-loop technique that allows you to use any preset and to create
customized presets freely. The multiplex power controller is adjusted via the
ITU412-
9
control (see step 1 on page 2-23
)
. Set it
OFF
if your country does not enforce the
standard.