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INTRODUCTION
ORBAN MODEL 8600Si
Transmission Levels
The transmission engineer is primarily concerned with the peak level of a program
to prevent overloading or over-modulation of the transmission system. This peak
overload level is defined differently, system to system.
In FM modulation (FM/VHF radio and television broadcast, microwave or analog sat-
ellite links), it is the maximum-permitted RF carrier frequency deviation. In AM mod-
ulation, it is negative carrier pinch-off. In analog telephone/post/PTT transmission, it
is the level above which serious crosstalk into other channels occurs, or the level at
which the amplifiers in the channel overload. In digital, it is the largest possible digi-
tal word.
For metering, the transmission engineer uses an oscilloscope, absolute peak-sensing
meter, calibrated peak-sensing LED indicator, or a modulation meter. A modulation
meter usually has two components — a semi-peak reading meter (like a PPM), and a
peak-indicating light, which is calibrated to turn on whenever the instantaneous
peak modulation exceeds the overmodulation threshold.
Line-Up Facilities
Metering of Levels and Subjective Loudness
The meters on the 8600Si show left/right input levels and composite modulation.
Left and right input level is shown on a VU-type scale
(
0 to –27 dB), while the meter-
ing indicates
absolute instantaneous peak
(much faster than a standard PPM or VU
meter). The input meter is scaled so that 0 dB corresponds to the absolute maximum
peak level that the 8600Si can accept (+27 dBu). If you are using the AES3 digital in-
put, the maximum digital word at the input corresponds to the 0 dB point on the
8600Si’s input meter.
If you have an 8600Si, you can switch the front-panel metering to indicate level and
gain reduction parameters relevant to the digital radio processing chain or to the
analog FM processing chain. Simultaneous metering of analog and digital radio pro-
cessing chains is available on 8600Si PC Remote Software.
Composite Output Level
The 8600Si controls instantaneous, absolute peak levels to a tolerance of approxi-
mately
±
0.1 dB. Composite modulation is indicated in percentage modulation, abso-
lute instantaneous peak indicating. 100% is calibrated to the highest composite
peak modulation level that the processing will produce, including the pilot tone,
under any program, processing, or setup condition (except when the processing is
switched to
B
YPASS
or when the final clipper is defeated in Setup). 100% ordinarily
corresponds to
±
75 kHz-carrier deviation.
Note that when the 8600Si’s subcarrier inputs are used, the meter will not indicate
the subcarriers’ effect on composite modulation because the subcarriers are mixed
Содержание OPTIMOD-FM 8600Si
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