Orban 8300J, Operating Manual

Page: 43 / 280

OPTIMOD-FM DIGITAL INTRODUCTION
1-19
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
satellite links), it is the maximum-permitted RF carrier frequency deviation. In AM
modulation, it is negative carrier pinch-off. In analog telephone / post / PTT trans-
mission, 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 digital 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
The meters on the 8300 show left/right input levels and composite modulation. Left
and right input level is shown on a VU-type scale

0 to –40 dB), while the metering
indicates absolute instantaneous peak (much faster than a standard PPM or VU me-
ter). The input meter is scaled so that 0 dB corresponds to the absolute maximum
peak level that the 8300 can accept (+26 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
8300’s input meter.
Composite Output Level
The Orban 8300 Audio Processor controls instantaneous, absolute peak levels to a
tolerance of approximately

0.1 dB. Composite modulation is indicated in percent-
age modulation, absolute 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
BYPASS ). 100% ordinarily corresponds to

75 kHz-carrier
deviation.
Note that if the 8300’s subcarrier inputs are used, the meter will not indicate the
subcarriers’ effect on composite modulation because the subcarriers are mixed into
the composite signal in the analog domain, after it is metered. Therefore, you must
mentally add the subcarriers to the meter indication, or refer to an external, cali-
brated modulation monitor.