OPTIMOD-FM 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 satel-
lite links), it is the maximum-permitted RF carrier frequency deviation. In AM modula-
tion, 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 digital word.
For metering, the transmission engineer uses an oscilloscope, absolute peak-sensing me-
ter, 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 modula-
tion exceeds the overmodulation threshold.
Line-Up Facilities
Metering of Levels
The meters on the 8400 show left/right input and output levels and composite modula-
tion. Left and right input level is shown on a VU-type scale (0 to –40 dB), while the me-
tering 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 8400 can accept, at the current setting of its
Clip Level control. If you are
using the AES/EBU digital input, the maximum digital word at the input corresponds to
the 0 dB point on the 8400’s input meter.
Left/Right Output Level
Left and right output level is shown on a VU-type scale (–10 to +3 dB), where the meter-
ing indicates absolute instantaneous peak (much faster than a standard PPM or VU me-
ter). The meter is scaled so that 0 dB is calibrated to the highest left and right peak modu-
lation level, before de-emphasis, that the processing will produce, under any program,
processing, or setup condition (except when the processing is switched to
Bypass). The
meter indication is not affected by the setting of the output level control.
Composite Output Level
The Orban 8400 Audio Processor controls instantaneous, absolute peak levels to a toler-
ance of approximately ±0.1 dB. Composite modulation is indicated in percentage modu-
lation, absolute instantaneous peak indicating. 100% is calibrated to the highest compos-
ite 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.
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