Line Up Facilities - Orban OPTIMOD 5750 Operating Manual

6-3
Orban 5750 Technical Manual Technical Data
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 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 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.

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Metering of Levels and Subjective Loudness
The meters on the 5750 show left/right input and output 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 meter). The input meter is scaled so that 0 dB on the scale corresponds +27 dBu, which
is the absolute maximum peak level that the 5750 can accept. If you are using the AES3 digital input, a full-scale
digital word corresponds to the 0 dB point on the 5750's input meter.
Figure 6-3: Metering of Levels and Subjective Loudness