Transmission Levels; Line-Up Facilities; Metering Of Levels; Fig. 1-1: Absolute Peak Level, Vu And Ppm Reading - Orban OPTIMOD-FM 5500 Operation Manual

OPTIMOD-FM DIGITAL
In facilities that use PPMs that indicate level directly in dBu, maximum program and
line-up level is often +6 dBu. Instantaneous peaks will reach +11 dBu or more.

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
modulation, it is negative carrier pinch-off. In analog telephone/post/PTT transmis-
sion, 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 pos-
sible 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 5500 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 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 5500 can accept (+27 dBu). If you are using the AES3 digital in-

Fig. 1-1: Absolute Peak Level, VU and PPM Reading

INTRODUCTION
ABSOLUTE PEAK
PPM
VU
1-21