Transmission Levels; Fig. 1-1: Absolute Peak Level, Vu And Ppm Reading - Orban OPTIMOD-FM 8500S Operating Manual

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INTRODUCTION ORBAN MODEL 8500S

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

The studio engineer is primarily concerned with calibrating the equipment to pro-
vide the required input level for proper operation of each device, and so that all de-
vices operate with the same input and output levels. This facilitates patching devices
in and out without recalibration.
For line-up, the studio engineer uses a calibration tone at a studio standard level,
commonly called line-up level, reference level, or operating level. Metering at the
studio is by a VU meter or PPM (Peak Program Meter). As discussed above, the VU or
PPM indication under-indicates the true peak level. Most modern studio audio de-
vices have a clipping level of no less than +21 dBu, and often +24 dBu or more. So
the studio standardizes on a maximum program indication on the meter that is
lower than the clipping level, so those peaks that the meter does not indicate will
not be clipped. Line-up level is usually at this same maximum meter indication. In fa-
cilities that use VU meters, this level is usually at 0VU, which corresponds to the stu-
dio standard level, typically +4 or +8 dBu.
For facilities using +4 dBu standard level, instantaneous peaks can reach +18 dBu or
higher (particularly if the operator overdrives the console or desk). Older facilities
with +8 dBu standard level and equipment that clips at +18 or +21 dBu will experi-
ence noticeable clipping on some program material.
In facilities that use the BBC-standard PPM, maximum program level is usually PPM4
for music, PPM6 for speech. Line-up level is usually PPM4, which corresponds to +4
dBu. Instantaneous peaks will reach +17 dBu or more on voice.
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.