About Transmission Levels And Metering; Meters; Studio Line-Up Levels And Headroom - Orban OPTIMOD-FM 5500 Operation Manual

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INTRODUCTION

About Transmission Levels and Metering

Meters

Studio Line-up Levels and Headroom

and decode cycles, because each cycle moves the added noise closer to the threshold
where the added noise is heard.
Studio engineers and transmission engineers consider audio levels and their meas-
urements differently, so they typically use different methods of metering to monitor
these levels. The VU meter is an average-responding meter (measuring the approxi-
mate RMS level) with a 300ms rise time and decay time; the VU indication usually
under-indicates the true peak level by 8 to 14 dB. The Peak Program Meter (PPM)
indicates a level between RMS and the actual peak. The PPM has an attack time of
10ms, slow enough to cause the meter to ignore narrow peaks and under-indicate
the true peak level by 5 dB or more. The absolute peak-sensing meter or LED indica-
tor shows the true peak level. It has an instantaneous attack time, and a release
time slow enough to allow the engineer to read the peak level easily. Fig. 1-1 shows
the relative difference between the absolute peak level, and the indications of a VU
meter and a PPM for a few seconds of music program.
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.
ORBAN MODEL 5500