About Transmission Levels And Metering; Meters; Studio Line-Up Levels And Headroom - Orban OPTIMOD 6300 Operating Manual

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INTRODUCTION

About Transmission Levels and Metering

Meters

Studio Line-up Levels and Headroom

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 14dB. The Peak Program Meter (PPM) in-
dicates 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 actual peak level of the digital samples. It has an instantaneous attack
time, and a release time slow enough to allow the engineer to read the peak level
easily. Figure 1-2 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 +21dBu, and often +24dBu 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 facilities
that use VU meters, this level is usually at 0VU, which corresponds to the studio
standard level, typically +4 or +8dBu. For digital transmission, the SMPTE standard
line-up level is –20 dBFS and the EBU standard line-up level is –18 dBFS.
Facilities complying with the ATSC A/85 or EBU R 128 loudness standards will align
their facilities by using an ITU-R BS.1770-2 compliant loudness meter, using the rec-
ommendations in these standards. For digital audio, BS.1770 also recommends using
a true peak level meter that oversamples the audio by at least 4x before it is applied
to the meter.
For facilities using +4dBu standard level, instantaneous peaks can reach +18dBu or
higher (particularly if the operator overdrives the console or desk). Older facilities
with +8dBu standard level and equipment that clips at +18 or +21dBu will experi-
ence noticeable clipping on some program material.
ORBAN MODEL 6300