Audio Principles 31Line up is often done using a 1-kHz sine wave generated at an agreed level such as
0 dB(u). If a receiving device does not display the same level, then its input sensitivity
must be adjusted. Tape recorders and other devices that pass signals through are usually
lined up so that their input and output levels are identical, that is, their insertion loss
is 0 dB. Line up is important in large systems because it ensures that inadvertent level
changes do not occur.
In measuring the level of a sine wave for the purposes of line up, the dynamics of the
meter are of no consequence, whereas on program material the dynamics matter a great
deal. The simplest (and least expensive) level meter is essentially an AC voltmeter with
a logarithmic response. As the ear is logarithmic, the defl ection of the meter is roughly
proportional to the perceived volume, hence the term volume unit (VU) meter.
In audio, one of the worst sins is to overmodulate a subsequent stage by supplying a
signal of excessive amplitude. The next stage may be an analog tape recorder, a radio
transmitter, or an ADC, none of which respond favorably to such treatment. Real audio
signals are rich in short transients, which pass before the sluggish VU meter responds.
Consequently, the VU meter is also called the virtually useless meter in professional
circles.
Broadcasters developed the peak program meter (PPM), which is also logarithmic,
but which is designed to respond to peaks as quickly as the ear responds to distortion.
Consequently, the attack time of the PPM is carefully specifi ed. If a peak is so short that
the PPM fails to indicate its true level, the resulting overload will also be so brief that
the ear will not hear it. A further feature of the PPM is that the decay time of the meter
is very slow so that any peaks are visible for much longer and the meter is easier to
read because the meter movement is less violent. The original PPM as developed by the
British Broadcasting Corporation was sparsely calibrated, but other users have adopted
the same dynamics and added dB scales.
In broadcasting, the use of level metering and line-up procedures ensures that the level
experienced by the listener does not change signifi cantly from program to program.
Consequently, in a transmission suite, the goal would be to broadcast recordings at a
level identical to that which was determined during production. However, when making
a recording prior to any production process, the goal would be to modulate the recording
as fully as possible without clipping as this would then give the best signal-to-noise ratio.
The level could then be reduced if necessary in the production process.