Recording Consoles 763as not, “ fray the nerves ” of those people working day after day in recording studios!
Instead a lower level is used for line-up, which actually has no physical justifi cation
at all. Instead it is cleverly designed to relate maximum signal level to the perceptual
mechanism of human hearing and to human sight as we shall see. Why sight? Because
it really isn’t practical to monitor the loudness of an audio signal by sound alone.
Apart from anything else, human beings are very bad at making this type of subjective
judgement. Instead, from the very earliest days of sound engineering, visual indicators
have been used to indicate audio level, thereby relieving the operator from making
subjective auditory decisions. There exist two important and distinct reasons to monitor
audio level.
The fi rst is to optimize the drive, the gain or sensitivity of a particular audio circuit,
so that the signal passing through it is at a level whereby it enjoys the full dynamic
range available from the circuit. If a signal travels through a circuit at too low a level,
it unnecessarily picks up noise in the process. If it is too high, it may be distorted or
“ clipped ” as the stage is unable to provide the necessary voltage swing, as shown in
Figure 27.1.
The second role for audio metering exists in, for instance, a radio or television continuity
studio where various audio sources are brought together for mixing and switching.
Listeners are justifi ably entitled to expect a reasonably consistent level when listening
to a radio (or television) station and do not expect one program to be unbearably loud
(or soft) in relation to the last. In this case, audio metering is used to judge the apparent
loudness of a signal and thereby make the appropriate judgements as to whether the next
contribution should be reduced (or increased) in level compared with the present signal.
The two operational requirements described earlier demand different criteria of the meter
itself. This pressure has led to the evolution of two types of signal monitoring meter, the
volume unit (VU) meter, and the peak program meter (PPM).
27.2.1 The VU Meter
A standard VU meter is illustrated in Figure 27.2(a). The VU is a unit intended to express
the level of a complex wave in terms of decibels above or below a reference volume, it
implies a complex wave—a program waveform with high peaks. A 0 VU reference level
therefore refers to a complex-wave power reading on a standard VU meter. A circuit
for driving a moving coil VU meter is given in Figure 27.2(b). Note that the rectifi ers