Recording Consoles 765To some extent, this is unjustifi ed because the VU may be used to monitor peak levels,
provided the action of the device is properly understood. The usual convention is to assume
that the peaks of the complex wave will be 10 to 14 dB higher than the peak value of a sine
wave adjusted to give the same reference reading on the VU meter. In other words, if a music
or speech signal is adjusted to give a reading of 0 VU on a VU meter, the system must have
at least 14 dB headroom—over the level of a sine wave adjusted to give the same reading—if
the system is not to clip the program audio signal. In operation, the meter needles should
swing only very occasionally above the 0 VU reference level on complex program.
27.2.2 The PPM Meter
Whereas the VU meter refl ects the perceptual mechanism of the human hearing system,
and thereby indicates the loudness of a signal, the PPM is designed to indicate the value
of peaks of an audio waveform. It has its own powerful champions, notably the BBC and
other European broadcasting institutions. The PPM is suited to applications in which
the balance engineer is setting levels to optimize a signal level to suit the dynamic range
available from a transmission (or recording) channel. Hence its adoption by broadcasters
who are under statutory regulation to control the depth of their modulation and therefore
fastidiously to control their maximum signal peaks. In this type of application, the
balance engineer does not need to know the “ loudness ” of the signal, but rather needs to
know the maximum excursion (the peak value) of the signal.
It is actually not diffi cult to achieve a peak reading instrument. The normal approach is
a meter driven by a buffered version of a voltage stored on a capacitor, itself supplied
by a rectifi ed version of the signal to be measured [see Figure 27.3(a) ]. In fact, the
main limitation of this approach lies with the ballistics of the meter itself, which, unless
standardized, leads to different readings. The PPM standard demands a defi ned and
consistent physical response time of the meter movement. Unfortunately, the simple
arrangement is actually unsuitable as a volume monitor due to the highly variable nature
of the peak to average ratio of real-world audio waveforms, a ratio known as crest factor.
This enormous ratio causes the meter needle to fl ail about to such an extent that it is
diffi cult to interpret anything meaningful at all! For this reason, to the simple arrangement
illustrated in Figure 27.3(a) , a logarithmic amplifi er is appended as shown at Figure
27.3(b). This effectively compresses the dynamic range of the signal prior to its display;
a modifi cation that (together with a controlled decay time constant) enhances the PPM’s
readability greatly—albeit at the expense of considerable complexity.