768 Chapter 27
loudness—leaving the operator to allow for peaks based on the known, probabilistic
nature of real audio signals. The PPM, however, indicates peak, leaving it to the operator
to base decisions of apparent level on the known stochastic nature of audio waveforms.
However, the latter presents a complication because, although the PPM may be used
to judge level, it does take experience. This is because the crest factor of some types
of program material differs markedly from others, especially when different levels of
compression are used between different contributions. To allow for this, institutions that
employ the PPM apply ad hoc rules to ensure continuity of level between contributions
and/or program segments. For instance, it is BBC practice to balance different program
material to peak at different levels on a standard PPM.
Despite its powerful European opponents, a standard VU meter combined with a peak-
sensing LED is very hard to beat as a monitoring device because it both indicates volume
and, by default, average crest factor. Any waveforms that have unusually high peak-to-
average ratio are indicated by the illumination of the peak LED. Unfortunately, PPMs do
not indicate loudness, and their widespread adoption in broadcast accounts for the many
uncomfortable level mismatches between different contributions, especially between
programs and adverts.
27.2.5 Polar CRT Displays
One very fast indicator of changing electrical signals is a cathode ray tube (CRT). With
this in mind, there has, in recent years, been a movement to use CRTs as a form of fast
audio monitoring, not as in an oscilloscope, with an internal timebase, but as a polar, or
XY display. The two-dimensional polar display has a particular advantage over a classic,
one-dimensional device like a VU or PPM in that it can be made to display left and right
signals at the same time. This is particularly useful because, in so doing, it permits the
engineer to view the degree to which the left and right signals are correlated; which is
to say the degree to which a stereo signal contains in-phase, mono components and the
degree to which it contains out-of-phase or stereo components.
In the polar display, the Y plates inside the oscilloscope are driven with a signal that is
the sum of the left and right input signal (suitably amplifi ed). The X plates are driven
with a signal derived from the stereo difference signal (R L), as shown in Figure 27.5.
Note that the left signal will create a single moving line along the diagonal L axis as
shown. The right signal clearly does the same thing along the R axis. A mono (L R )