920 Chapter 25
top of the dim switches; thus, they follow monitoring. A
further pair permanently hung across the main stereo mix
output is optional. It’s customary to provide metering
facilities on each channel; in this design the feed is taken
following the monitor path source/return switching. This
allows level indication of what is going to a tape track
during recording and an “all is well” playback display.
Gazing at a row of meters hanging off a multitrack play-
back, it’s surprisingly easy to tell what each is indicating.
This is an important cue to a mixing engineer.
There are two basic types of meters, both evolving
around the same period on opposing sides of the
Atlantic. Each tells the observer entirely different
things. Nearly every other sort of audio level indicator
(LCD displays, rows of LEDs etc.) nods in style toward
one or other of these, (see Chapter 26).
25.14.6.1 VU Meters
Volume unit (VU) meters evolved as a standard in the
United States by Bell Telephone Laboratories. A need
was shown for a consistent instrument to measure audio
levels on lines; it is pictured in Fig. 25-93. The VU
meter has a quite tightly defined specification, even
down to the buff color of the scale! It is the ubiquitous
style of meter that finds itself everywhere, from broad-
cast consoles to cassette players, but with very few of
the interpretations actually bearing much resemblance
to the original Bell Laboratories’ intentions. It might
only consist of a light termination, a rectifier, and a
moving-coil meter, but at least the characteristics of all
were well defined; enough to be called a standard. Its
inception was in Ma Bell’s self-defense; she needed
some sort of consistency to the levels being hurled
down her lines.
In essence, it was a meter only valid for hanging
across 600: transmission lines; the 0 VU marking
indicates an actual line power level of +4 dBm. The
attack and decay times (the time taken for the meter to
indicate a steady input signal of 0 VU accurately and
the time taken for the needle to fall back afterwards) are
some 300 ms. This time is based predominantly on the
physical meter ballistics and happens to correlate quite
nicely with the level-sensing integration time of the
human ear. The VU is intended to give an approxima-
tion of how subjectively loud different pieces of
program material are in order to match them evenly.
This it does quite well. What it doesn’t do is give any
idea of the actual signal level. The relatively leisurely
integration time misses most transients altogether with
the consequence that a VU meter will underindicate
actual signal level; depending on program material this
can be by as much as 20 dB (on impulses and tran-
sients—snare drums spring to mind), 12–15 dB on
piano, and 8–10 dB on spoken voice.
The underread is unimportant in the respect that the
VU does allow subjective level matching and is very
easy to read and use. On a purely technical level, the
rectifier in a neat, unbuffered VU meter hung straight
across a 600: line imbues a serious amount of distor-
tion (some 0.3%) to the program material. This has
become more and more of an embarrassment over the
years. It is less of a problem with zero-impedance feeds,
but, unbuffered, it is still evident.25.14.6.2 Peak Program Meter (PPM)The peak program meter, Fig. 25-94, was the British
Broadcasting Corporation’s answer to the same
problem—the BS4297 spec. PPM differs from the VU
in three very important respects:- The PPM is a peak-reading instrument, capable of
accurately displaying signal transients. Correspond-
ingly, it has a very short attack time, coupled with a
long fallback decay time to give a chance to see the
peaks once it has captured them. - The PPM is black.
- The PPM has a logarithmic scale, allowing accurate
signal-level measurements to be made over all the
scale range.
The scale consists of seven marks, numbered 1 to 7,
each division representing 4 dB level change. PPM 4,
the middle mark, is set to indicate 0 dBm/0 dBu. The
normal operational maximum signal limit is PPM 6, or
+8 dBu.
As accurate and as useful as the PPM is, operators
have to consult a list of peak levels for different types of
program material (e.g., PPM 5 to 5½ for speech and
PPM 4 for heavily compressed pop music) in order to
perform the same function as a VU meter, which is
subjective level matching. A VU meter makes these
adjustments automatically since, although it’s worthless
for peaks, it follows program density, or loudness, well.
Virtually every other level-indicating device
emulates either the VU or the PPM characteristics—or
both. There are other European meters—peak reading
and log scales over a very wide (40 dB) range and with
a longer fallback time than the PPM. Using one of these
for the first time is unnerving; audience noise and even
studio noise often make the meter dither some way off
the bottom stop—a very unusual sight to eyes used to
VUs and PPMs.