Dorrough 10-A, Instruction Manual, Page: 2 / 19

1
INTRODUCTION
Today’s audio requires careful attention to precise level control. The con-
sumer now has audio playback systems that stretch the limits of program
dynamics. With all this new equipment, the modern listener is able to hear
subtle differences in level and distortion within program material and make
critical program choices based on the quality of sound.
Now it is more critical for the engineer to obtain the maximum loudness with
the minimum of distortion components in order to fully utilize the dynamic
range available. With the dynamic range that digital audio offers, and the
bandwidth now available to film and video productions, it has become of
paramount importance that there be a new method of monitoring and
establishing the maximum safe level at which a system can operate.
The Weston (VU) Meter
The Weston meter, introduced in 1939, was the standard for level monitoring.
It met the then new standard for VU (Volume Unit) meters, and those same
specifications are still the standard for all VU meters in use today. VU
meters are really nothing more than voltmeters calibrated in power, and
their ballistics were chosen to represent program material of the early days of
radio and film.
A VU meter is classified as a quasi-average reading device. It almost com-
pletely ignores peak waveforms. With the somewhat arbitrarily chosen rise
time of 300
µ
s, engineers have developed certain compensations, such as
riding dialogue 3 to 5 dB below music, to control the peak levels in an
attempt to attain consistent listening levels. The standard broadcast practice
is to allow for a crest factor of at least 10 dB to cover the peak excursions of
the waveform that the VU meter is too slow to indicate.
The VU meter was never intended to provide acoustical comparisons be-
tween processed and unprocessed program material. Nor was it ever
intended to provide indications of peak excursions. Clearly, the VU meter
has some dramatic shortcomings with today’s dynamic program material .
The PPM Meter
The PPM (Program Peak Meter) meter is not quite as old as the VU meter,
being only about a forty-year-old standard, but like the old VU meter, the
original standards are still on the books today. The PPM meter was the first
answer to the obvious shortcomings of the VU meter, but, because it displays
and holds only the peak level of the waveforms, the integration time of
quasi-average program level is almost completely ignored.
The PPM also has a scale that does not relate directly to measured program
levels, but only indicates relative levels. The numbers are arbitrary and
could be just as well represented by fruit symbols or stick figures. Modern
technology has eliminated the need to stare at a narrow mechanical needle
oscillating in an analog motion to set level.
The American Standard...
The European Standard...
80
100 60
40
20
0
-2 -1
VU
-3
-5
-7
-10
-20
+3
+1
+2
4
5
6
7
3
2
1