|
It is beneficial for the sound system operator to have some visual
feedback regarding the operating levels of sound system components.
Unfortunately, the monitoring of audio program material is not a
trivial task. Audio program material is very complex and constantly
changing in an unpredictable manner. A number of methods exist for
monitoring a waveform, each with its own pros and cons. Ironically,
all of the meters shown in this article will produce different indications
when fed the same waveform!
There are at least three significant parameters that can be monitored
for audio waveform. These include peak, RMS and average.
Peak
The peak value is simply the maximum instantaneous amplitude of
the waveform. It’s value is important because all system components
must be able to pass it without deformation. “Clipping”
occurs when a component does not have sufficient dynamic range to
pass the waveform. The maximum peak output voltage of an audio component
is usually specified by measuring the RMS value of the largest single
frequency sine wave that the component can pass. This becomes the
“full scale” output voltage of the device, and is usually
specified in decibels referenced to 1 Volt (dBV) or 0.775 Volts
(dBu). Many PC-based wave editors use the actual peak voltage (rather
than the RMS value of a sine) as the full-scale reference. Others
allow the user to decide, so be sure to check before you start analyzing
waves.
| The Wave Editor |
 |
 |
A wave editor for a personal computer provides the most
complete view of a waveform. Statistics about the wave (peak,
RMS, etc.) can be easily determined by calculation. Meters
that display the entire dynamic range of the waveform can
be constructed in software (left). This type of waveform monitoring
is not normally a “real-time” process, limiting
its use to the recording studio. Wave editors are useful for
testing the properties of real-time meters. |
RMS
The RMS value of the waveform describes its energy content. RMS
or Root Mean Square is a method of determining the area of the waveform.
This value is sometimes referred to as the “effective”
value of the waveform, and describes the equivalent DC voltage over
some time span that would generate the same power as the complex
audio waveform. This “heating value” of a waveform is
of particular importance in sound system work because it determines
how hot the loudspeaker will get. “True RMS” voltmeters
are now commonplace, due primarily to low-cost AID converters and
processors. Beware that the time span used for the computation,
as well as the display refresh rate may vary from meter to meter.
| The Oscilloscope |
 |
|
The oscilloscope displays amplitude variations as a function
of time. While useful for repetitive waves such as sine and
square, it is of limited usefulness for viewing real-time
audio program material..
|
 |
|
You
may also want to read these related stories:
|
|
|
