Handbook for Sound Engineers

1224 Chapter 33

This makes such lines immune to the detrimental effects
of reflections and standing waves. Radio, digital, and
telephone engineers are not so fortunate, and impedance
matching is required at component interfaces. Constant
voltage (sometimes called bridged) interfaces are inher-
ently simpler than their impedance matched cousins.
Advantages include the ability for a single output to
drive multiple high-impedance inputs (in parallel) with-
out loss of signal or degradation. Also, the constant
voltage interface does not require that manufacturers
standardize their input and output impedances. As long
as the output impedance is low (typically less than
1000 :) and the input impedance is high (typically
greater than 10 k:) then the two devices are compati-
ble. In practice most output impedances are fairly low
(<100:), allowing a single low-impedance output to
drive several high-impedance inputs, Fig. 33-3.

If the source impedance is large when compared to
the load, then a constant current interface is formed. In
this topology, the current from the source is determined
by the source impedance and is independent of the load
impedance. Constant current interfaces are not often
used to interface electronic components, and are usually
reserved for specialized applications, such as the
construction of impedance meters. We will not consider
this interface further in this chapter.

33.2 Audio Waveform Fundamentals

In a sound reinforcement system, program sources pro-
vide information that is to be reinforced and presented
to a listener. This information can originate in the form
of an acoustical wave (acoustic musical instruments or
human voices) or an electrical wave (electronic instru-
ments or storage media such as compact disc). In either
case, the waveform must be in the electromagnetic
domain prior to being presented to the sound system.
Acoustic signals must be converted into electromag-
netic signals with an appropriate transducer such as a

microphone or accelerometer. We will refer to electro-

magnetic waves within the bandwidth of the human

auditory system as audio waveforms. Typical audio

waveforms are quite complex and are continuously

changing in value over time. This makes it difficult to

describe them numerically. Several parameters can be

used to describe the characteristics of an audio wave-

form. These include the following:

Peak-to-Peak Voltage. The number of volts between

the largest positive and largest negative peak of the

waveform.

Peak Voltage. The highest peak of the waveform,

regardless of whether it is positive or negative. For a

waveform with amplitude symmetry, it will be one-half

the peak-to-peak voltage.

Average Voltage. The average of all ± amplitude

values of the waveform.

Root-Mean-Square (rms) Voltage. Sometimes called

the effective value of the waveform, rms describes the

ac voltage in terms of the equivalent dc voltage that

would produce the same amount of heat into a resistive

load. Rms is useful because it indicates the heating

value of the waveform. The rms level of a complex

audio waveform is also related to its perceived loud-

ness if it is used to drive a loudspeaker. For a sine wave,

the rms voltage is 0.707 times the peak voltage.

Complex waveforms also have an rms voltage, but

finding it requires integration of the waveform over

time. The peak-to-rms ratio of a waveform is called its

crest factor. Crest factors must be described in terms of

a finite span of time. A span of 50 ms correlates well

with the integration time of the human hearing system,

but other values can be used.

Fig. 33-4 shows a sine wave and speech waveform

on a common plot of amplitude versus time. An oscillo-

scope provides this representation of the data, as does a

wave editor application for a personal computer. A

Figure 33-3. Constant voltage bridged interface driving multiple inputs.

100 7

Source

Es

E 1

E 2

Load

Interface 10 k 7 E 3

Load

40 k 7 E 4

Load

20 k 7 Es = E 2 = E 3 = E 4