THE THREE BASIC IMPAIRMENTS AND HOW THEY AFFECT THE END-USER
propagation of the medium and, of course, the length of the medium. The value can vary
from 10,000 mi/sec (16,000 km/sec) to 186,000 mi/sec (297,600 km/sec). The former
value is for heavily loaded
subscriber pair cable. This latter value is the velocity of
propagation in free space, namely, radio propagation.
The velocity of propagation also tends to vary with frequency because of the electrical
characteristics associated with the network. Again, the biggest culprit is filters. Consid-
ering the voice channel, therefore, the velocity of propagation tends to increase toward
band center and decrease toward band edge. This is illustrated in Figure 3.4.
The finite time it takes a signal to pass through the total extension of the voice channel
or through any network is called delay. Absolute delay is the delay a signal experiences
while passing through the channel end-to-end at a reference frequency. But we have
learned that propagation time is different for different frequencies with the wavefront of
one frequency arriving before the wavefront of another frequency in the passband. A
modulated signal will not be distorted on passing through the channel if the phase shift
changes uniformly with frequency, whereas if the phase shift is nonlinear with respect to
frequency, the output signal is distorted with respect to frequency.
In essence we are dealing with phase linearity of a circuit. If the phasefrequency
relationship over a passband is not linear, phase distortion will occur in the transmitted
signal. Phase distortion is often measured by a parameter called envelope delay distortion
(EDD). Mathematically, envelope delay is the derivative of the phase shift with respect to
frequency. The maximum variation in envelope delay over a band of frequencies is called
envelope delay distortion. Therefore, EDD is always a difference between the envelope
delay at one frequency and that at another frequency of interest in the passband. It should
be noted that envelope delay is often defined the same as group delay --that is, the ratio
of change, with angular frequency,
of phase shift between two points in the network
Figure 3.4 shows that absolute delay is minimum around 1700 and 1800 Hz in the
voice channel. The figure also shows that around 1700 and 1800 Hz, envelope delay
Typical differential delay across a voice channel.
Wire-pair loading is discussed in Chapter 5.
Angular frequency and just the term frequency are conceptually the same for this text. Actually, angular
frequency is measured in radians per second. There are 2
radians in 1 Hz.