ALBEDO AT.2048
6
1.2.2 Distortion
Distortion produces a change in the original shape of the signal at the receiver end.
There are two types: amplitude distortion and delay distortion.
•
When the impairments affect the amplitudes of the frequency components of
the signal differently, this is said to produce
amplitude distortion
(sometimes
called absorption). Amplitude distortion is caused because the transmission
channel is limited to certain frequencies (see Figure 5). To overcome this prob-
lem amplifiers must equalize the signal, separately amplifying each band of
frequencies.
1
•
When the velocity of propagation of a signal varies with the frequency, there is
said to be
delay distortion
(sometimes called dispersion). Delay distortion is
particularly disturbing in the digital transmission producing
intersymbol inter-
ference
(ISI), where a component of the signal of one bit is misplaced in the
time slot reserved for another bit. ISI limits the capacity to extract digital infor-
mation from the received signal.
Harry Nyquist showed that the maximum transmission capacity (C) is limited by
ISI and depends on the channel bandwidth (B) and the number of signal elements
(M) coding the information.
1.
Note that attenuation is a specific case of amplitude distortion that equally affects all
frequencies of the signal.
Figure 5
The two basic transmission channels. In the frequency domain the channel transfer
function H(f) determines the attenuation of each frequency and consequently the
amplitude distortion.
f
f
f
o
f
2
f
1
Bandwidth
Bandwidth
H(f)
H(f)
Lowpass
Bandpass
C
bps
2Blog
2
M
=