Hardware Reference
In-Depth Information
pure sine-wave signal at a frequency of 1000 to 2000 Hz, called a
carrier
, can be
transmitted with relatively little distortion, however, and this fact is exploited as the
basis of most telecommunication systems.
Time
01001011000100
V2
(a)
V1
High
amplitude
Low
amplitude
(b)
High
frequency
Low
frequency
(c)
(d)
Phase change
Figure 2-38.
Transmission of the binary number 01001011000100 over a tele-
phone line bit by bit. (a) Two-level signal. (b) Amplitude modulation. (c) Fre-
quency modulation. (d) Phase modulation.
Because the pulsations of a sine wave are completely predictable, a pure sine
wave transmits no information at all. However, by varying the amplitude, frequen-
cy, or phase, a sequence of 1s and 0s can be transmitted, as shown in Fig. 2-38.
This process is called
modulation
and the device that does it is called a
modem
,
which stands for
MOdulator DEModulator
.In
amplitude modulation
[see
Fig. 2-38(b)], two different voltage levels are used, for 0 and 1, respectively. A
person listening to digital data transmitted at a very low data rate would hear a loud
noise for a 1 and no noise for a 0.
In
frequency modulation
[see Fig. 2-38(c)], the voltage level is constant but
the carrier frequency is different for 1 and 0. A person listening to frequency mod-
ulated digital data would hear two tones, corresponding to 0 and 1. Frequency
modulation is often referred to as
frequency shift keying
.
In simple
phase modulation
[see Fig. 2-38(d)], the amplitude and frequency
do not change, but the phase of the carrier is reversed 180 degrees when the data
switch from 0 to 1 or 1 to 0. In more sophisticated phase-modulated systems, at
the start of each indivisible time interval, the phase of the carrier is abruptly shifted
