Information Technology Reference
In-Depth Information
15.9 Digital modulation
Digital modulation changes the characteristic of a carrier according to binary information.
With a sine wave carrier the amplitude, frequency or phase can be varied. Figure 15.11 illus-
trates the three basic types: amplitude-shift keying (ASK), frequency-shift keying (FSK) and
phase-shift keying (PSK).
15.9.1 Frequency-shift keying (FSK)
FSK, in the most basic case, represents a 1 (a mark) by one frequency and a 0 (a space) by
another. These frequencies lie within the bandwidth of the transmission channel.
On a V.21, 300 bps, full-duplex modem the originator modem uses the frequency 980 Hz
to represent a mark and 1180 Hz a space. The answering modem transmits with 1650 Hz for a
mark and 1850 Hz for a space. The four frequencies allow the caller originator and the an-
swering modem to communicate at the same time; that is, full-duplex communication.
FSK modems are inefficient in their use of bandwidth, with the result that the maximum
data rate over normal telephone lines is 1800 bps. Typically, for rates over 1200 bps, other
modulation schemes are used.
1
1
0
1
0
ASK
PSK
FSK
Figure 15.11
Waveforms for ASK, PSK and FSK
15.9.2 Phase-shift keying (PSK)
In coherent PSK a carrier gets no phase shift for a 0 and a 180
°
phase shift for a 1, as given
next:
0
⇒
0
°
1
⇒
180
°
Its main advantage over FSK is that as it uses a single frequency it uses much less band-
width. It is thus less affected by noise. It has an advantage over ASK because its information
is not contained in the amplitude of the carrier, thus again it is less affected by noise.
