Time Domain Representation of Continuous and Discrete Signals 21
R.1.54 Another important class of signals are the signals used in the transmission and
processing of information such as voice, data, and video, referred to as telecom
(telecommunication) signals. Telecom signals are, broadly speaking, composed of
a. Modulated signals
b. Multiplex signals
R.1.55 An exponential analog-modulated sinusoidal signal is given byf(t) = Aeat cos(t + )where the sinusoidal term is called the carrier, and the exponential Aeat is called the
envelope of the carrier that can represent a message or in general information.
R.1.56 An exponential discrete modulated sinusoid sequence is defi ned byf(n) = acn cos(2n/N + )Recall that a, c, N, and α were defi ned in R.1.38 and R.1.41.
R.1.57 Modulated signals are used extensively by electrical, telecommunication, com-
puter, and information system engineers to deliver and process information.
The modulation process involves two signals referred as
a. The carrier (a high-frequency sinusoidal)
b. The information signal (i.e., the message that can be audio, voice, data, or video)
R.1.58 The modulation process is accomplished by varying one of the variables that defi nes
the carrier (amplitude, frequency, or phase) in accordance with the instantaneous
changes in the information signal. Information such as music or voice (audio) con-
sists typically of low frequencies and is referred to as a base band signal. Base band
signals cannot be transmitted in its maiden form because of physical limitations
due to the distances involved in the transmission path, such as attenuation. Hence,
to obtain an economically viable system that can, in addition, support a number of
additional information channels (multiplexing), the information signal has to be
boosted to higher frequencies through the modulation process.
R.1.59 AM is the process in which the amplitude of the high-frequency carrier is varied
in accordance with the instantaneous variations of the information signal. This
process is accomplished by multiplying the high-frequency carrier by the low-
frequency component of the information signal.
AM signals present a constant frequency (which corresponds to the carrier’s fre-
quency) and phase variation.
A special type of AM signal used in the transmission of digital information is the
amplitude shift keying (ASK) signals also known as on-off keying (OOK).
R.1.60 FM is a type of modulation in which the frequency of the carrier is varied in accor-
dance with the instantaneous variations of the amplitude of the information signal.
These types of signals present a constant magnitude and phase.
A special type of FM signal used in the transmission of digital information is
the frequency shift keying (FSK) signal employed to modulate information that is
originated from digital sources such as computers.
Modulators and demodulators used to transmit digital information are referred
to as modems that stand for modulator–demodulator.
R.1.61 PM is a technique in which the phase of the carrier signal is varied in accordance
with the instantaneous changes of the information signal.