Introduction to signals - Continuous and Discrete Time Signals and Systems - page 3

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Introduction to signals

Signals are detectable quantities used to convey information about time-varying
physical phenomena. Common examples of signals are human speech, temper-
ature, pressure, and stock prices. Electrical signals, normally expressed in the
form of voltage or current waveforms, are some of the easiest signals to generate
and process.
Mathematically, signals are modeled as functions of one or more independent
variables. Examples of independent variables used to represent signals are time,
frequency, or spatial coordinates. Before introducing the mathematical notation
used to represent signals, let us consider a few physical systems associated
with the generation of signals. Figure 1.1 illustrates some common signals and
systems encountered in different fields of engineering, with the physical sys-
tems represented in the left-hand column and the associated signals included in
the right-hand column. Figure 1.1(a) is a simple electrical circuit consisting of
three passive components: a capacitor C , an inductor L , and a resistor R .A
voltage v ( t ) is applied at the input of the RLC circuit, which produces an output
voltage y ( t ) across the capacitor. A possible waveform for y ( t ) is the sinusoidal
signal shown in Fig. 1.1(b). The notations v ( t ) and y ( t ) includes both the depen-
dent variable, v and y , respectively, in the two expressions, and the independent
variable t . The notation v ( t ) implies that the voltage v is a function of time t .
Figure 1.1(c) shows an audio recording system where the input signal is an audio
or a speech waveform. The function of the audio recording system is to convert
the audio signal into an electrical waveform, which is recorded on a magnetic
tape or a compact disc. A possible resulting waveform for the recorded electri-
cal signal is shown in Fig 1.1(d). Figure 1.1(e) shows a charge coupled device
(CCD) based digital camera where the input signal is the light emitted from a
scene. The incident light charges a CCD panel located inside the camera, thereby
storing the external scene in terms of the spatial variations of the charges on the
CCD panel. Figure 1.1(g) illustrates a thermometer that measures the ambient
temperature of its environment. Electronic thermometers typically use a therm al
res istor , known as a thermistor , whose resistance varies with temperature. The
fluctuations in the resistance are used to measure the temperature. Figure 1.1(h)

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