Telecommunications uses both analog and digital signals. We are most familiar with analog signals, because humans are analog devices. That is, the sensations we experience, such as sounds, images, tastes, and touches, vary greatly and in a continuous fashion. The term “continuous” means that they vary smoothly, and can have arbitrarily large or small changes. One sound can be a tiny bit louder than another; a leaf may be slightly greener than another or it might be brown. However, information is increasingly sent in a digital form because it is more reliable to do so and the quality received is much higher and more consistent (think of the contrast in quality between vinyl records and a CD). While the principles of digital transmission have long been known, the equipment to digitize and de-digitize analog signal for transmission was until recently prohibitively expensive for many uses, particularly for high-quality video.
Nevertheless, most signals we are interested in as humans, such as voices, music, and television, start out as analog signals and must be finally received as analog signals, whether or not they are carried between sender and receiver in an analog or digital form. Thus, we must understand the properties of both kinds of signals .
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