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In this chapter we will survey various topics in signal processing for communications. Communications, like signal processing itself, is commonly divided into analog and digital varieties. Analog communications consist of techniques for transmitting and receiving speech, music or images as analog signals, as in telephones, broadcast radio and television. Digital communications are methods of transferring digital information, usually in the form of bit streams. | Digital Signal Processing A Computer Science Perspective Jonathan Y. Stein Copyright 2000 John Wiley Sons Inc. Print ISBN 0-471-29546-9 Online ISBN 0-471-20059-X Part IV Applications Digital Signal Processing A Computer Science Perspective Jonathan Y. Stein Copyright 2000 John Wiley Sons Inc. Print ISBN 0-471-29546-9 Online ISBN 0-471-20059-X 18 Communications Signal Processing In this chapter we will survey various topics in signal processing for communications. Communications like signal processing itself is commonly divided into analog and digital varieties. Analog communications consist of techniques for transmitting and receiving speech music or images as analog signals as in telephones broadcast radio and television. Digital communications are methods of transferring digital information usually in the form of bit streams. Digital communications are often between computers or between human and computer although increasingly digital communications are being used between people as well email . Both analog and digital signal processing may be used for various portions of both analog and digital communications systems. A device that takes an analog input signal and creates an analog communications signal is called a transmitter while a receiver inputs an analog communications signal and attempts to recover as accurately as possible the original analog message signal. A device that takes a digital input and creates a digital communications signal is usually called a modulator while a demodulator inputs a digital communications signal and attempts to recover with as few bit errors as possible the original digital message. Transmitters and receivers are sometimes packaged together and called transceivers for digital communications it is almost universal to package the modulator and demodulator together and to call the combined device a modem. Digital communications systems include such diverse objects as fax machines telephone-grade modems local area networks wide .