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Chapter 8:
Signal Flow, Quality, and Noise
Abstract
Whether as users or providers of telecommunications services, we are concerned with the quality of the signal. In general, there is a correlation between quality and cost, and each user and provider must decide the relative importance of each. For a human-related signal, such as a telephone call or television picture, there is first the receiver’s subjective perceived impression of quality. This is imprecise and relative, and varies with the experience, expectations, and expertise of the receiver. Engineers attempt to find more objective numerical measurements that hopefully correlate with the users’ subjective opinions. If there were nothing to interfere with our intended signal, we could in theory send a perfectly good signal at very low power. In the real world, however, we are faced with numerous sources of interference, to which we give the generic term noise. The signal is whatever it is that we wish to transmit, and the noise is anything that we do not want. Note that “noise” is any unwanted signal competing with the one that you want. Even a brilliant speech or beautiful music is noise if it interferes with what you are doing by leaking into your signal.
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CHAPTER 8
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