Topical Guide 67
Communication: Signal and Noise
Eliza turned the radio on, and she began twisting the dial, exploring for signals. For much of the time while she explored, she was between stations, and the living room was full of the noises that lie between stations on a radio dial, noises that are drowned out when we come upon a strong signal. Some of those noises come from within the receiver itself, produced by the operation of the receiver’s circuits, noises from within the machine. Other noises come from outside the receiver. The sources of some of those are local, familiar, homely. These may, for example, be produced by the ignition systems of passing Studebakers or by the motor in a refrigerator or by a toaster. The sources of others, however, are distant, exotic, intriguing. These may, for example, be produced by stations too far away for a clear signal to reach us, stations calling from God knows where, with voices as weak as that of a boy calling against the wind. Or they may originate in electrical discharges from the sun, from other stars, other galaxies: the pervasive and indecipherable, eternal and inestimable noise, the static of the spheres.
While Eliza and I are curled up on the floor twiddling the dial, searching for a signal, let me pause for a moment to plant in your mind the notion that our senses, like radio receivers, pick up lots of noise, and that in our perception of events the truth is sometimes nearly buried by static. Let me suggest, too, that in remembering the things that have happened to us, the people who have spoken to us, the things that they have said, we introduce new static, and that as time goes by we may even find, as I did with the whine from my little Philco, that the noise has become stronger than the signal.
Little Follies, “The Static of the Spheres”
Much of the above is accurate.
The International Telecommunication Union cites as sources of radio noise external to the radio receiving system itself “radiation from lightning discharges (atmospheric noise due to lightning); unintended radiation from electrical machinery, electrical and electronic equipments, power transmission lines, or from internal combustion engine ignition (man-made noise); emissions from atmospheric gases and hydrometeors; the ground or other obstructions within the antenna beam; and radiation from celestial radio sources.” (ITU-R P.372-13 (09/2016) “Radio noise”)
The classic signal-and-noise paper is Claude Shannon’s “Communication in the Presence of Noise,” in which:
A method is developed for representing any communication system geometrically. Messages and the corresponding signals are points in two “function spaces,” and the modulation process is a mapping of one space into the other. Using this representation, a number of results in communication theory are deduced concerning expansion and compression of bandwidth and the threshold effect. Formulas are found for the maximum rate of transmission of binary digits over a system when the signal is perturbed by various types of noise. Some of the properties of “ideal” systems which transmit at this maximum rate are discussed. The equivalent number of binary digits per second for certain information sources is calculated.
A brief non-technical explanation of the paper is available in the January 19, 2010, issue of MIT News:
Shannon, who taught at MIT from 1956 until his retirement in 1978, showed that any communications channel — a telephone line, a radio band, a fiber-optic cable — could be characterized by two factors: bandwidth and noise. Bandwidth is the range of electronic, optical or electromagnetic frequencies that can be used to transmit a signal; noise is anything that can disturb that signal.
Larry Hardesty, “Explained: The Shannon Limit”
Memory: Sources of Inaccuracy
Our imagination, and our dreams, are forever invading our memories; and since we are all apt to believe in the reality of our fantasies, we end up transforming our lies into truths. Of course, fantasy and reality are equally personal, and equally felt, so their confusion is a matter of only relative importance. . . . I am the sum of my errors and doubts as well as my certainties.
[Eric Kraft will be taking a break from posting from August 16 through August 20 while he prepares the launch of the Personal History podcast—every episode, read to you, by him. So, my next Topical Guide entry will appear on Monday, August 23, 2021. — Mark Dorset]
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