Improving the noise performance of communication systems: radio and telephony developments of the 1920s [History of Communications]

This article discusses the early pioneering work of both telephone and radio engineers in effecting improvements in the noise performance of communication systems. This work led ultimately to the explosive growth of communication activities following World War II. Radio engineers during the 1920s were most concerned with reducing the impact of externally generated "static," and showed this could be accomplished by the use of directional antennas and moving to higher-frequency transmission. Telephone engineers during that period of time, most prominently John R. Carson of AT&T, were led to include the impact of "fluctuation noise" (shot and thermal noise) as well. Carson, using the then novel concept of noise frequency spectrum, showed how the appropriate choice of bandwidth and frequency of transmission could be used to improve the signal-to-noise ratio, anticipating the concept of a "matched filter" introduced 20 years later during radar developments of World War II. This early work on improving the noise performance of communication systems led, in the early 1930s, to Edwin H. Armstrong's spectacular leap ahead with his invention of wide-deviation low-noise frequency modulation (FM), followed a few years later by the invention by Alec Reeves of pulse code modulation (PCM), the first low-noise digital communication system of the modern era.