Modulation Methods

This paper covers briefly the early radio telephone experiments with efforts by the various inventors to modulate spark, arc and alternator outputs with microphones of various kinds, and also with variable reactances controlled by voice signals. The development of the high-vacuum triode provided a basis for a completely new art, both in sustained wave generation, and in modulation. A few of the methods of modulation by the triode, as well as the development of the first practical radio telephone which led the way to early broadcast transmitters, are outlined. The discovery of sidebands with its early influence on transmitter circuits, which led to channel spacing, and also to single sideband use for ether space conservation, is mentioned. The early fumbling with frequency modulation and then the eventual discovery of how it could be used to advantage is outlined. Pulse-modulation systems, which have not yet reached their full development, are covered briefly. Nothing is given about modulation methods for the infrared optical waves for which methods of generation have lately been discovered, but for which modulation methods have not yet been published.