Single-sideband modulation

Lore & Background

Single-sideband modulation was derived directly from amplitude modulation by removing redundancy. In conventional AM, an audio signal controls the amplitude of a radio-frequency carrier, producing a carrier plus two mirror-image sidebands, each containing a complete copy of the original information while the carrier itself conveys none. SSB transmits only one sideband and usually suppresses the carrier, requiring approximately half the bandwidth of AM and using transmitter power more efficiently. SSB signals are typically generated at low power using filtering or phase-cancellation techniques and then amplified linearly.

The first U.S. patent application for SSB modulation was filed on December 1, 1915, by John Renshaw Carson. The U.S. Navy experimented with SSB over its radio circuits before World War I. SSB first entered commercial service on January 7, 1927, on the longwave transatlantic public radiotelephone circuit between New York and London, with high-power transmitters at Rocky Point, New York, and Rugby, England, and receivers in Houlton, Maine, and Cupar, Scotland.

SSB was also used in carrier telephony over long-distance telephone lines using frequency-division multiplexing (FDM), pioneered by telephone companies in the 1930s. With this technology, many simultaneous voice channels could be transmitted on a single physical circuit, with channels spaced usually 4,000 Hz apart, offering a speech bandwidth of nominally 300 Hz to 3,400 Hz. Amateur radio operators began serious experimentation with SSB after World War II, and the Strategic Air Command established SSB as the radio standard for its aircraft in 1957.

Reader's Guide

Single-sideband modulation's significance lies in its efficient use of both bandwidth and transmitter power compared to conventional amplitude modulation. By transmitting only one sideband and suppressing the carrier, SSB occupies approximately half the bandwidth of an equivalent AM signal and avoids wasting power on the carrier and redundant sideband. This efficiency made SSB the de facto standard for long-distance voice radio transmissions after its adoption by the Strategic Air Command in 1957 and its widespread use in amateur radio, where prior to widespread digital voice, most HF voice operation moved from AM to SSB. The technique also enabled frequency-division multiplexing in long-distance telephone lines, allowing many simultaneous voice channels on a single physical circuit. However, because the carrier is suppressed, SSB reception requires reinsertion of a locally generated carrier and greater frequency stability than AM, which can be generated and received with relatively simple equipment. The mathematical formulation of SSB is a special case of quadrature amplitude modulation where one baseband waveform is derived from the other via the Hilbert transform. In December 1956, the Proceedings of the IRE devoted a full issue to single-sideband transmission, covering its history, theory, and practical implementation across spectrum management, transmitters, receivers, filtering, power amplifiers, and military, commercial, and amateur applications.