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AT&T Archives: Single Sideband, a 1977 film about microwave transmission

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SSB (Single Sideband) is an obscure but very important way to communicate via radio. It is used primarily for two-way voice communication by ham radio operators, aircraft and air traffic control (ATC), ships at sea, military and spy networks. Occasionally some shortwave broadcast stations use this format.

In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modulation, it uses transmitter power and bandwidth more efficiently. Amplitude modulation produces an output signal the bandwidth of which is twice the maximum frequency of the original baseband signal. Single-sideband modulation avoids this bandwidth increase, and the power wasted on a carrier, at the cost of increased device complexity and more difficult tuning at the receiver.

This short film, made by Bell Laboratories in 1977, gives a basic overview of what single-sideband transmission consists of, and why it was implemented in the phone system that year. Single sideband transmission allowed the Bell System to carry a greater number of data, voice, and broadcast signals along the microwave network.

Single Sideband transmission was first recognized during the first overseas radio transmission tests conducted by the Bell System in concert with the U.S. Navy, in 1915. These tests took place at the U.S. Naval Radio Station in Arlington, Virginia, and were overseen by Harold D. Arnold of Bell Telephone (Bell Laboratories was formed later, in 1925). The tests built on scientific work by Raymond A. Heising and John Renshaw Carson at Bell Telephone and Western Electric.

The first cross-country microwave network call was placed in 1951. By 1977, the Bell System’s microwave network carried 70% of the long-distance transmissions in the United States.

In 1974-77, new single sideband systems were tested in Georgia and Massachusetts: the plan was to bring this increased transmission capacity to the long distance network by 1980.

The microwave network continued to shoulder the bulk of the company’s transmission capacity for another decade or so, but required a line-of-sight connection. As fiber optic transmission research and digital switching technologies made great strides in the 1980s, it became more feasible to fit a greater amount of information through fiber optic lines rather than relying on the microwave system.

The 1977 Massachusetts single sideband tests were of the AR6A system. A 1983 issue of the Bell System Technical Journal explains this technology and how it came about.

Footage Courtesy of AT&T Archives and History Center, Warren, NJ

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