Sunday, September 20, 2009

Early telephone multiplex systems & Bell Labs

During a recent morning QSO on 3968 we talked about early frequency division multiplex technologies for carrying multiple voice channels on individual metallic circuits. Probably the earliest example was the O-carrier system deployed on open wire routes. A few photos are shown at this web site.

Information about these FDM systems is sparse on the web. The above referenced article on FDM does include a link about L-carrier systems deployed on coaxial cable.

N-carrier systems were used on twisted-pair cables starting about 1950. According to two books in my library, N-carrier was used for routes ranging from 15 to 250 miles. They used two frequency bands: 36 to 140 kHz (low group) and 164 to 268 kHz (high group) in opposite directions, on two cable pairs. At successive repeaters the signals were translated between the low group and high group to promote equalization across the entire route. Depending on the version of the system, either 12 or 24 channels were carried on 2 cable pairs.

Searching the web for more information about these carrier systems also yielded a nice article about Bell Labs. Scroll down to find entries grouped by decades.

The Bell Ringers web site also includes some good historical links on the Links page.

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