Bell Utilizes PCM Multiplexing

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The first successful commercial PCM (pulse code modulation) system, developed at Bell Labs, was put into operation in 1962. It vastly increased the capacity of telecom infrastructure through digitization and time-division multiplexing.

British scientist Alec H. Reeves invented the concept of pulse code modulation in 1937 while working at the Paris Laboratories of the International Telephone and Telegraph Corporation. Reeves sought to create a method for sending digital signals across analog communications channels. PCM works by sampling the amplitude of analog signals at uniform intervals. Each sample is quantized (mapped and rounded) at the nearest value within a range of digital steps.

After obtaining a patent in France and the United States, however, Reeves shelved the idea. Practical applications for PCM awaited the dawn of the semiconductor age. Using solid state components, Bell Labs was able to apply the PCM principle to transit digital signals. This technology, known as ESSEX, allowed engineers to switch signals without returning each conversion path to an analog transmission. The system consisted of 24 digital channels running at a format known as T1.

See also: Digital Signal Processing