|Reference||STATE OF THE ART||Stan Augarten|
|The First Telephone Coder-Decoder on a Chip|
Until recently, telephone conversations were always transmitted by analogue signals. A vibrating diaphragm in the telephone mouthpiece converted the speaker's voice into a continuous electrical wave of varying intensity, or amplitude, while another diaphragm in the receiver changed the wave back into sound. For all their simplicity, analogue telephone systems had a major drawback: each telephone line could carry only one conversation at a time, which meant that a large telephone exchange needed a vast array of wires to handle the thousands of calls that took place during the height of the business day. At other times, however, those lines lay idle.
In the 1970s, engineers developed a system of chips which converted analogue signals into digital data that could be interwoven, many conversations at a time, onto a single line. The system used a technique know as pulse-code modulation (PCM). Through PCM, the amplitude of a wave is measured every eight-thousandth of a second or so and converted, or coded, into digital pulses (ons and offs). By sampling many conversations in rapid succession, a PCM system can transmit dozens of phone calls simultaneously over the same wire. Because the system samples each call many times a second, it obtains enough data to reproduce the speakers' voices.
As IC technology progressed, engineers eventually found a way to pack all the PCM circuitry onto a single chip called a coder-decoder or codec. For the phone company, the advent of the codec brought a marked increase in communications efficiency and a significant drop in equipment cost; as a result, a growing number of phone calls are now being transmitted digitally. The 2910 was the first of these revolutionary chips and is still one of the most widely used. It was designed by Ted Hoff, one of the inventors of the microprocessor (p. 30), and fellow Intel engineers John Huggins and Ben Warren.
|The 2910, shown here on a wafer of 2910 chips, can process about two million bits of data a second and is used widely in telephone systems. The gridded structures are registers. The bright red colors were created by lights aimed at the wafer. Actual size: 0.133 x 0.171 inches.|
|STATE OF THE ART
©Copyright Stan Augarten
|This book is provided for general reference. The National Museum of American History and the Smithsonian Institution make no claims as to the accuracy or completeness of this work.|
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