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Education and Career in the U.S.A.


Nyquist moved to the United States in 1907. He came to the University of North Dakota, Grand Forks, in 1912, where earned his Bachelor of Science in Electrical Engineering degree in 1914 and his Master of Science in Electrical Engineering degree in 1915. Nyquist continued his graduate studies at Yale University, New Haven, Conn., where he received the Ph.D. in physics in 1917. He was employed at American Telephone and Telegraph Company (AT&T) from 1917 to 1934, in the Department of Development and Research of Transmission, where he was concerned with studies on telegraph picture and voice transmission.

From 1934 to 1954 he was with the Bell Telephone Laboratories, Inc., where he continued in the work of communications engineering, especially in transmission engineering and systems engineering. At the time of his retirement from Bell Telephone Laboratories in 1954, Nyquist was Assistant Director of Systems Studies.

During his 37 years of service with the Bell System, he received 138 U.S. patents and published twelve technical articles. His many important contributions to the radio art include the first quantitative explanation of thermal noise, signal transmission studies which laid the foundation for modern information theory and data transmission, the invention of the vestigial sideband[5] transmission system now widely-used in television broadcasting, and the well-known Nyquist diagram for determining the stability of feedback systems.

Some of Nyquist's best-known work was done in the 1920s and was inspired by telegraph communication problems of the time. Because of the elegance and generality of his writings, much of it continues to be cited and used. In 1924 he published "Certain Factors Affecting Telegraph Speed," an analysis of the relationship between the speed of a telegraph system and the number of signal values used by the system. His 1928 paper "Certain Topics in Telegraph Transmission Theory" refined his earlier results and established the principles of sampling continuous signals to convert them to digital signals. The Nyquist sampling theorem showed that the sampling rate must be at least twice the highest frequency present in the sample in order to reconstruct the original signal. These two papers by Nyquist, along with one by R.V.L. Hartley, are cited in the first paragraph of Claude Shannon's classic essay "The Mathematical Theory of Communication" (1948), where their fundamental role in the development of information theory is acknowledged.

In 1927 Nyquist provided a mathematical explanation of the unexpectedly strong thermal noise studied by J.B. Johnson. The understanding of noise is of critical importance for communications systems. Thermal noise is sometimes called Johnson noise or Nyquist noise because of their pioneering work in this field.

In 1932 Nyquist discovered how to determine when negative feedback amplifiers are stable. His criterion, generally called the Nyquist stability theorem, is of great practical importance. During World War II it helped control artillery employing electromechanical feedback systems.

His remarkable career included advances in the improvement of long-distance telephone circuits, picture transmission systems, and television. Dr. Nyquist's professional, technical, and scientific accomplishments are recognized worldwide. It has been claimed that Dr. Nyquist and Dr. Claude Shannon, another signal procession pioneer, are responsible for virtually all the theoretical advances in modern telecommunications. He was credited with nearly 150 patents during his 37-year career. His accomplishments underscore the excellent preparation in engineering that he received at the University of North Dakota. In addition to Nyquist's theoretical work, he was a prolific inventor and is credited with 138 patents relating to telecommunications.

Nyquist and FAX


In 1918 H. Nyquist began investigating ways to adapt telephone circuits for picture transmission. By 1924 this research bore fruit in "telephotography" - AT&T's fax machine. The principles used in 1924 were the same as those used today, though the technology was comparatively crude. A photographic transparency was mounted on a spinning drum and scanned. This data, transformed into electrical signals that were proportional in intensity to the shades and tones of the image, were transmitted over phone lines and deposited onto a similarly spinning sheet of photographic negative film, which was then developed in a darkroom.

The first fax images were 5x7 photographs sent to Manhattan from Cleveland and took seven minutes each to transmit.

Date: 2015-12-24; view: 573

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EDWIN HENRY COLPITTS | Nyquist's Signal Sampling Theory
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