Otto Julius Zobel (October 20, 1887 – January 1970) was an electrical engineer who worked for the American Telephone & Telegraph Company (AT&T) in the early part of the 20th century. Zobel's work on filter design was revolutionary and led, in conjunction with the work of John R. Carson, to significant commercial advances for AT&T in the field of frequency-division multiplex (FDM) telephone transmissions. Although much of Zobel's work has been superseded by more modern filter designs, it remains the basis of filter theory and his papers are still referenced today. Zobel invented the m-derived filter and the constant-resistance filter, which remain in use. Zobel and Carson helped to establish the nature of noise in electric circuits, concluding that—contrary to mainstream belief—it is not even theoretically possible to filter out noise entirely and that noise will always be a limiting factor in what is possible to transmit. Thus, they anticipated the later work of Claude Shannon, who showed how the theoretical information rate of a channel is related to the noise of the channel.
Life Otto Julius Zobel was born on October 20, 1887, in Ripon, Wisconsin. He was the son of Oscar Ewald "Herman" Zobel, who had emigrated to the United States from his native Germany in 1860, and his wife Ernestine, née Kahl. Zobel had seven siblings. After attending Ripon High School, he first studied at Ripon College, where he received his BA in 1909 with a thesis on Theoretical and experimental treatment of electrical condensers. He later received a Distinguished Alumnus Award from Ripon. He then went to the University of Wisconsin and graduated with an MA in physics in 1910. Zobel stayed at the University of Wisconsin as a physics instructor from 1910 to 1915, and graduated with his PhD in 1914; his dissertation concerned "Thermal Conduction and Radiation". This followed his 1913 co-authoring of a book on the subject of geophysical thermodynamics. From 1915 to 1916 he taught physics at the University of Minnesota. Having moved to Maplewood, New Jersey, he joined AT&T in 1916, where he worked on transmission techniques. In 1926, still with the company, he moved to New York and in 1934, he transferred to Bell Telephone Laboratories (Bell Labs), the research organisation created jointly by AT&T and Western Electric a few years earlier. He retired from Bell Telephone in 1952. The last of Zobel's prolific list of patents occurred for Bell Labs in the 1950s, by which time he was residing in Morristown, New Jersey. He was a fellow of the American Physical Society and of the Acoustical Society of America. He died in Morristown of a heart attack in January 1970. He had married Irene Staab on May 28, 1949; she was still living when he died but a son predeceased him.
Thermal conduction
Zobel's early work on heat conduction was not pursued in his later career. There are, however, some interesting connections. Lord Kelvin in his early work on the transmission line derived the properties of the electric line by analogy with heat conduction. This is based on Fourier's law and the Fourier conduction equation. Ingersoll and Zobel describe the work of Kelvin and Fourier in their book and Kelvin's approach to the representation of transmission functions would consequently have been very familiar to Zobel. It is therefore no surprise that in Zobel's paper on the electric wave filter a very similar representation is found for the transmission function of filters. Solutions to the Fourier equation can be provided by Fourier series. Ingersoll and Zobel state that in many cases the calculation involved makes the solution "well-nigh impossible" by analytical means. With modern technology such a calculation is trivially easy, but Ingersoll and Zobel recommend the use of harmonic analysers, which are the mechanical counterpart of today's spectrum analysers. These machines add together mechanical oscillations of various frequencies, phases and amplitudes by combining them through a set of pulleys or springs: one for each oscillator. The reverse process is also possible, driving the machine with the function and measuring the Fourier components as output.
Background to AT&T research After the work of John R. Carson in 1915 it became clear that multiplexed telephone transmissions could be greatly improved by the use of single sideband suppressed carrier (SSB) transmission. Compared to basic amplitude modulation (AM) SSB has the advantage of half the bandwidth and a fraction of the power (one sideband can have no more than 1/6 of the total power and would typically be a lot less). AM analysed in the frequency domain consists of a carrier and two sidebands. The carrier wave in AM represents the majority of the transmitted power but contains no information whatsoever. The two sidebands both contain identical information so only one is required, at least from an information transmission point of view. Up to this point filtering had been by simple tuned circuits. However, SSB required a flat response over the sideband of interest and maximum rejection of the other sideband with a very sharp transition between the two. As the idea was to put another (completely different) signal in the slot vacated by the unwanted sideband it was important that all traces of it were removed to prevent crosstalk. At the same time minimum distortion (i.e. flat response) is obviously desirable for the sideband being retained. This requirement led to a big research effort in the design of electric wave filters.
George A. Campbell and Zobel worked on this problem of extracting a single sideband from an amplitude-modulated composite wave for use in multiplexing telephone channels and the related problem of extracting (de-multiplexing) the signal at the far end of the transmission.
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