George Ashley Campbell
Encyclopedia
George Ashley Campbell was a pioneer in developing and applying quantitative mathematical methods to the problems of long-distance telegraphy and telephony. His most important contributions were to the theory and implementation of the use of loading coil
s and the first wave filters designed to what was to become known as the image method. Both these areas of work resulted in important economic advantages for the American Telephone and Telegraph Company (AT&T).
in 1893. He was awarded a fellowship which enabled him to spend three years on graduate work; one year studying advanced mathematics under Felix Klein
at Göttingen, one year studying electricity and mechanics under Ludwig Boltzmann
in Vienna, and one year studying under Henri Poincaré
in Paris. Campbell received a doctorate from Harvard in 1901 with his dissertation being on the subject of his loading coil
research at AT&T.
s into the line at carefully calculated intervals to increase the inductance
. Engineer Michael I. Pupin also patented a similar system and AT&T paid Pupin a very large sum for his patents, so that development would continue without a legal battle. In fact, neither man was the first to suggest the idea of loading coils, that credit goes to Oliver Heaviside
in a 1887 article. Heaviside, however, never patented the idea; indeed, he took no commercial advantage of any of his brilliant work. Despite the rather arcane legal arguments surrounding this, it is unquestionable that Campbell was the first to actually construct a telephone circuit using loading coils.
Campbell was aware of Heaviside's work in discovering the Heaviside condition
, in which the specification for distortionless transmission of signals is formulated, but apparently was not aware of Heaviside's suggestion of using loading coils to force a line to meet it. Campbell initially attacked the problem from a completely different basis. Campbell was tasked by AT&T to investigate the possibility of improving line quality with the use of iron-copper bimetallic cable invented by John S. Stone
, another AT&T engineer. This cable of Stone's would similarly increase line inductance and had the potential to meet the Heaviside condition. However, Campbell was struggling to set up a practical demonstration over a real telephone route with the budget he had been allocated. After considering that his artificial line simulators used lumped
components rather than the distributed
quantities found in a real line, he wondered if he could not insert the inductance with lumped components instead of using Stone's distributed line. When his calculations showed that the manholes on telephone routes were sufficiently close together to be able to insert the loading coils without the expense of either having to dig up the route or lay in new cables he changed to this new plan. The very first demonstration of loading coils on a telephone cable was on a 46-mile length of the so-called Pittsburgh cable (the test was actually in Boston, the cable had previously been used for testing in Pittsburgh) on September 6, 1899 carried out by Campbell himself and his assistant. The first telephone cable using loaded lines put into public service was between Jamaica Plains and West Newton in Boston on May 18, 1900.
The invention was of enormous value to AT&T. Telephone cables could now be used to twice the distance previously possible, or alternatively, a cable of half the previous quality (and cost) could be used over the same distance. When considering whether to allow Campbell to go ahead with the demonstration, their engineers had estimated that they stood to save $700,000 in new installation costs in New York and New Jersey alone. It has been estimated that AT&T saved $100 million in the first quarter of the 20th century. Heaviside, who began it all, came away with nothing. He was offered a token payment but would not accept, wanting the credit for his work. He remarked ironically that if his prior publication had been admitted it would "interfere . . . with the flow of dollars in the proper direction . . .".
in the line response, whose value could be predicted with a knowledge of the line capacitance
and coil inductance
. An unloaded line had no such behavior, the attenuation
simply steadily increased with frequency. This behavior, and the lumped element
networks being used to create artificial lines for test purposes, suggested to Campbell a possible topology
for a filter with similar characteristics.
This work on filtering was begun in 1910. Using a ladder network of inductors and capacitors in appropriate configurations he produced low-pass, high-pass and band-pass filters. These filters could be designed to pass frequencies in any specified range and reject those in other ranges. This class of filter was later to be dubbed the constant k filter
by Otto Zobel working for AT&T in New York.
The sharpness of transition from the passband
to the stopband
, and the depth of rejection in the stopband were determined by the number of sections in the ladder. If a tighter specification was required for the filter, all that was necessary was to add more inductors and capacitors to the ladder in exactly the same circuit configuration as those for a less stringent specification.
The purpose of filtering a telephone channel so precisely was that AT&T were attempting to use the same wires for many telephone conversations simultaneously using the technique of frequency division multiplexing (FDM) and it was important for reasons of privacy, as well as intelligibility, that there was no crosstalk between the channels. Filters were also required to separate out the various conversations at the far end of the cable. Initially, a passband of 200 Hz
to 2.5 kHz was used for the voice baseband
, but soon the International Telecommunication Union
(ITU) established the world standard of 300 Hz to 3.4 kHz with 4 kHz spacing between channels.
These filter designs, which Zobel was later to improve upon, were of great economic value to AT&T. The ability to send multiple conversations over the same wires resulted in very substantial savings in cable installation costs. The modulation system used (single-sideband suppressed-carrier transmission) and the ITU standard remained the primary method of telephone service distribution until it began to be supplanted by digital techniques from the 1980s onwards.
Loading coil
In electronics, a loading coil or load coil is a coil that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance. The need was discovered by Oliver Heaviside in studying the disappointing slow speed of the Transatlantic telegraph cable...
s and the first wave filters designed to what was to become known as the image method. Both these areas of work resulted in important economic advantages for the American Telephone and Telegraph Company (AT&T).
Education
He graduated from MIT in 1891, and then received a master’s degree from Harvard UniversityHarvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
in 1893. He was awarded a fellowship which enabled him to spend three years on graduate work; one year studying advanced mathematics under Felix Klein
Felix Klein
Christian Felix Klein was a German mathematician, known for his work in group theory, function theory, non-Euclidean geometry, and on the connections between geometry and group theory...
at Göttingen, one year studying electricity and mechanics under Ludwig Boltzmann
Ludwig Boltzmann
Ludwig Eduard Boltzmann was an Austrian physicist famous for his founding contributions in the fields of statistical mechanics and statistical thermodynamics...
in Vienna, and one year studying under Henri Poincaré
Henri Poincaré
Jules Henri Poincaré was a French mathematician, theoretical physicist, engineer, and a philosopher of science...
in Paris. Campbell received a doctorate from Harvard in 1901 with his dissertation being on the subject of his loading coil
Loading coil
In electronics, a loading coil or load coil is a coil that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance. The need was discovered by Oliver Heaviside in studying the disappointing slow speed of the Transatlantic telegraph cable...
research at AT&T.
Work on loading coils
In 1897 Campbell went to work for AT&T in Boston. He developed a method for transmitting analog telephony over much greater distances than had previously been possible by the insertion of loading coilLoading coil
In electronics, a loading coil or load coil is a coil that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance. The need was discovered by Oliver Heaviside in studying the disappointing slow speed of the Transatlantic telegraph cable...
s into the line at carefully calculated intervals to increase the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
. Engineer Michael I. Pupin also patented a similar system and AT&T paid Pupin a very large sum for his patents, so that development would continue without a legal battle. In fact, neither man was the first to suggest the idea of loading coils, that credit goes to Oliver Heaviside
Oliver Heaviside
Oliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
in a 1887 article. Heaviside, however, never patented the idea; indeed, he took no commercial advantage of any of his brilliant work. Despite the rather arcane legal arguments surrounding this, it is unquestionable that Campbell was the first to actually construct a telephone circuit using loading coils.
Campbell was aware of Heaviside's work in discovering the Heaviside condition
Heaviside condition
The Heaviside condition, due to Oliver Heaviside , is the condition an electrical transmission line must meet in order for there to be no distortion of a transmitted signal...
, in which the specification for distortionless transmission of signals is formulated, but apparently was not aware of Heaviside's suggestion of using loading coils to force a line to meet it. Campbell initially attacked the problem from a completely different basis. Campbell was tasked by AT&T to investigate the possibility of improving line quality with the use of iron-copper bimetallic cable invented by John S. Stone
John Stone Stone
John Stone Stone was an American mathematician, physicist and inventor. He labored as an early telephone engineer, was influential in developing wireless communication technology, and holds dozens of key patents in the field of "space telegraphy".-Early years:Stone was born in Dover, now Manakin...
, another AT&T engineer. This cable of Stone's would similarly increase line inductance and had the potential to meet the Heaviside condition. However, Campbell was struggling to set up a practical demonstration over a real telephone route with the budget he had been allocated. After considering that his artificial line simulators used lumped
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...
components rather than the distributed
Distributed element model
In electrical engineering, the distributed element model or transmission line model of electrical circuits assumes that the attributes of the circuit are distributed continuously throughout the material of the circuit...
quantities found in a real line, he wondered if he could not insert the inductance with lumped components instead of using Stone's distributed line. When his calculations showed that the manholes on telephone routes were sufficiently close together to be able to insert the loading coils without the expense of either having to dig up the route or lay in new cables he changed to this new plan. The very first demonstration of loading coils on a telephone cable was on a 46-mile length of the so-called Pittsburgh cable (the test was actually in Boston, the cable had previously been used for testing in Pittsburgh) on September 6, 1899 carried out by Campbell himself and his assistant. The first telephone cable using loaded lines put into public service was between Jamaica Plains and West Newton in Boston on May 18, 1900.
Legal battle
AT&T fought a legal battle with Pupin over his claim. Pupin was first to patent but Campbell had already conducted practical demonstrations before Pupin had even filed his patent (December 1899), Campbell's delay in filing being due to the slow internal machinations of AT&T. The claim Pupin makes in his autobiography that he had previously thought of the idea while climbing a mountain in 1894 is widely doubted and there is no evidence for this either documentary or in the subsequent activities of Pupin and his students. However, AT&T foolishly deleted from Campbell's proposed patent application all the tables and graphs detailing the exact value of inductance that would be required before the patent was submitted. Since Pupin's patent contained a (less accurate) formula, AT&T was open to claims of incomplete disclosure. Fearing that there was a risk that the battle would end with the invention being declared unpatentable, they decided to buy an option on Pupin's patent for a yearly fee so that AT&T would control both patents. By January 1901 Pupin had been paid $200,000 and by 1917, when the AT&T monopoly ended and payments ceased, he had received a total of $455,000.The invention was of enormous value to AT&T. Telephone cables could now be used to twice the distance previously possible, or alternatively, a cable of half the previous quality (and cost) could be used over the same distance. When considering whether to allow Campbell to go ahead with the demonstration, their engineers had estimated that they stood to save $700,000 in new installation costs in New York and New Jersey alone. It has been estimated that AT&T saved $100 million in the first quarter of the 20th century. Heaviside, who began it all, came away with nothing. He was offered a token payment but would not accept, wanting the credit for his work. He remarked ironically that if his prior publication had been admitted it would "interfere . . . with the flow of dollars in the proper direction . . .".
Work on filters
One of the important results of the work on loading coils was that the loading caused a cut-off at a definite frequencyFrequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
in the line response, whose value could be predicted with a knowledge of the line capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
and coil inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
. An unloaded line had no such behavior, the attenuation
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
simply steadily increased with frequency. This behavior, and the lumped element
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...
networks being used to create artificial lines for test purposes, suggested to Campbell a possible topology
Electronic filter topology
Electronic filter topology defines electronic filter circuits without taking note of the values of the components used but only the manner in which those components are connected....
for a filter with similar characteristics.
This work on filtering was begun in 1910. Using a ladder network of inductors and capacitors in appropriate configurations he produced low-pass, high-pass and band-pass filters. These filters could be designed to pass frequencies in any specified range and reject those in other ranges. This class of filter was later to be dubbed the constant k filter
Constant k filter
Constant k filters, also k-type filters, are a type of electronic filter designed using the image method. They are the original and simplest filters produced by this methodology and consist of a ladder network of identical sections of passive components...
by Otto Zobel working for AT&T in New York.
The sharpness of transition from the passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
to the stopband
Stopband
A stopband is a band of frequencies, between specified limits, through which a circuit, such as a filter or telephone circuit, does not allow signals to pass, or the attenuation is above the required stopband attenuation level...
, and the depth of rejection in the stopband were determined by the number of sections in the ladder. If a tighter specification was required for the filter, all that was necessary was to add more inductors and capacitors to the ladder in exactly the same circuit configuration as those for a less stringent specification.
The purpose of filtering a telephone channel so precisely was that AT&T were attempting to use the same wires for many telephone conversations simultaneously using the technique of frequency division multiplexing (FDM) and it was important for reasons of privacy, as well as intelligibility, that there was no crosstalk between the channels. Filters were also required to separate out the various conversations at the far end of the cable. Initially, a passband of 200 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
to 2.5 kHz was used for the voice baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
, but soon the International Telecommunication Union
International Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
(ITU) established the world standard of 300 Hz to 3.4 kHz with 4 kHz spacing between channels.
These filter designs, which Zobel was later to improve upon, were of great economic value to AT&T. The ability to send multiple conversations over the same wires resulted in very substantial savings in cable installation costs. The modulation system used (single-sideband suppressed-carrier transmission) and the ITU standard remained the primary method of telephone service distribution until it began to be supplanted by digital techniques from the 1980s onwards.
Publications
- Loaded lines in telephonic transmission (1903)
- Cisoidal oscillations (1911)
- Physical theory of the electric wave-filter (1922)
- Fourier integrals for practical applications (1931)
People
- Oliver HeavisideOliver HeavisideOliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
- John Stone StoneJohn Stone StoneJohn Stone Stone was an American mathematician, physicist and inventor. He labored as an early telephone engineer, was influential in developing wireless communication technology, and holds dozens of key patents in the field of "space telegraphy".-Early years:Stone was born in Dover, now Manakin...
- Mihajlo Idvorski Pupin
- Otto Julius ZobelOtto Julius ZobelOtto Julius Zobel was a design engineer who worked for the American Telephone & Telegraph Company 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...
Filtering terms
- Low-pass filterLow-pass filterA low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
- High-pass filterHigh-pass filterA high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...
- Band-pass filterBand-pass filterA band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
- PassbandPassbandA passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
- StopbandStopbandA stopband is a band of frequencies, between specified limits, through which a circuit, such as a filter or telephone circuit, does not allow signals to pass, or the attenuation is above the required stopband attenuation level...