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User:WhaleFarm/Carrier telephony2

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Early carrier telephony circuits

Multiple voice signals can be transmitted over a single pair of wires using frequency-division multiplexing (FDM), a technique historically known as carrier telephony. In early telephone systems, each conversation required its own physical circuit. By 1910, a second conversation was demonstrated by modulating a voice signal onto a higher-frequency carrier and combining it with another signal on the same line. At the receiving end, the signals were separated and demodulated in a manner similar to radio transmission and reception; early systems were sometimes described as "wired wireless".

After World War I, commercial carrier systems were introduced. In these systems, each voice channel (typically 300–3000 Hz) was translated to a distinct frequency band using modulators and filters, allowing many channels to coexist without interference. At the receiving end, corresponding filters and detectors separated and recovered the individual signals.

Carrier systems expanded through the 20th century as vacuum tubes, and later semiconductor devices, improved in performance and cost. Advances in circuit design, filtering, and manufacturing enabled increasingly dense channel packing. By the 1970s, a single long-distance system could carry more than 100,000 simultaneous telephone conversations over thousands of miles on a pair of coaxial cables.

History

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As demand for telephony services increased, carrier transmission enabled multiple telephone calls to share a single wire. Early systems used frequency-division multiplexing, first demonstrated in 1910 by George Owen Squier, who transmitted two simultaneous voice signals over a single circuit using a high-frequency carrier for one channel with what he termed "wired wireless".[1]

Commercial carrier multiplexing systems were introduced by AT&T in 1918. Contemporary and later accounts describe these systems as strongly influenced by Squier's earlier work, which had prompted renewed investigation within the Bell System after initial skepticism.[2]

Type A and Type B systems

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In 1921, E. H. Colpitts and O. B. Blackwell published a description of several frequency-division multiplexed (FDM) systems in use for telephony.[3]: 205–300  These systems were used on open-wire transmission circuits. The labels Type A and Type B were applied in later publications. In test systems put into use in 1914, satisfactory operation was described on communications between South Bend, Indiana and Toledo, Ohio, as well as between Chicago and Toledo. Demand for telephony during World War I resulted in installation of the test system between Pittsburgh and Baltimore.

In one carrier system[3]: 254–255  a ordinary telephony signal is carried in the 300 Hz - 3000 Hz band. 3 other 2-way circuits are assigned to carrier channels, with signals one way occupying the 10 kHz - 20 kHz band, and the other way the 20 kHz to 30 kHz band. Additionally, 8 two-way telegraph signals are multiplexed into the 3333 Hz - 10 kHz band.

Commercial systems were described, one of which transmitted the carrier with each modulated voice signal, used for the Detroit - Harrisburg circuit. It carried three carrier signals in addition to one normal telephony circuit over 596 miles, quadrupeling the prior capacity.[3]: 280 

Another system supressed the carrier, and tranmitted as a base frequency signal to the oposite end. Removal of the carrier made the transmission more efficient. This was used in the Harrisburg - Chicago system. It carried one normal circuit and four carrier circuits on one line.[3]: 287 

Early systems also added multiple telegraph signals onto lines used for telephony.[4]

Type C systems

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In a 1928 paper, the four carrier channels described in the 1921 paper were called Type A, and the three carrier channels without carrier suppression were called Type B. A further improvement in the system is described as Type C.[5]: 1360–1387  From 1926 to 1928, usage increased from 130,000 channel-miles to 230,000 channel-miles.

The Type C system was similar to the Type A system in that it used single-sideband with suppressed carrier. Two different frequency-planning strategies minimized crosstalk. Each modulator and demodulator used three vacuum tubes: one as an oscillator and two as a balanced mixer. The oscillator, an LC resonant type, maintained a 20 Hz maximum variation between modulation and demodulation oscillators.

Type D systems

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Frequency allocations for the Type D system

The Type D system was for shorter circuits, 50–200 miles. It added one carrier circuit to a plain telephony circuit, doubling the capacity. The system was low cost. It used 10.3 kHz and 6.87 kHz as the carrier frequencies for the two directions.[6]

Type K systems

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In 1934, Bell Labs proposed arranging channels in groups of 12 with 4 kHz spacing, or 48 kHz total bandwidth. It was also proposed that those groups be combined in groups of five to form a supergroup of 60 channels occupying 240 kHz. Operation above 1 MHz was reported on coaxial cable.[7]

In 1938, Bell published a 12-channel multiplexing system for either buried or above-ground cables. This was labeled Type K and was useful for distances of 100 to 4000 miles. Repeaters were required approximately every 17 miles. The system used a pair of wires for each direction, for a total of four wires for each 12-channel full-duplex system. As the same frequency sets were used for both directions, the system was not suitable for open-wire systems, where coupling between lines would be problematic. Channel carriers were spaced every 4 kHz from 12 kHz to 56 kHz.

The double-balanced mixers were implemented with copper-oxide rectifiers.[8]

The voice signals were first translated to the 60 kHz to 108 kHz band, filtered with crystal filters,[9] and then modulated as a group back to 12 kHz to 60 kHz. This two-step arrangement allowed the use of crystal filters with steep band edges.[10]

The local oscillator signals were provided by a 4 kHz tuning-fork oscillator feeding a harmonic generator.[11] Harmonics were separated with filters. The harmonics were generated with non-linear magnetics.

Type J systems

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In 1939, Bell Telephone Laboratories published a 12-channel system for open-wire lines.[12]: 351–360  As in the Type K system, channels were modulated in groups of 12. In the Type J system, both directions shared the same lines but used different frequency bands. West-to-east transmission used frequencies from 36 kHz to 84 kHz; east-to-west transmission used 92 kHz to 140 kHz.

Similar circuitry to the 4 kHz tuning-fork system was used as in the Type K system.

The Type J system was placed into commercial service in late 1938 by AT&T. The six initial systems were Dallas–Houston; two on Oklahoma City–Los Angeles; Oklahoma City–Albuquerque; and two between Charlotte and West Palm Beach, creating a total of about 55,000 channel-miles. The Charlotte circuits were continued to New York using Type K systems.[12]: 360 

Type L1 system

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In 1941, the L1 system, based on paired coaxial cables, was introduced. It used 60 12-channel groups, for up to 720 telephone circuits.[13]

Type-42 48 channel system

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In 1950, the Lenkurt Electric Company announced a 48-channel carrier system. The paper's author described the history of carrier systems, including the J, K, and L systems. The life cycle of an invention was discussed: an initial phase demonstrating usefulness; a second phase of improvement, sometimes at high cost; a third phase focused on manufacturability; and a final phase realizing the full advantages of the method[14]

The Type-42 system first translated all 48 input signals, each occupying 200–3600 Hz, to 7800–4400 Hz by mixing with an 8 kHz oscillator and selecting the lower sideband. Each signal was then translated to its final frequency and combined with the other signals. This two-step method simplified filtering, requiring only LC filters. The hardware for each 12 channel group is separate, giving redundacy for reliability.[14]

Type N-1 systems

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The N-1 system, described in 1951 by the Bell System, was a 12-channel vacuum-tube-based system.[15][16] It used two frequency ranges, 164 kHz to 260 kHz and 44 kHz to 140 kHz. The system was designed for ranges between 15 and 200 miles. Transmission used amplitude modulation with both sidebands and the carrier transmitted, with carriers spaced at 8 kHz intervals.

Signals were companded, with amplitude compression before transmission and expansion at reception. This improved dynamic range and reduced noise for low-level signals.

Type O systems

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Type O was a 4-circuit system for 15 to 150 mile segements of open-wire lines, described in 1952.[17]

Type ON2

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24 single-sideband signals using the same fequency plan as the N-1 system, utilizing components from the N-1 and the O systems.[18]

Type L3 system

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The 1953 L3 system was designed to operate on the same coaxial cable as the L1 system so that the wiring could be reused. It provided 1,860 telephone circuits on each pair of coaxial cables, or 600 circuits and one television circuit.[19] Distances could be up to 4,000 miles. A total of 155 groups of 12 channels occupied frequencies up to 8.32 MHz. The design used vacuum tubes.

Lenkurt 45BN system

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The 1955 Lenkurt system was coordinated with the N and ON carrier systems. It provided 24 voice channels using single-sideband in a 96 kHz band, either at 40–140 kHz or 164–264 kHz. The N allocations were 44–140 kHz and 164–260 kHz, and the O allocations were 40–136 kHz and 168–264 kHz. These systems could operate on the same cables.

A pilot tone corrected any frequency errors when operated with type N repeaters.[20]

In 1958, a repeater, compatible with the type N system, was described. It used transistors for low-level processing, and tubes as final amplifiers.[21]

K24A system

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In 1960, ITT Kellogg described a solid-state carrier system, the K24 synchroplex.[22]

Type N2 system

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Direct distance dialing (DDD) drove the need for more circuits. The Bell System began using N2 systems for short-haul lines (up to 200 miles) in 1962. It was a solid-state, 12-channel double-sideband system. It was used to supplement and replace the N1 system. Western Electric produced over 9,600 N2 terminals in 1964.[23]

The input amplifiers were constructed with PNP germanium transistors, and silicon NPN transistors were used for the output stages. The silicon transistor was developed specifically for this design to permit high-power operation at elevated temperatures. The compressor and expander, used to increase dynamic range and reduce noise, required the development of a special diode[24] to be used as a variable resistance, termed a "variolosser".[25] They were designed to CCITT recommendations.[23]

The double-sideband frequency conversion was accomplished with a square-wave crystal oscillator driving an NPN switch-type unbalanced mixer.[25]

General Dynamics 12 channel system

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In 1963, General Dynamics reported a solid-state 12-channel carrier system for open-wire and cable applications. The architecture and frequencies were chosen to maintain line compatibility with Type J and Type O systems. It used amplitude modulation up to 350 kHz, with optional companding, primarily for higher-frequency carriers. Frequencies were staggered with 8, 12, and 16 kHz spacing to simplify filter design. Modulation mixers were simple four-diode single-balanced designs. Demodulation used a single-diode AM detector, with carrier level feedback to set gain using a diode attenuator. Filters included LC with ferrite, LC air-core, and crystal types.[26]

Type N3 system

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The Type N3 carrier system was reported in 1966. It was a 24-channel single-sideband system designed for distances of 35 to 200 miles. Single-sideband operation was reported to be more economical for distances greater than 35 miles.[18]

The 24 channels were modulated at 4 kHz intervals into a 96 kHz band from either 36–132 kHz or 172–268 kHz. A phase-locked loop corrected for any frequency drift between the modulation and demodulation oscillators.

The mixers were single-balanced transistor modulators, built with a germanium transistor designed specifically for this circuit, having high reverse gain. Crystal filters provided an audio bandwidth of 200–3450 Hz.[27]

Type L4 system

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Placed in service in 1967, the L4 system provided 3,600 telephone circuits on each coaxial pair. It was a solid-state system, with repeaters spaced approximately 2 miles apart, and operated with signals up to 17.5 MHz.[28] Six hundred-channel mastergroups were defined as the standard within the Bell System, and L4 used six mastergroups.

Two families of transistors were developed for the L4 system: one for low-power, low-noise circuits, and another for medium-power applications. A new diode was also developed for use in fully balanced ring modulators up to 18 MHz.[29]

Lenkhurt 60 channel system

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In 1972, Lenkurt published a design in which 60-channel supergroups were modulated directly, rather than being formed from 12-channel groups.[30] The design used integrated circuits and custom crystal bandpass filters built with multiple quartz blanks for harmonic generation and bandpass filtering.[31]

Type L5 system

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Placed in service on January 3, 1974, the L5 system supported 10,800 telephone circuits on a coaxial pair. A cable with 10 coaxial pairs could support 108,000 simultaneous telephone conversations.[32] The channels were arranged in six jumbogroups, each containing six mastergroups, each mastergroup containing 600 channels. Repeaters were spaced at approximately 1 mile intervals. Frequencies above 60 MHz were used.[33]

Ultralinear transistors were developed for the L5 system.[34]

References

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  1. ↑ Squier, George D. (May 1911). "Multiplex telephony and telegraphy by means of electric waves guided by wires". Proceedings of the American Institute of Electrical Engineers. 30 (5): 857–905. doi:10.1109/PAIEE.1911.6660586. ISSN 0097-2444.
  2. ↑ Schwartz, M. (May 2008). "The origins of carrier multiplexing: Major George Owen Squier and AT&T". IEEE Communications Magazine. 46 (5): 20–24. doi:10.1109/MCOM.2008.4511639. ISSN 0163-6804.
  3. 1 2 3 4 Colpitts, E. H.; Blackwell, O. B. (January 1921). "Carrier Current Telephony and Telegraphy". Transactions of the American Institute of Electrical Engineers. XL: 205–300. doi:10.1109/T-AIEE.1921.5060708. ISSN 0096-3860.
  4. ↑ Hamilton, B. P.; Nyquist, N.; Long, M. B.; Phelps, W. A. (January 1925). "Voice-Frequency Carrier Telegraph System for Cables". Transactions of the American Institute of Electrical Engineers. XLIV: 327–332. doi:10.1109/T-AIEE.1925.5061114. ISSN 0096-3860.
  5. ↑ Affel, H. A.; Demarest, C. S.; Green, C. W. (October 1928). "Carrier Systems on Long Distance Telephone Lines". Transactions of the American Institute of Electrical Engineers. 47 (4). doi:10.1109/T-AIEE.1928.5055151. ISSN 0096-3860.
  6. ↑ Black, H. S.; Almquist, M. L.; Ilgenfritz, L. M. (January 1929). "Carrier Telephone System for Short Toll Circuits". Transactions of the American Institute of Electrical Engineers. 48 (1): 117–139. doi:10.1109/T-AIEE.1929.5055185. ISSN 0096-3860.
  7. ↑ Espenschied, Lloyd; Strieby, M. E. (October 1934). "Wide Band Transmission Over Coaxial Lines". Transactions of the American Institute of Electrical Engineers. 53 (10): 1371–1380. doi:10.1109/T-AIEE.1934.5056525. ISSN 0096-3860.
  8. ↑ Caruthers, R. S. (April 1939). "Copper Oxide Modulators in Carrier Telephone Systems*". Bell System Technical Journal. 18 (2): 315–337. doi:10.1002/j.1538-7305.1939.tb03579.x. ISSN 0005-8580.
  9. ↑ Lane, C. E. (January 1938). Bell System Technical Journal Crystal Channel Filters for the Cable Carrier System (Lane, C. E.). pp. 125–136.
  10. ↑ Green, C. W.; Green, E. I. (May 1938). "A Carrier Telephone System for Toll Cables". Transactions of the American Institute of Electrical Engineers. 57 (5): 227–236. doi:10.1109/T-AIEE.1938.5057804. ISSN 0096-3860.
  11. ↑ Peterson, E.; Manley, J. M.; Wrathall, L. R. (August 1937). "Magnetic Generation of a Group of Harmonics". Transactions of the American Institute of Electrical Engineers. 56 (8): 995–1001. doi:10.1109/T-AIEE.1937.5057679. ISSN 0096-3860.
  12. 1 2 Kendall, B. W.; Affel, H. A. (July 1939). "A 12-Channel Carrier Telephone System for Open-Wire Lines". Transactions of the American Institute of Electrical Engineers. 58 (7): 351–360. doi:10.1109/T-AIEE.1939.5057972. ISSN 0096-3860.
  13. ↑ "L5 Coaxial-Carrier Transmission System: Foreword". Bell System Technical Journal. 53 (10): 1897–1899. December 1974. doi:10.1002/j.1538-7305.1974.tb02724.x. ISSN 0005-8580.
  14. 1 2 Erickson, L. G. (January 1950). "A Simplified 48-Channel Carrier Telephone System". Transactions of the American Institute of Electrical Engineers. 69 (2): 1493–1500. doi:10.1109/T-AIEE.1950.5060324. ISSN 0096-3860.
  15. ↑ Caruthers, R. S. (January 1951). "The Type N-1 Carrier Telephone System: Objectives and Transmission Features". Bell System Technical Journal. 30 (1): 1–32. doi:10.1002/j.1538-7305.1951.tb01364.x. ISSN 0005-8580.
  16. ↑ Kahl, W. E.; Pedersen, L. (April 1951). "Some Design Features of the N-1 Carrier Telephone System". Bell System Technical Journal. 30 (2): 418–446. doi:10.1002/j.1538-7305.1951.tb03664.x. ISSN 0005-8580.
  17. ↑ Edwards, Paul G.; Montfort, L. R. (July 1952). "The Type-O Carrier System". Bell System Technical Journal. 31 (4): 688–723. doi:10.1002/j.1538-7305.1952.tb01402.x. ISSN 0005-8580.
  18. 1 2 Bleisch, G. W.; Irby, C. W. (1966-07-08). "The N3 Carrier System: Objectives and Transmission Features". Bell System Technical Journal. 45 (6): 767–799. doi:10.1002/j.1538-7305.1966.tb04221.x. ISSN 0005-8580.
  19. ↑ Elmendorf, C. H.; Ehrbar, R. D.; Klie, R.H.; Grossman, A. J. (July 1953). "The L3 Coaxial System". Bell System Technical Journal. 32 (4): 781–832. doi:10.1002/j.1538-7305.1953.tb03713.x. ISSN 0005-8580.
  20. ↑ "New type 45BN cable carrier system". The Lenkurt demodulator. 4 (6): 1–8. June 1955.
  21. ↑ Babin, V.; Fish, R. (March 1958). "A transistorized repeater for use with the 45BN cable carrier system". Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics. 77 (1): 41–49. doi:10.1109/TCE.1958.6372756. ISSN 0097-2452.
  22. ↑ Coetsee, B. G.; Curtis, G. L.; Halina, J. W. (January 1960). "The K24A syncroplex telephone carrier system". Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics. 78 (6): 1044–1054. doi:10.1109/TCE.1960.6368515. ISSN 0097-2452.
  23. 1 2 Boyd, R. C.; Herr, F. J. (1965-05-06). "The N2 Carrier Terminal - Objectives and Analysis". Bell System Technical Journal. 44 (5): 731–759. doi:10.1002/j.1538-7305.1965.tb04157.x. ISSN 0005-8580.
  24. ↑ Gardner, K. R.; Robillard, T. R. (September 1967). "Gold Doped Silicon Compandor Diodes For N2 and N3 Carrier Systems". Bell System Technical Journal. 46 (7): 1451–1477. doi:10.1002/j.1538-7305.1967.tb02470.x. ISSN 0005-8580.
  25. 1 2 Lundry, W. R.; Willey, L. F. (1965-05-06). "The N2 Carrier Terminal - Circuit Design". Bell System Technical Journal. 44 (5): 761–785. doi:10.1002/j.1538-7305.1965.tb04158.x. ISSN 0005-8580.
  26. ↑ Jorgensen, O. A. (November 1963). "New 12-channel carrier system for extended-area trunk and subscriber service". IEEE Transactions on Communication and Electronics. 82 (5): 643–647. doi:10.1109/TCE.1963.6373278. ISSN 0536-1532.
  27. ↑ Haner, R. L.; Wood, I. E. (1966-07-08). "Circuit Design of the N3 Carrier Terminal". Bell System Technical Journal. 45 (6): 801–844. doi:10.1002/j.1538-7305.1966.tb04222.x. ISSN 0005-8580.
  28. ↑ Klie, R. H. (April 1969). "The L-4 Coaxial System". Bell System Technical Journal. 48 (4): 819–820. doi:10.1002/j.1538-7305.1969.tb04252.x. ISSN 0005-8580.
  29. ↑ Chaplin, Norman J.; Dodson, Graydon A.; Jacobs, Richard M. (April 1969). "Solid State Devices". Bell System Technical Journal. 48 (4): 983–992. doi:10.1002/j.1538-7305.1969.tb04257.x. ISSN 0005-8580.
  30. ↑ Reading, A.; Martin, R. (February 1972). "A New Modulation Concept in Channel Translating Equipment". IEEE Transactions on Communications. 20 (1): 67–72. doi:10.1109/TCOM.1972.1091100. ISSN 0096-2244.
  31. ↑ Sheahan, D. F. (1975). "Polylithic Crystal Filters". 29th Annual Symposium on Frequency Control: 120–127. doi:10.1109/FREQ.1975.200072.
  32. ↑ Cite error: The named reference :5 was invoked but never defined (see the help page).
  33. ↑ Kelcourse, F. C.; Herr, F. J. (December 1974). "L5 System: Overall Description and System Design". Bell System Technical Journal. 53 (10): 1901–1933. doi:10.1002/j.1538-7305.1974.tb02725.x. ISSN 0005-8580.
  34. ↑ D'Altroy, F. A.; Jacobs, R. M.; Nacci, J. M.; Panner, E. J. (December 1974). "L5 System: Ultralinear Transistors". Bell System Technical Journal. 53 (10): 2195–2202. doi:10.1002/j.1538-7305.1974.tb02735.x. ISSN 0005-8580.
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