Carrier telephony

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Short description: Analog multiplexing technique used in early telephone systems


Carrier telephony is a long-distance telephone transmission method that uses frequency-division multiplexing (FDM) to carry multiple telephone conversations over the same media. Each voice signal is shifted by modulation to a separate frequency band, combined with other channels for transmission, and separated again at the receiving end by filters and demodulators.

This allowed long-distance lines to carry more traffic without putting up more wire. Early telephone circuits used direct voice-frequency transmission on pairs of wires strung between poles, one pair of wires per conversation. In 1910, George Owen Squier demonstrated the transmission of more than one conversation on a single line by using a high-frequency carrier, a method he described as "wired wireless". Commercial carrier systems were introduced after World War I.

Carrier telephony became one of the main technologies of 20th-century telephone communication. Early open-wire systems carried a few additional circuits, later increasing to 12-channel systems. Cable, coaxial, microwave, and submarine systems built on the 12-channel group to form larger multiplexing hierarchies in the 1930s. By the 1970s, high-capacity coaxial systems could carry more than 100,000 simultaneous calls in a multi-pair coaxial cable.

Carrier telephony also changed the practice of electrical engineering. Filters had to separate signals only a few kilohertz apart. Amplifiers for cable and coaxial lines had to add gain that matched the loss of the cable, without distortion. On the longest routes where hundreds or thousands of repeaters were in the path, a repeated small error grew to a failure. Consumers expected telephone service simply to work, but behind that apparent simplicity were thousands of miles of cable, thousands of circuits, and new approaches to accuracy, maintenance, testing, and reliability. Much of the later theory of filters, feedback amplifiers, equalizers, and communication channels grew out of this work.

Analog carrier systems were gradually replaced by digital transmission systems using pulse-code modulation (PCM), including the T-1 system introduced in 1962. Many organizing ideas of carrier telephony, including channel banks, multiplexing hierarchies, transmission levels, pilot tones, and network management, carried over into digital telephone networks.

History

CXR terminology

In Bell System usage, "CXR" was an abbreviation for carrier; field drawings therefore used forms such as "L CXR" for L-carrier facilities. A 1958 Long Lines route interconnection drawing, for example, labels an "L CXR switch" and "Lenkurt 45BX CXR terminal equipment" in a route using Type L1 carrier and 45BX Lenkurt carrier equipment.[1]

Background

Prior to carrier techniques, voice circuits were carried on open wire pairs. Two pairs, or four wires, could also add a third circuit, a phantom pair, using a pair of pairs. The east coast to Denver was the longest line used in this native mode, in 1915 amplification was added with repeaters to reach coast to coast.[2]: 4–5 

Early carrier telephony (1910–1920s)

In 1910, George Owen Squier demonstrated that more than one voice signal could be sent over a single wire circuit by placing a second signal on a high-frequency carrier. He described the method as "wired wireless".[3]

AT&T introduced commercial carrier systems in 1918, after further development of the method for telephone lines.[4] The early systems used open-wire lines and added carrier channels above the ordinary voice-frequency circuit. More detailed commercial systems were described by Colpitts and Blackwell in 1921.

These early systems established the basic architecture of carrier telephony: frequency translation of baseband signals, bandpass filtering, and recombination on a shared transmission medium, reversing the process at the receiving end.

Early commercial systems (1920s)

Carrier growth to 1928

In 1921, E. H. Colpitts and O. B. Blackwell published a description of several frequency-division multiplexed (FDM) systems in use for telephony.[5]: 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[5]: 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, quadrupling the prior capacity.[5]: 280 

Single-sideband transmission was later used in the transatlantic radiotelephone system of 1926. It was also used in the Harrisburg–Chicago carrier system, which suppressed the carrier and transmitted a single sideband to improve spectral and power efficiency. That system carried one normal circuit and four carrier circuits on a single line.[5]: 287 

In 1915, J. R. Carson showed that a modulated signal could be separated into two sidebands, and that only one sideband was required to recover the original telephony signal at the receiving end.[6] This became known as single-sideband modulation.[7]

Ralph Hartley later analyzed the relation between carrier and sideband components, showing that transmitting only one sideband save power and bandwidth, allowing more signals to share the line. He also described a balanced modulator circuit for suppressing the carrier component.[8]

Early systems also added multiple telegraph signals onto lines used for telephony.[9]

Type C system from 1928. Three carrier signals were transmitted in addition to one voice-band signal.

Long-distance open-wire systems (late 1920s–1930s)

In a 1928 paper, the four-carrier-channel system described in 1921 was identified as Type A, and the three-carrier-channel system without carrier suppression as Type B. The same paper described a later open-wire system, Type C, which added three bidirectional carrier circuits to a bidirectional voice-band circuit on the same open-wire pair.[10]: 1360–1387 

The Type C system was intended for long open-wire routes, from about 150 miles to more than 1,000 miles. It used single-sideband transmission with suppressed carrier, as in Type A, and used alternate frequency plans to reduce objectionable crosstalk between carrier systems on the same pole route. From 1926 to 1928, Bell System use of carrier systems increased from about 130,000 channel-miles to 230,000 channel-miles.[10]

Four-crossarm telephone pole using Type C and Type D carrier systems with ordinary voice-band circuits and telegraph circuits.

The Type D system was developed for shorter toll circuits of about 50 to 200 miles. It added one carrier circuit to a plain voice-frequency circuit, doubling the capacity of a pair at lower cost than the long-distance Type C equipment. For circuits longer than about 125 miles, an added amplifier produced the D-A version.[11]

Type C and Type D systems could be used together on the same pole routes. One Bell System example showed a four-crossarm pole line carrying six long-distance Type C circuits, fourteen shorter Type D circuits, thirty voice-band circuits, and eighty telegraph circuits.[11]

The last Type C circuit in the United States was taken out of service around 1980.[2]: 6 

Morristown cable carrier trial (1933)

In 1933, Bell Telephone Laboratories conducted an experimental cable carrier installation at Morristown, New Jersey. The trial used a 25-mile length of underground cable with repeaters at 25-mile intervals to simulate an 850-mile circuit. Nine carrier telephone channels were transmitted using frequencies from 4 kHz to 40 kHz. The trial demonstrated "very excellent" transmission quality and manageable crosstalk.[12]

For test purposes, several carrier links were connected in tandem to represent circuits as long as 7,650 miles. The simulated circuit had approximately 1,300 dB of total attenuation, which was compensated by the repeaters. Stable operation depended on negative-feedback amplifiers, developed from work by H. S. Black.[12][13] Bell Labs later described negative feedback as a key requirement for developing long-distance cable carrier systems.[2]: 73 

Although not adopted immediately for commercial service because of economic conditions during the Great Depression, the Morristown trial established the viability of cable carrier systems. It bridged earlier open-wire carrier systems and the standardized multi-channel cable systems introduced later in the 1930s.

The 12-channel group systems (late 1930s–early 1940s): Types J, K, and L

By the late 1930s, Bell System carrier telephony had standardized on the 12-channel group as a basic multiplexing unit. The group combined twelve voice channels into a band 48 kHz wide, using 4 kHz channel spacing and single-sideband transmission. Standardizing the group allowed much of the same translating equipment to be used on open-wire lines, paired cable, coaxial cable, and later microwave radio-relay systems.

Type K, introduced in 1938, was used on buried and aerial cable. It used separate wire pairs for the two directions of transmission and was designed for long-haul routes, with repeaters spaced about 17 miles apart.[14]

Type J, introduced in 1939, adapted the 12-channel group to open-wire lines. Because both directions shared the same wires, the two directions were separated by frequency. Initial installations included several long-distance routes totaling about 55,000 channel-miles, and Type J systems were often interconnected with Type K cable systems.[15]

Type L extended carrier telephony to coaxial cable. Development of coaxial carrier systems began in the mid-1930s, and the L1 system entered service in 1941 for very long-haul trunk routes. Coaxial systems carried many 12-channel groups by combining them into larger multiplexing structures, allowing much higher capacities than open-wire or paired-cable systems.[16][17][18]

By the late 1940s, coaxial carrier systems had expanded rapidly, with thousands of miles of L1 cable in service in the United States. The same coaxial facilities could also carry television signals for network broadcasting.[19] The 12-channel group later became part of the international multiplexing hierarchy standardized through the CCITT.[20]

British Post Office coaxial system

In 1937, the British General Post Office (GPO) described a wideband coaxial carrier system developed for trunk telephony and television transmission between London and Birmingham.[21] The design was similar to Bell System coaxial systems, using frequency-division multiplexing on coaxial cable pairs with negative-feedback repeater along the route.

The system initially used 5 kHz channel spacing, with provision for later conversion to 4 kHz spacing. Eight-channel groups occupied the band from 60 to 100 kHz, with provision for expansion to ten channels within the same frequency range. Crystal filters were used in the channel-group translating equipment. Five groups formed a 200 kHz supergroup, and eight supergroups occupied frequencies from approximately 500 kHz to 2.1 MHz, allowing up to 320 telephone circuits on a single coaxial system.[21]

Unlike contemporary Bell System practice, all carriers were derived from a master 400 kHz signal transmitted on the cable using frequency dividers and multipliers.[22] Terminal stations were located in Faraday Building in London and Telephone House in Birmingham. Twelve new repeater buildings were constructed along the route, with additional repeater equipment housed in existing telephone properties.[23] The equipment was placed in service on April 12, 1938, and was used both for trunk telephone service and experimental television transmission. [24]

Postwar refinement and solid-state transition (1950s–1960s)

Type-42 48 channel system

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[25]

Type N-1 systems

The N-1 system, described in 1951 by the Bell System, was a 12-channel vacuum-tube-based system.[26][27] 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. Unlike the J, K, and L systems, transmission used amplitude modulation transmitting both sidebands and the carrier. This used twice the bandwidth of a J, K, or L system, but simplified the circuitry.

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

Repeaters in N-carrier systems were typically spaced approximately 6 miles (10 km) apart, depending on wire gauge. These systems commonly operated on unloaded toll pairs in two-wire configuration. Transmission in opposite directions was separated in frequency, and repeaters alternated between low- and high-frequency bands in successive sections, a technique known as "frequency frogging", which enabled equivalent four-wire operation over a single pair of wires.[28] Repeaters were spaced approximately 6 miles (10 km) apart, depending on wire gauge

The twelve channel scheme, in order to maintain some bandwidth and routing compatibility, was carried through to the short haul carriers, as well, as they started developing to eliminate voice band open wire trunk lines in the 1950s.

Type O systems

Type O was a 4-circuit system for 15 to 150 mile segments of open-wire lines, described in 1952.[29]

Type ON2

24 single-sideband signals using the same frequency plan as the N-1 system, utilizing components from the N-1 and the O systems.[30]

Lenkurt 45BN system

In 1955, the Lenkurt Electric Company introduced the Type 45BN cable carrier system. It was a 24-channel single-sideband system designed to operate on cable routes using Bell System Type N and Type O carrier allocations. The system used a 96 kHz band, operating either from 40 to 140 kHz or from 164 to 264 kHz. These allocations allowed it to share cable routes with Type N or Type O carrier systems.[31]

When operated with Type N repeaters, the 45BN system used a pilot tone to correct frequency errors.[31] A compatible repeater described in 1958 used transistors for low-level stages and vacuum tubes for the final amplifiers. The design reflected the limits of available transistors, which were not yet considered suitable for the higher-power output stages.[32]

Lenkurt later became part of the General Telephone and Electronics equipment business. A 1968 review of the communications equipment industry described General Telephone as vertically integrated in a manner similar to the Bell System, with manufacturing affiliates including Automatic Electric, Lenkurt, and Leich supplying equipment to affiliated telephone companies.[33]

K24A system

In 1960, ITT Kellogg described the K24A Syncroplex, a solid-state carrier system for short-haul telephone service.[34]

Type N2 system

The growth of direct distance dialing (DDD) increased the need for circuits. The Bell System began using Type N2 systems in 1962 for routes up to 200 miles. N2 was a solid-state, 12-channel double-sideband system, used to supplement and replace the Type N1. Western Electric produced more than 9,600 N2 terminals in 1964.[35]

General Dynamics 12 channel system

In 1963, General Dynamics described a solid-state 12-channel carrier system for open-wire and cable applications. It's architecture and frequency plan maintained line compatibility with Type J and Type O systems. The system used amplitude modulation up to 350 kHz, with optional companding. Carrier 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.[36]

Type N3 system

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.[30]

High-capacity coaxial systems (1950s–1970s)

In 1929, ATT applied for a patent to multiplex telephony signals onto a coaxial cable. It described telephone with video capabilites, and up to 100s of voice channels.[37]

Type L3 system

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.[38] 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.

Type L4 system

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.[39] 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.[40]

Lenkurt 60 channel system

In 1972, Lenkurt published a design in which 60-channel supergroups were modulated directly, rather than being formed from 12-channel groups.[41] The design used integrated circuits and custom crystal bandpass filters built with multiple quartz blanks for harmonic generation and bandpass filtering.[42]

Type L5 system

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.[18] 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.[43]

Ultralinear transistors were developed for the L5 system.[44]

Technology

Influence on communications engineering

The demand for more telephone circuits made carrier telephony a major driver of circuit design and communications theory. E. H. Colpitts and O. B. Blackwell applied frequency-division multiplexing to telephone transmission. Closely spaced telephone channels required accurate filters, as developed by George Ashley Campbell, Otto Julius Zobel, and others. Long carrier routes required many repeaters in tandem, requiring low distortion and noise. This motivated Harold Black's negative-feedback amplifier, Harry Nyquist's stability criterion, and later equalizer and network-design methods by Hendrik Wade Bode and Sidney Darlington. The noise measurements and theory of John B. Johnson and Nyquist helped define the thermal-noise limits of repeaters and transmission systems.

The same telephone and radio transmission problems also shaped early information theory. Ralph Hartley's work on the transmission of information and Claude Shannon's mathematical theory of communication grew from questions of signal transmission, bandwidth, noise, and channel capacity.

Frequency planning

Frequency allocations for the Type D system, showing coordination with the Type C carriers and with telegraphy.

Carrier telephony systems used carefully planned frequency assignments so that many telephone channels could share the same wires without objectionable crosstalk. Crosstalk occurs when signals from one circuit couple into another through the mutual inductance and capacitance between nearby wires. Depending on the severity, crosstalk could appear as faint conversations, distorted or "Donald Duck" speech, whistles, tones, or background noise. This was especially important on open-wire pole routes, where many circuits ran close together for hundreds of miles. Because line construction was expensive, engineers tried to fit as many channels as possible into the available frequency range.

In the Type A system, three carrier channels in one direction occupied the 10 to 20 kHz band, while channels in the opposite direction occupied the 20 to 30 kHz band.[5]: 205–300 

The Type C system added three carrier channels in each direction in addition to the baseband voice circuit, increasing the capacity of a line to four simultaneous bidirectional telephone conversations. The system was designed for open-wire lines, where crosstalk between adjacent pairs was significant. Because LC filters provided poorer selectivity at higher frequencies, wider spacing was used for the upper channels.

Two related frequency plans, known as CS and CN, were developed so that crosstalk between neighboring carrier systems would be less objectionable. Telephone equipment and the human ear are most sensitive to interference near 1 kHz. The CS and CN allocations were staggered so that the 1 kHz components of neighboring systems would not overlap directly. In the overlapping portions of the spectrum, opposite sidebands were used so that crosstalk appeared spectrally inverted and was less intelligible as speech.[10]

forward

carrier kHz

sideband frequency range

of channel kHz

1 kHz image

kHz

reverse

carrier kHz

sideband Frequency range

of channel kHz

1 kHz image

kHz

CN 7.6 LSB 7.4 - 4.9 6.6 16.1 USB 16.3 - 18.8 17.1
10.6 LSB 10.4 - 7.9 9.6 19.9 USB 20.1 - 22.6 20.9
14.0 LSB 13.8 - 11.3 13.0 23.4 USB 23.6 - 26.1 24.4
CS 6.3 USB 6.5 - 9.0 7.3 20.7 LSB 20.5 - 18.0 19.7
9.5 USB 9.7 - 12.2 10.5 24.2 LSB 24.0 - 21.5 23.2
12.9 USB 13.1 - 15.6 13.9 28.2 LSB 28.0 - 25.5 27.2
D 6.87 LSB 6.67 - 4.17 5.87 10.3 LSB 10.1 - 7.6 9.3

The Type D system was designed as a lower-cost system compatible with the CS frequency plan, but not with the alternative CN allocation.[10]

As carrier systems increased in capacity during the 1930s, more standardized frequency plans were adopted. The J, K, and L systems used 4 kHz channel spacing, with carrier frequencies placed at multiples of 4 kHz. This tighter spacing required sharper filters than the earlier A, B, C, and D systems. The initial channel modulation stages were similar in the J, K, and L systems, using double-balanced modulators and crystal filters. Individual voice channels were first translated into a standard 60 to 108 kHz 12-channel group, which could then be shifted to different transmission bands as required.

The Type J system, introduced in 1939, adapted the 12-channel group for open-wire lines.[15] To reduce crosstalk between directions on the same line, west-to-east transmission used frequencies from 36 to 84 kHz while east-to-west transmission used 92 to 140 kHz. The large separation between the two bands minimized coupling between transmission directions on long open-wire routes.

The Type K system was designed for cable transmission systems, where crosstalk was lower than on open-wire lines but attenuation was substantially higher because of cable capacitance. Separate cable pairs were normally used for each transmission direction. The carrier allocations occupied the 12 to 60 kHz range and were derived from the standard 60 to 108 kHz groups by frequency translation.[14]

The Type L systems were developed for coaxial cable transmission. Unlike open-wire and cable carrier systems, coaxial systems normally operated in one direction per cable. Five 12-channel groups were combined into a 60-channel supergroup occupying approximately 240 kHz. Higher-capacity systems were formed by additional stages of frequency translation and grouping.[18]

The L5 system used a digitally controlled precision oscillator with a stated accuracy of 50 × 10−12.[45] It formed part of the L5 jumbogroup frequency supply, which used a digitally controlled frequency-locking loop to keep the system synchronized with the national frequency reference.[46] When a confirmed frequency difference was detected, digital control logic stepped the oscillator setting up or down. The frequency supply was fully redundant.

Frequency control

Early carrier systems depended on the accuracy of their local carrier oscillators. In SSB system, a mismatch in the mod and demod clocks shifts the frequency of the speech. The Type A and Type B systems used LC oscillators.[5] In the Type A system, which used single-sideband transmission, frequency matching between the transmitting and receiving oscillators was important because any error shifted the recovered speech frequencies. The Type B system transmitted the carrier with the sidebands, and was therefore less sensitive to oscillator mismatch.

The Type C system also used LC oscillators. Because the highest carrier frequency was about 30 kHz, keeping the transmitting and receiving frequencies within 20 Hz required what Affel, Demarest, and Green described as an oscillator "of exceptional stability".[10]

The Type D system used an LC oscillator, but reduced tube count by combining the oscillator and modulator functions. Its two mixer tubes operated in parallel as the oscillator and differentially as the balanced modulator.[11]

Bell called the generation of the carriers as the "carrier supply". The later J, K, and L systems used a more systematic approach to generating the. carrier supply. Channel carrier frequencies were derived from a common 4 kHz source and multiplier chains, so that all channel translations were tied to one reference. The 4k signal was passed through a symetric pulse generator to created odd harmonics, and a bridge rectifier to produce even harmonics. Needed harmonics were separated with narrow band filters. In the Type J open-wire and Type K cable systems, the reference oscillator used a tuning fork. This was accurate enough for the lower carrier frequencies used in those systems.[14]

The Type L coaxial system used a different arrangement because its higher carrier frequencies required closer frequency agreement between terminals. Its frequency supply was based on crystal-controlled oscillators and a frequency-locked loop. The adjustable oscillator was not a voltage-controlled oscillator in the later electronic sense, but a motor-controlled oscillator in which a motor drove a variable capacitor to keep the frequency locked to the reference.[17]

Modulation

Example of frequency translation from 1921 Colpitts paper

The initial signal modulation to form the groups was identical in the J, K and L systems, and was based on double-balanced mixers made with copper oxide rectifiers[47] and crystal filters. The 12 signals are first translated to a band from 60 to 108 kHz, where as a group, they were then translated to the final frequency as needed.[17] The systems all operated from a master, 4 kHz clock, and a frequency multiplier system to synthesize all of the needed carriers. In the J and K systems, the oscillators were open loop, using tuning forks for frequency references. In the L system, given the high accuracy requirements for the greater frequencies involved (.25 ppm relative accuracy between the two terminals), crystal oscillators with a frequency locked loop was used. The variable frequency oscillator was implemented with a motor driving a variable capacitor.[17]

In the D system, two triodes built both the local oscillator and the mixer. The two tubes operated in parallel for the oscillator, and differentially for the single-balanced mixer. This reduced cost and power dissipation. An terminal unit used 5 triodes, 2 for each frequency converter, and one for the ringer circuit.

Single balanced mixer used in 1921 carrier systems.

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 redundancy for reliability.[25]

Amplification

Type C push-pull amplifier. Used for both the repeater and the terminal equipment.

The Type C system used the same amplifier design in both terminal equipment and line repeaters. Low second-harmonic distortion was important because harmonics generated in the lower-frequency bands could fall inside higher-frequency carrier channels. A balanced push-pull circuit reduced second-harmonic distortion by approximately 15 to 20 dB.[10] Although balanced amplifiers theoretically cancelled even-order harmonics completely, practical vacuum tubes rarely matched closely enough for perfect cancellation. Harold Black later described these limitations as one of the factors that motivated his work on negative feedback amplifiers.[48]

The Type D system usually operated without dedicated repeaters, since the modulators themselves provided enough gain for shorter routes. For lines longer than about 125 miles, a two-triode push-pull repeater amplifier was used.[11]

In 1929, Harold Black applied for a patent on the negative feedback amplifier.[49] The circuit was designed to allow carrier systems to have a span of up to 4000 miles.

Black demonstrated that an amplifier using a feedback network matched to the loss characteristics of a cable could compensate for transmission loss while maintaining stable gain. In the Morristown carrier telephony trials, 29 such amplifiers were operated in tandem over long-distance test circuits. The mathematical theory of feedback stability was later developed further by Hendrik Bode[50][51] and Harry Nyquist.[52]

Black's negative-feedback amplifier was later recognized as a major enabling technology for wideband carrier systems. In remarks quoted when Black received the 1957 Lamme Medal, Mervin Kelly of Bell Laboratories described the invention as "one of the two inventions" of broadest importance in electronics and communications during the preceding half-century, and linked it directly to the feasibility of long-distance telephone, television, and transoceanic cable networks.[48]

In the late-1930s Type K cable systems, repeater amplifiers were spaced about 17 miles apart. The repeaters used cascaded pentode amplifiers with negative feedback networks chosen to match the characteristics of particular cable types and lengths. By placing a simulated cable network in the feedback path, the combined amplifier-and-cable response could be made nearly uniform over the transmission band. Because cable loss varied with temperature, automatic gain regulation was provided using a pilot tone transmitted through another dedicated wire pair on the cable together with a motor-driven variable capacitor in the feedback network.[14] The Type J open-wire system used similar pilot-tone regulation techniques, although the pilot frequency was transmitted on the same wire pair as the carrier channels, as a extra open-wire pair for the pilot was impractical.[15]

A revised K2 system entered service during World War II and was described publicly in 1947. By that time approximately 250,000 miles each of K1 and K2 circuits were in operation, using roughly 30,000 negative-feedback repeaters. The K2 system used embedded pilot tones and automatic non-mechanical gain regulation employing thermistors and heaters as variable control elements. Both positive and negative feedback were used in the regulating circuits.[53]

As the Type L coaxial systems evolved, wider bandwidths made the line loss higher and more frequency-dependent. Repeaters therefore had to be spaced more closely, while their amplifiers required more precise gain shaping and automatic regulation.

L1
1941 		L3
1957 		L4
1967 		L5
1974
Repeater spacing 8 miles 4 miles 2 miles 1 mile
Channels per coax 600 1,800 3,600 10,800
Amplifier technology Vacuum tube Vacuum tube Transistor Hybrid IC
Maximum frequency 2.5 MHz 8 MHz 20 MHz 66 MHz

The L3 amplifier was described in 1953. It used two amplifier stages with tubes designed for coaxial carrier service. The repeater operated to about 8 MHz and supplied gain shaped to match coaxial cable loss, from about 10 dB at low frequencies to about 45 dB at 8 MHz.[54] Negative-feedback networks were used both to shape the gain and to match the input and output impedances of the cable. A regulating stage between the two amplifier stages used a thermistor/heater combination to track changes in cable loss with temperature. The equalization and feedback design drew on network theory developed by H. W. Bode[55][51][56] and Sidney Darlington.[57][58]

The L4 coaxial system extended the line bandwidth to 20 MHz. Its repeaters used solid-state amplifiers spaced at 2-mile intervals on routes up to about 4,000 miles. The regulating system followed the same general approach as the L3 vacuum-tube system, using thermistors as control elements.[59][60]

The L5 system increased the useful bandwidth to more than 60 MHz and reduced repeater spacing to about 1 mile. Its solid-state repeaters used highly linear transistor circuits, with parallel matched transistors used to reduce distortion. Gain regulation again used thermistor control elements, while the implementation used thin-film hybrid construction.[61] The L5 repeatered line used 28 bands of frequency equalization, each adjusted by an equalizer based on Bode's method.[62]

Filter development

LC filter for carrier telephony. Solid line show attenuation matching schematic, dashed line show simple coupled LC network.

The development of carrier telephony systems was a major driver in the advancement of electrical wave filter theory and selective filter design during the early 20th century.[63] George Campbell described the theory and practical construction of electric wave filters in a 1922 paper in the Bell System Technical Journal.[64] Campbell described both ladder and lattice filter structures for separating signals by frequency, using networks of inductors and capacitors (LC). Otto Zobel[65] and K. S. Johnson[66] expanded on these efforts. These filters became standard components in Bell System carrier telephony equipment, including the channel filters used in the Type A through D carrier systems.[67]

In 1922, Walter Cady proposed the use of quartz resonators as filter elements.[68] Building on this work, W. P. Mason developed crystal lattice filter networks for Bell System carrier telephony applications.[69] Mason's wideband lattice filters provided much sharper selectivity than earlier LC filters. The lattice configuration used complementary network arms to produce cancellation outside the passband while maintaining transmission within the desired channel group, allowing sharply selective channel filters suitable for 12-channel carrier systems operating from 60 to 108 kHz.[70]

The sharp cutoff characteristics of crystal lattice filters made 4 kHz channel spacing practical and economical in large carrier systems.[71] The standard 60 to 108 kHz group band was influenced by the practical frequency range of early crystal filters. In many carrier systems, voice channels were first translated into this band for filtering and grouping, then shifted again to the higher frequencies used on open-wire, cable, or coaxial systems. The J, K, and L systems mainly differed in these later frequency translations and higher-level grouping arrangements.

Similar work took place in the United Kingdom, where Post Office engineers developed crystal channel filters for carrier telephony systems based on the same lattice and quartz-resonator techniques pioneered at Bell Laboratories.[72]

In 1946, Bell Laboratories introduced a redesigned crystal channel filter using eight crystal elements in a single lattice structure, reducing the size and weight of earlier filter assemblies.[73]

Lower-cost short-haul carrier systems such as the Bell System Type N carriers returned to LC filters and double-sideband modulation. Eliminating single-sideband channel filters reduced equipment cost and complexity, while the wider 8 kHz channel spacing allowed the use of simpler receive filters. These compromises were practical for shorter-distance systems where spectrum efficiency was less important than equipment cost.

In the late 1960s and early 1970s, Bell Laboratories developed monolithic crystal filters for carrier telephony channel banks, including the A6 channel bank system.[74][75] These designs replaced earlier discrete lattice crystal filter assemblies that had been used for more than 40 years. In these systems, voice channels were translated to frequencies above 8 MHz using 4 kHz spacing then combined and translated into the standard 60 to 108 kHz carrier group. The compact monolithic filters reduced the size and cost limitations that had previously restricted widespread use of crystal channel filters.

Other manufacturers developed related technologies. Lenkurt's 60-channel carrier systems used polylithic crystal filters for compact multiplex equipment.[76]

Semiconductor systems

In the N2 system, 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[77] to be used as a variable resistance, termed a "variolosser".[78] They were designed to CCITT recommendations.[35]

The N2 double-sideband frequency conversion was accomplished with a square-wave crystal oscillator driving an NPN switch-type unbalanced mixer.[78] In the N3 system, 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 were used, providing an audio bandwidth of 200–3450 Hz.[79]

12-channel channel banks

The Bell System and the CCITT standardized the basic analog FDM group as twelve voice channels occupying the band from 60 to 108 kHz. The same final group band could be produced in several ways, depending on the filter technology used in the channel bank. The most selective filters were the individual channel filters, since they had to separate adjacent voice channels spaced only 4 kHz apart.[80]

The 12-channel arrangement was formalized internationally in C.C.I.T.T. Recommendation G.232, which specified 12-channel translating equipment for carrier systems.[81]

Different national systems used different translating arrangements. Some European and Japanese channel banks used LC single-sideband filters by first translating three channels into the 12 to 24 kHz range, then combining four such groups into the standard 60 to 108 kHz band. Other systems in Europe, the United States, and Japan first translated each voice channel to the 48 to 52 kHz range before shifting the channels into the group band. In the United States and Great Britain, some channel banks used direct modulation into the 60 to 108 kHz band, with twelve LC, crystal, or mechanical channel filters operating in parallel. European mechanical-filter systems could instead use an intermediate range around 200 to 204 kHz before final translation to 60 to 108 kHz.[80]

The Bell System A1 through A5 channel banks used direct modulation into the 60 to 108 kHz group band. The later A6 channel bank used an intermediate frequency above 8 MHz so that monolithic crystal filters could be used before the completed group was translated down to the standard 60 to 108 kHz band.[80]

A-type channel banks

The Bell System A-type channel banks translated individual voice circuits to and from the standard 12-channel group occupying 60 to 108 kHz. These banks provided the channel modulation, demodulation, filtering, and level control used with the J, K, L, and later broadband carrier systems. The first A-type channel banks were developed for open-wire, cable, and coaxial carrier systems; later versions were also used with microwave systems such as TD-2 and related Bell System radio relay equipment.

A-type channel bank development
A1 A2 A3 A4 A5 A6
Channels per equipment bay 18 24 24 38 108–120 —
Year 1934 — — 1944 1962 1972
Gain element Vacuum tube Vacuum tube Vacuum tube Vacuum tube PNP germanium transistor NPN silicon transistor integrated circuits
Modulator Copper-oxide Copper-oxide Copper-oxide Copper-oxide Copper-oxide Two-transistor balanced modulator
Filter Two-stage crystal lattice Two-stage crystal lattice Two-stage crystal lattice One-stage lattice One-stage lattice Monolithic crystal
Conversion method Direct, one-stage Direct, one-stage Direct, one-stage Direct, one-stage Direct, one-stage Two-stage, with intermediate conversion above 8 MHz

[75][82]

The A5 channel bank, introduced in the early 1960s, was a transistorized replacement for earlier vacuum-tube channel banks. Bell System authors described it as a "radically new version" of the A-type bank.[82] Its transmitting path used passive copper-oxide modulator bridges and crystal filters, with the twelve channel units operating in parallel. The receiving path used a crystal filter, copper-oxide demodulator bridge, and a three-transistor negative-feedback voice amplifier with low-frequency equalization to compensate for channel-filter rolloff near the lower edge of the voice band. Compared with the A4 bank, the A5 bank reduced size and power requirements while improving operating characteristics.[82]

The A6 channel bank, described in 1972, used a two-stage conversion system with an intermediate frequency above 8 MHz, since monolithic crystal filters were practical only above about 5 MHz. This allowed the earlier multi-crystal lattice filters to be replaced by monolithic crystal filters, in which several mechanically coupled resonators were formed in a single small quartz element. The filters remained electromechanical devices, but were much smaller and better suited to manufacture than assemblies built from several separate crystal units. The A6 bank also used thin-film hybrid circuits and silicon integrated circuitry.[75]

Supergroup and mastergroup multiplexing

High-capacity carrier systems combined 12-channel groups into larger multiplexing structures. This was used in Type L coaxial systems and in microwave radio-relay systems such as TD-2, TH, TJ, and TL, where a single transmission path carried far more than one 12-channel group.[83]

In the Bell System L600 multiplex equipment, five 12-channel groups were first combined into a 60-channel supergroup occupying 312 to 552 kHz, a bandwidth of 240 kHz. Ten supergroups were then combined to form a 600-channel mastergroup. The mastergroup occupied the band from 60 to 2788 kHz; the difference between the ten 240 kHz supergroups and the full mastergroup band provided guard spaces. The resulting 600-channel structure was compatible with the CCITT multiplex hierarchy.[84]

The L1860 terminal extended the same hierarchy by translating 31 supergroups into the band from approximately 312 kHz to 8284 kHz.

Pilot tones were transmitted at 92 kHz with each of the groups and were used to regulate transmission levels through the system.[84]

See also

References

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  2. ↑ 2.0 2.1 2.2 A History of Engineering and Science in the Bell System: Transmission technology (1925-1975). Internet Archive. [Short Hills, N.J.] : The Laboratories. 1985. ISBN 978-0-932764-08-9. http://archive.org/details/transmissiontech0000unse. 
  3. ↑ 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. https://ieeexplore.ieee.org/document/6660586/. 
  4. ↑ 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. http://ieeexplore.ieee.org/document/4511639/. 
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  7. ↑ Bray, John (2002). Innovation and the communications revolution: from the Victorian pioneers to broadband Internet. London: Institution of Electrical Engineers. pp. 62. ISBN 978-0-85296-218-3. 
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  42. ↑ Sheahan, D. F. (1975). "Polylithic Crystal Filters". 29th Annual Symposium on Frequency Control: 120–127. doi:10.1109/FREQ.1975.200072. http://ieeexplore.ieee.org/document/1537012/. 
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  49. ↑ , Harold S."Wave translation system" patent US2102670A, issued 1937-12-21
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  53. ↑ Black, H. S.; Brooks, F. A.; Wier, A. J.; Wilson, I. G.; Tidd, W. H.; Rosen, S.; Wenk, H. A. (July 1947). "Cable carrier developments". Electrical Engineering 66 (7): 650–656. doi:10.1109/EE.1947.6443612. ISSN 0095-9197. https://ieeexplore.ieee.org/document/6443612/. 
  54. ↑ Morris, L. H.; Lovell, G. H.; Dickinson, F. R. (July 1953). "The L3 Coaxial System". Bell System Technical Journal 32 (4): 879–914. doi:10.1002/j.1538-7305.1953.tb03715.x. ISSN 0005-8580. https://doi.org/10.1002/j.1538-7305.1953.tb03715.x. 
  55. ↑ , Hendrik W."Coupling network" patent US2337965A, issued 1943-12-28
  56. ↑ Bode, Hendrik W. (1945). Network Analysis and Feedback Amplifier Design. New York: D. Van Nostrand Company. pp. 285. http://archive.org/details/dli.ernet.15701. 
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  67. ↑ Buckley, Oliver E. (1937). "The Evolution of the Crystal Wave Filter". Journal of Applied Physics 8 (1): 40–47. https://pubs.aip.org/aip/jap/article-abstract/8/1/40/174602/The-Evolution-of-the-Crystal-Wave-Filter. 
  68. ↑ Cady, W.G. (April 1922). "The Piezo-Electric Resonator". Proceedings of the IRE 10 (2): 83–114. doi:10.1109/JRPROC.1922.219800. ISSN 0096-8390. http://ieeexplore.ieee.org/document/1666707/. 
  69. ↑ Mason, W. P. (July 1934). "Electrical Wave Filters Employing Quartz Crystals as Elements". Bell System Technical Journal 13 (3): 405–452. https://dn710306.ca.archive.org/0/items/bstj13-3-405/bstj13-3-405.pdf. 
  70. ↑ Mason Warren P. (1942). Electromechanical Transducers And Wave Filters. pp. 263-280. http://archive.org/details/in.ernet.dli.2015.13601. 
  71. ↑ Kinsman, Robert G. (1998). "A History of Crystal Filters". 
  72. ↑ Stanesby, H.; Broad, E. R. (January 1939). "Channel Filters employing Crystal Resonators". The Post Office Electrical Engineers' Journal 31: 254–264. 
  73. ↑ Willis, E. S. (March 1946). "A New Crystal Channel Filter for Broad Band Carrier Systems". Transactions of the American Institute of Electrical Engineers 65 (3): 134–138. doi:10.1109/T-AIEE.1946.5059316. ISSN 0096-3860. http://ieeexplore.ieee.org/document/5059316/. 
  74. ↑ Lloyd, P. (1971). "Monolithic for Frequency Crystal Filters Division Multiplex". 25th Annual Symposium on Frequency Control (IEEE): 280–286. doi:10.1109/FREQ.1971.199868. http://ieeexplore.ieee.org/document/1536808/. 
  75. ↑ 75.0 75.1 75.2 Bleisch, G. (April 1972). "The A6 Channel Bank" (in en). IEEE Transactions on Communications 20 (2): 196–201. doi:10.1109/TCOM.1972.1091124. ISSN 0096-2244. http://ieeexplore.ieee.org/document/1091124/. 
  76. ↑ Sheahan, D. F. (1975). "Polylithic Crystal Filters". 29th Annual Symposium on Frequency Control: 120–127. doi:10.1109/FREQ.1975.200072. http://ieeexplore.ieee.org/document/1537012/. 
  77. ↑ 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. https://doi.org/10.1002/j.1538-7305.1967.tb02470.x. 
  78. ↑ 78.0 78.1 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. https://doi.org/10.1002/j.1538-7305.1965.tb04158.x. 
  79. ↑ 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. https://doi.org/10.1002/j.1538-7305.1966.tb04222.x. 
  80. ↑ 80.0 80.1 80.2 Kurth, C. (1973). "Analog and digital filtering in multiplex communication systems". IEEE Transactions on Circuit Theory 20 (4): 408–415. doi:10.1109/TCT.1973.1083688. ISSN 0018-9324. http://ieeexplore.ieee.org/document/1083688/. 
  81. ↑ C.C.I.T.T. (1965). Blue Book, Volume III: Line Transmission. Geneva: International Telecommunication Union. pp. 99–109. https://www.itu.int/rec/T-REC-G.232-196403-I/en. 
  82. ↑ 82.0 82.1 82.2 Blecher, F. H.; Hallenbeck, F. J. (January 1962). "The Transistorized A5 Channel Bank for Broadband Systems" (in en). Bell System Technical Journal 41 (1): 321–359. doi:10.1002/j.1538-7305.1962.tb03271.x. https://ieeexplore.ieee.org/document/6773568. 
  83. ↑ Hallenbeck, F. J.; Mahoney, J. J. (March 1963). "The New L Multiplex - System Description and Design Objectives". Bell System Technical Journal 42 (2): 207–221. doi:10.1002/j.1538-7305.1963.tb00499.x. ISSN 0005-8580. https://doi.org/10.1002/j.1538-7305.1963.tb00499.x. 
  84. ↑ 84.0 84.1 Graham, R. S.; Adams, W. E.; Powers, R. E.; Bies, F. R. (March 1963). "New Group and Supergroup Terminals for L Multiplex". Bell System Technical Journal 42 (2): 223–278. doi:10.1002/j.1538-7305.1963.tb00500.x. ISSN 0005-8580. https://doi.org/10.1002/j.1538-7305.1963.tb00500.x. 



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