Reference / Paper · 1957
Bell Laboratories Record, December 1957 (A Modern Crossbar PBX: the 756A)
Overview of the 756A crossbar PBX: capacity, modules, 48 V power, numbering and trunks.
Reference / Paper · 1957
Overview of the 756A crossbar PBX: capacity, modules, 48 V power, numbering and trunks.
Bell Laboratories Volume XXXV RECORD Number 12 December 1957 ls¡i. ai Ú967 Sharpness in Color Television Bell Laboratories RECORD CONTENTS Simulating Sharpness in Color Television, M. W. Baldwin, Jr. A Modern Crossbar PBX, O. H. Williford 481 485 The "TNistor" Memory Device 488 Telephone Communications for RAPCON Centers, G. A. Giddings 490 Propagation of Millimeter Waves Through the Atmosphere, A. B. Crawford and D. C. Hogg 494 Microdeviometer for Evaluating Periodic Structures 498 Cable Splicing Goes Modern, W. C. Kleinfelder 499 F. R. Kappel Addresses Independents' Association 502 Automatic Calculation of Transmission Deviations, R. C. Nance 503 Welding at Relay Contacts 508 Lightning Protection of TD -2 Stations, D. W. Bodle 509 A Printer -Comparer-Scanner for AMA, Miss M. E. Pilliod 512 THE COVER: Converging beams of red, blue and green light produce image for Bell Laboratories' study of subjective aspects in color television. (See opposite page.) The BELL LABORATORIES RECORD is published monthly by Bell Telephone Laboratories, Incorporated, 463 West Street, New York 14, N. Y.. M. J. KELLY, President; M. B. LONG, Secretary and Treasurer. Subscriptions: $2.00 per year; Foreign, $2.60 per year. Checks should be made payable to Bell Laboratories Record and addressed to the Circulation Manager. Printed in U. S. A. © Bell Telephone Laboratories, Incorporated, 1957. EDITORIAL BOARD F. J. Singer, Chairman J. A. Hornbeck F. A. Korn E. T. Mottraml R. J. Nossaman W. E. Reichle A. H. White EDITORIAL STAFF G. E. Schindler, Jr., Editor W. W. Mines, Assistant Editor, Murray Hill A. G. Tressler, Assistant Editor, Murray Hill A. B. Hilton, Assistant Editor R. L. Shepherd, Production Editor T. N. Pope. Circulation Manager Simulating Sharpness in COLOR TELEVISION Color television bears out well the adage, "beauty is in the eye of the beholder," since only the human eye can measure "sharpness." This purely subjective quality of video depends on the many complex parameters of a television system. Because Bell System transmission facilities are frequently a part of such systems, a color television simulator has been devised at the Laboratories to help answer some of the basic questions about sharpness. M. W. BALDWIN, JR. Visual and Acoustics Research From time immemorial man has made pictures in color. Before modern times all pictures were made by hand, and any faults in form or color could be attributed to the artist's lack of skill. As the scientific understanding of picture -making increased, this better understanding led to the introduction of new techniques. Complete freedom from hand reproduction became possible about a century ago, with the invention of photography. This revolutionary advance brought with it, of course, new kinds of picture faults that required increased scientific understanding. Photography has progressed rapidly nowadays we make pictures that move, we make them in full color, and most of us take them quite for granted. The youngest of the arts of picture -making is color television. This new art has released us from some of the limitations imposed by the dyes and processes used in color photography, but not without cost. The very complexity of television apparatus results in a host of new picture faults to be understood and overcome. To increase our understanding of some of these faults, the Visual and Acoustics Research Department at Bell Laboratories has for some time used a "defocusing" projector. The chief function of this projector is to make color pictures that are measurably blurred. This unusual research tool can also produce a full color image that looks like the picture one might see on a color television receiver. The blurring that results from out -of -focus projection can then he used to simulate the blurring that results when a color television picture is transmitted over a circuit of limited bandwidth. In the future, the Bell System may be called upon to transmit television signals which will pro- - duce color pictures sharper than those presently broadcast for home entertainment. By being able to simulate such color television pictures right now, it is possible to study some of the factors which influence sharpness. Color television pictures are produced by a three color additive process. The picture tube produces a separate and complete image in each of the primary colors red, green and blue and these three pictures are superimposed on the screen of the tube. The controlled addition of three colors at every point on the screen can produce whatever color is required at each point to build up a full color image on the viewing screen. The defocusing projector, shown in Figure 1, also makes color pictures by an additive process, - - Front view of the defocusing projector Fig. .1 showing the four individual projector lenses. Fig. 2 - The vertical reference lines on the screen show that these two pictures are the same size. as a proper simulator should. The projector, or more accurately, the projection machine, is actually composed of four projectors, with independent focusing adjustments, directed at a common projection screen. The machine projects separate pictures, one in red light, one in green light and one in blue light onto the screen, where the additive process takes place. The viewer sees only the sum of these three pictures, or a full color image. The fourth projector is available for showing a comparison picture if needed. A basic requirement, when separate images are projected to make a color picture, is that the separate images remain in good register on the screen. The machine would not be useful for sharpness studies if the effects of focus adjustment were to be contaminated by misregistration. In other words, image size and centering must both remain fixed while the focus changes. The optical principle used to maintain constant magnification with variable focus is a simple one. Taking an ordinary home slide projector as an example, we know that the picture gets larger as the projector is moved farther from the screen. We also know that the picture goes in and out of focus as the lens is moved back and forth on the projector. Likewise, movement of the lens causes the picture to change size. If one could move the whole projector back and at the same time move the lens forward on the projector by the proper distance, he could keep the picture from changing size. The increase in size due to moving the projector back 482 would cancel out the decrease in size due to moving the lens forward, yet the picture would go out of focus. Figure 2 illustrates projections of the same slide and shows that the two images are exactly the same size, even though the picture on the right is obviously out of focus. To make possible the precise motions and optical relationships necessary to get a constant -size picture of variable focus, a well -known mechanism FIXED MEMBER SCREEN PROJECTOR A IJIIIIIIIIIIIIII I-------------- INHEREUS.\\. IJIIIIIIIIIIIIII±I--= - _ PROJKOTO SB,. HERE DISTANCE MOVED: FIXED MEMBER - --BY PROJECTOR --BY LENS Pantograph maintains spacing of projector and lens for in- and out -of -focus projection. Fig. 3 BELL LABORATORIES RECORD - the pantograph - is used. The pantograph, which is simply a geometric parallelogram with pivots at the corners, provides exactly the motions required. Figure 3 shows how the pantograph was adapted to this use. The pantographs as they appear in the simulator are shown in Figure 5. A bellows, seen between the slide carrier and the lens, accommodates the relative motion of these parts as the projector moves in its focusing and defocusing action. The bellows stretches about % of an inch when the whole projector moves back two inches. The two projectors on the other side are mounted in the same way. The four projectors are mounted so that the projection lenses and lantern slides are all squared onto, or held parallel to, the screen. The centers of the lenses are closer together than the centers of the slides. The resulting projection is known - Side view shows top pantograph as it actually looks in the projection machine. Fig. 5 - Fig. 4 The observer, E. Averbach, tells the operator, G. Nielsen, when he thinks the sharpness of this picture matches that of a comparison picture. as off -axis projection, because the center of the image is a few degrees off the optical axis of each lens. This kind of projection insures perfect registration of the images on the screen at all times and eliminates "keystone" distortion an image irregularity that would result if the projectors were not squared onto the screen. The keystone effect, to describe it simply, is the tapered shape a rectangu- - lar picture assumes when projected at an angle. The slightly tapered keystone shape, which is ordinarily not objectionable, would make image registration impossible. The illuminating systems are "toed -in" so that they all point toward the center of the screen to improve the uniformity of coloring over the field. DECEMBER, 1957 This machine performs well because all of its parts have been made and aligned with precision. The lengths of corresponding sides of the pantographs differ by less than one thousandth of an inch. The lantern slides and the lenses are squared onto the screen to better than five minutes of arc. The in -focus magnifications of the four projectors are equalized to better than one part in 2,000. As a result, the separate images on the screen, whether in sharp focus or not, are in register to within the limits of unaided vision. A most important part of this projection machine is its control system. Each of the four projectors is actuated by small hydraulic controls which move the projectors slowly and smoothly and hold them firmly in position. Small indicating units monitor the individual projector positions and transmit these positions to indicator dials mounted on the control panel. The indicator dials, in effect, relate the focus condition of individual projectors to the sharpness of the color picture on the screen. The sharpness purely subjecof a three -color additive picture by an observer. tive quality can only be judged In a typical experiment, the observer examines a picture in the "test condition" ( the three projectors out of focus by different amounts) and then examines a picture in the "gauging condition" ( the three projectors out of focus by the same amount). The observer then adjusts the gauging condition to be equally as sharp as the test condition. Fig- - -a 483 ure 4 shows the projector as it is used with an observer. The indicator dial readings called for by the observer in making this sharpness match are then recorded by the operators and can be translated into bandwidth information by a straightforward calibration procedure. MAXIMUM EQUIVALENT BANDWIDTH -, IN MEGACYCLES m 50 0 6.3 MC 30 PER 40 CENT OF ALL 50 BANDWIDTHO 70 FOR GREEN 80 9 100 - Equivalent bandwidths of a 10 -mc, threecolor picture show the line of maximum sharpness. Fig. 6 A three -dimensional plot of the sharpness -match data from ten observers, statistically analyzed and put into terms of bandwidth, is presented in Figure 6. The percentages of the total band of ten megacycles allotted to blue and green are shown on the base plane. Red would, of course, occupy the remainder of the band. The equivalent bandwidth of the gauging picture ( all three projectors focused identically ) is plotted vertically. All of the points on the vertical, or equivalent-bandwidth, axis form the top surface of the plot. Wherever the surface lies above ten megacycles, the color picture looks sharper than would the picture transmitted by a ten -megacycle band equally apportioned. The maximum sharpness equivalent to a 15.6 -megacycle picture occurs when 63 per cent of the total band is allotted to green. In other words, a ten -megacycle test picture ( each projector out of focus by a different amount) with 6.3 megacycles devoted to green looks just as sharp as a 15.6 -megacycle picture with 5.2 megacycles devoted to each color. It is also interesting to note that the remaining 37 per cent of the total band can be allotted to red and blue in any proportion. Many experiments of this type have been performed by engineers at the Laboratories and elsewhere, some of them earlier than this one, and they all point toward the same conclusion: television in three colors can be made to look just as sharp as television in monochrome without paying the price of three times the bandwidth. Such information is the foundation of the latest system of color television transmission adopted by the Federal Communications Commission. In practical terms, television researchers want to know whether the color picture sent by one television system will look sharper than that sent by another. This color television simulator is helping to fit together some of the pieces of this larger puzzle. In addition to increasing our understanding of some of the complex problems involved in the transmission of color television signals, this projection machine has also made some valuable contributions in the field of visual acuity in color. - - THE AUTHOR N1. W. BALDWIN, Ja3., a native of Portland, Maine, received the E.E. degree from Cornell in 1925 and the M.A. degree from Columbia in 1928. He joined Bell Laboratories in 1925, and has been associated with the Research Department since that time. His primary concern has been television research, with particular emphasis on subjective aspects. During the war he was involved in the development of an automatic tracking anti -aircraft radar. He is a member of Eta Kappa Nu and of Tau Beta Pi. He is a Fellow of the I.R.E., and is currently Chairman of its Standards Committee. 484 BELL LABORATORIES RECORD A Modern Crossbar PBX O. H. WILLIFORD Special Systems Engineering l Private Branch Exchange (PBX) customers differ widely in their telephone some have only a few extensions and others have thousands requirements of extensions in hotels, department stores, and large industrial establishments. Most businesses, however, require less than sixty extensions, and demand has been increasing for a modern dial PBX system to serve such customers. The new 756A PBX is a small "packaged" design that matches the decor of modern offices. In addition to the conventional PBX services, the 756A provides a "camp -on" feature to permit automatic connection when a line becomes free. - To most people, a PBX is a telephone switchboard, frequently located in or near the reception room of a business establishment and used for servicing calls over the company's telephone extensions. In a dial PBX, however, the switchboard is only one part of the total equipment; much of the work of interconnecting telephones is done auto- matically by electromechanical switches. The switchboard in a dial installation might thus be considered as a sort of "control panel," with the relays, switches, and other equipment out of sight. In small manual PBX systems, much or even all of the "behind -the- scenes" equipment can be mounted within the switchboard itself. On the other hand, even a small dial system requires switching equipment and power equipment mounted in additional cabinets. As a consequence, the installation of present -day systems occasionally offers difficulties because a particular building may have narrow doors, too small an elevator. or insufficient floor bracing. These problems have been solved by the new 756A crossbar dial PBX developed at the Laboratories. Many telephone customers requiring dial PBX service have between 20 and 60 extension lines and DECEMBER, 1957 10 or fewer central -office trunks, and the most economical arrangement for serving this field is to provide only one or two "packaged" systems. The 756A is a small packaged design supplied in capacities of either 40 or 60 lines and equipped for 6 central- office trunks. Four additional central -office trunks can be added as desired. The basic switching components are crossbar switches and wire -spring relays. The 756A is also the first equipment to use the recently -developed "2 -in -1" wire -spring relay that provides two relays in the space normally required for one. Physically, the entire PBX is housed in two modular cabinets of steel and aluminum ( Figure 1 ) The depth and height are similar to those of standard office filing cabinets, and all maintenance is done from the front. Each module contains three small relay racks arranged so that they can be pulled forward out of the module like vertical file drawers. Each rack mounts on ball-bearing telescoping slides. Interlocking latches prevent more than one rack being pulled out at a time and, since each weighs about the same, the combined weight of the other two racks is sufficient to keep the cabinet steady. For this reason there is no . 485 need for floor bolts when installing this system. The PBX cabinets are designed for mounting in regular office space, along with file cabinets or other office furniture. Since an extended rack projects no further than a standard file drawer, the aisles do not need to be widened. The cabinets are of modern styling, finished in a shade of beige -gray that has become popular in office furniture. Sound -absorbing material reduces the noise of the switching equipment. The PBX is connected to the office wiring by plugs and pre- arranged jacks. A total of three equipment modules are provided, from which the twin- module 40 -line or 60 -line 756A may be constructed. One module is common to both 40 and 60 lines, and the second contains additional equipment for either the 40 -line or the 60 -line system. A new modern attendant's console, shown in the illustration at the head of this article, has sufficient capacity to handle all ordinary calls requiring the assistance of an attendant. This console features push-button keys and simplified operation. Since it is assumed that the attendant will have other duties as a secretary or receptionist, the console is small enough for table- or desk -top mounting. Smaller systems may use a six -button telephone set instead of the console. With some installations a switchboard may be more desirable than either the console or the six- button set. In such cases the Fig. 1 486 - The 75Ót1 crossbar PBX dial equipment. 60 -line system can be used with a switchboard. The power supply is completely self -contained, and is mounted in the first or basic module. Commercial 115 -volt, 60 -cycle ac is stepped down in voltage, rectified, and filtered to produce 48 volts dc for use in the PBX. The filter contains a large capacitor that stores sufficient electrical energy to maintain the voltage during momentary interruptions of the commercial supply. Where interruptions are apt to be of appreciable length, an additional modular cabinet can be supplied. This cabinet contains storage batteries as a reserve power source, and a rectifier keeps the batteries charged. Ringing current, signaling tones, and flashing interruptions are generated by equipment in the power -supply unit regularly furnished. While most present -day dial PBX's use step -bystep switches, the 756A achieves small size and faster operation by using crossbar switches and common -control principles. It operates essentially as a small crossbar office; register circuits connect to calling lines and trunks to supply dial tone and receive called numbers. The called numbers are passed to a marker which determines the busy and idle state of called points, performs an idle -hunting function for grouped lines and trunks, and controls the establishment of talking connections. The crossbar switches establish all connections. Although only one marker is supplied, it actually contains two independent marker channels, used alternately on successive calls. Should trouble be encountered, the marker "times out" and makes a second trial using the other channel. The two registers are also used alternately unless one is busy. Busy tone is normally supplied by a busy-tone trunk but, should this trunk itself be busy, the marker operates a relay in the appropriate register, and the register supplies busy tone. Troubles are indicated by lamp displays. All conventional PBX services are available with the 756A. Calls to extensions are placed by dialing two digits; with two to seven as the number in the initial digit. Calls requiring the attendant are placed by dialing zero. Dialing an initial 9 causes connection to an idle central -office trunk and the calling station line receives a second dial tone from the central office. Where certain extensions are restricted from direct outside dialing, users must dial 0 for the attendant and ask her to get the number. If they dial 9, they will be intercepted and transferred to the attendant. Also, certain extensions, while not restricted from dialing local outside calls, will also be intercepted if used to dial BELL LABORATORIES RECORD the code reserved for long- distance telephone calls. Inward calls from a central office first cause the attendant to be signaled. She answers a trunk by its key appearance on the console and asks for the desired called party. She then operates a key to summon a register which furnishes dial tone, and then either keys or dials the desired extension number into the PBX equipment. The trunk is connected to the desired station which receives ringing current, and subsequent talking battery from the central office. The attendant may split the connection when desired until after conversation with the called party begins. After the attendant releases her keys, she may be recalled by the called party operating his switch -hook momentarily. Disconnects take place automatically without involving the attendant. A "camp -on" feature has been included in the 756A for inward calls from a central office. If the called station is busy, the attendant is so notified and can advise the calling party that he may wait if he wishes. After the attendant releases from the call, the trunk will "camp -on" until the line is free. As soon as the first call disconnects, the trunk is automatically cut through to the called line and applies ringing current. No further assistance is required from the attendant. This feature reduces waiting time for inward calls to busy lines and frees the attendant for other duties. The central office provides talking current for both inward and outward calls, and also provides current to ring the PBX attendant on inward calls. For intra -PBX calls those between two extensions ringing and talking current are supplied by the PBX. An additional circuit, called an "intercom" - - The 756A PBX circuitry being inspected at the Cleveland, Ohio, trial installation. - trunk, or junctor, is brought in on this type of call to supply the necessary currents and signals. Outward trunks to other PBX's are reached by dialing 8, and calls over these trunks are handled in the same manner as outgoing calls. This new PBX, modern in design and appearance, should fill the telephone needs of today's smaller businesses. The ease of installation and maintenance, the adaptability of its size and modular construction to the requirements of modern offices, and the new features offered should make the 756A system attractive both to customers and to the Operating Telephone Companies. THE AUTHOR O. H. WILLIFORD, whose home town is Greenwood, Miss., joined the Laboratories in 1920 and initially engaged in laboratory and field testing of step -bystep, panel, manual and No. I crossbar systems. During World War II, he was associated with various military projects, and at the close of the war he became concerned with the development of the No. 5 crossbar system, particularly in the design of the maintenance facilities. He was an active member of the group representing the Laboratories during the installation and cutover of the first No. 5 system in Media, Pa. Transferring to the Systems Engineering Department, he prepared engineering information on the No. 5 system for the use of Operating Company engineers in planning central offices. He was active in the Englewood DDD trial and thereafter in engineering studies related to the expansion of the customer dialing network. He is now in charge of a Special Systems Engineering group studying requirements for new PBX features and systems. DECEMBER, 1957 487 The "Twistor" Memory Device A. H. Bobeck tests memory characteristics of a "Twistor" device, which incorporates magnetic wires. A new concept in memory devices has recently been announced at Bell Laboratories. As a result of exploratory work by A. H. Bobeck of the Device Development Department, it may now be possible to design memory systems that are simpler to fabricate and more economical to manufacture than existing systems. The new concept, which has been given the name "Twistor ", may find extensive application in computers and electronic switching systems where rapid- access, high- capacity memories are necessary. With the "Twistor" concept, a memory array could be constructed in the familiar grid -like or coordinate pattern. This is the way present magnetic- core memories are arranged horizontal and vertical wires are interwoven, with a donut- shaped magnetic core encircling each intersection. A pulse of energy along one wire is chosen to be insufficient for magnetizing a core, but coincident pulses along both a horizontal and a vertical wire will magnetize the core in a circular direction. Pulses of opposite polarity magnetize the core in the reverse direction around the donut. In this way, one "bit" of information is stored at one coordinate point in the array. - ELIMINATES CORES I; TWIST Set -up for testing magnetic wire: 1. supplies circular field and I, in solenoid supplies longitudinal field, resulting in helical magnetization of the wire. 488 A "Twistor" array would be similar in appearance to the magnetic -core memory, but would have no cores. It would be constructed merely by interweaving copper wires with wires made of a magnetic material, much as window screen is woven. The "Twistor" gets its name from a characteristic of magnetic wire. Normally, such wire can be magnetized most easily in the longitudinal direction (in a straight line along its length). But if a torsional force is applied to the wire, the magnetization will prefer to lie along a helical, rather than longitudinal, path. Torsion may not have to be applied to the magnetic wire in a final device. The preference for helical magnetization may be "frozen" into the wire during processing. In a "Twistor" memory, the required helical field can be obtained by applying coincident pulses of BELL LABORATORIES RECORD current through the copper and magnetic wires. Since current causes a circular magnetic field around a conductor, the circular component is readily supplied with a pulse of current through the magnetic wire. The field resulting from current in the copper conductor is also circular, but at an intersection where the copper wire touches the magnetic wire at right angles, this field has a component longitudinal to the magnetic wire. Thus, to "write" information into 10 x 10 array, for example, current could be applied to wire number 7 of 10 vertical magnetic wires, and to wire number 3 of 10 horizontal copper wires. This would produce helical magnetization in the magnetic wire in the immediate vicinity of the (7,3) coordinate point. In this way, a "bit" of information can be stored in the "direction" of the helical magnetization. Although two pulses of current are needed to "write" information into a "Twistor" memory, only a single pulse is required for "readout ". The current is applied to one of the copper wires so that it will overdrive the longitudinal field in the reverse direction at any intersection where a bit is stored. A "readout" pulse of voltage will then be induced in the associated magnetic wire. Because the lines of magnetic flux along the helical path "wrap" the magnetic wire many times, a favorable increase in output signal is obtained. Thus, the magnetic wire is used both as the storage medium and as the sensing element. TEN BITS PER INCH Investigations are now under way to determine optimum size and composition for the magnetic wires. It appears that a conductor plated with magnetic material will be most useful. Diameters as small as one thousandth of an inch appear to be usable. At least 10 bits per inch may be stored along such a magnetic wire without adverse interaction. In conventional magnetic -core memory devices, conductors must be threaded through the cores to make up a suitable matrix. When a ferrite sheet is used, either a threading or a plating operation is necessary for suitably locating the conductors. However, with the "Twistor ", the ferrite material is completely eliminated and no threading or plating is necessary. Speed of operation and output are comparable to those of ferrite systems. Present indications are that the drive circuits for a "Twistor" array can be readily transistorized. Thus, a memory system using the "Twistor" concept will retain all of the advantages of ferrite -core or sheet systems, and will be much simpler and more economical to fabricate. An experimental version of the "Twistor" memory: information- storage section in center consists only of horizontal magnetic wires and vertical copper wires. Magnetic wire serves for both storage and sensing. DECEMBER, 1957 489 Telephone Communications for RAPCON Centers G. A. GIDDINGS Station Apparatus Development To handle air traffic in the vicinity of air bases, the Air Force is installing a number of Radar Approach Control (RAPCON) Centers. These Centers, which maintain high safety standards of aircraft control, require highly specialized communications service, both internally and with other control points. A modified version of the Bell System's 102A key equipment is being installed to provide safer, more efficient air -traffic service of this type. Since World War II the United States Air Force has greatly extended the use of radar for controlling military air traffic. Personnel operating a radar-control system are given a graphic display of the location, direction of travel and speed of all aircraft within the range of the radar set. To use the advantages of radar methods, Radar Approach Control ( RAPCON ) Centers have been installed at many Air Force Bases. These centers use radar for departure and landing control of all traffic within the traffic pattern of the air base, and extend this control from the visual limit of an airport control tower to about 50 miles from the base. The chief purpose of air-traffic control is to provide safe separation of aircraft in the control area, especially those under instrument flight rules. Modern aircraft compound the safety problem by virtue of the complexity of operation. The time element has become increasingly important because many jet planes now fly over 600 miles per hour. At this speed, the aircraft is traveling one mile every six 490 seconds, or ten miles per minute. Another important consideration is that the fuel consumption of jet aircraft is very high, particularly below 20,000 feet. Positive and definite landing preparations are therefore necessary, since even a lapse of a few seconds in traffic- control procedures could mean the difference between safety and disaster. The success with which RAPCON Centers control these high speed aircraft depends to a large degree on a rapid and efficient voice- communication system. RAPCON Centers are furnished with the No. 102A key equipment system. With this system, several attendants, or controllers, can establish direct communication with any one of many internal and external lines. This key equipment was originally designed for the Civil Aeronautics Administration to control civilian air traffic without the use of radar. Certain features have been added for the switching and other arrangements necessary to meet the operational procedures required by the RAPCON Center method of air -traffic control. BELL LABORATORIES RECORD No. 102A key equipment was originally arranged for one attendant at an operating position who had access to telephone lines only. Two controllers a duty controller and an assistant are normally required at each RAPCON Center position, and they have a relatively large number of direct air ground radio contacts. Thus the two attendants must be able to share the telephone facilities and to integrate closely the telephone and radio equipment. They can independently use either the same or separate telephone and key circuits, and either the same or separate radio equipment at the console. Each attendant has a transfer key so that the same telephone headset may be used on either telephone lines or Air Force -owned radio equipment; rapid transfer is possible from one to the other. The convenience and savings in time gained by this arrangement are of great value in the over-all operations. The transfer keys, line key units, and attendant's telephone-set jacks are mounted in the Air Force -owned consoles as shown in the illustration on page 490. - - Rapid and positive intercommunication between the operating personnel in a RAPCON Center is imperative. For example, when the controller in charge of departures ( departure controller ) finds that an aircraft has been inadvertently taxied onto an active runway, he must immediately contact the controller in charge of incoming traffic ( final controller ) to warn him of the existing hazardous condition. The final controller can then direct any aircraft on final approach to a temporary holding point to avoid a collision. An "override" feature permits this fast and positive intercommunication between operating personnel. It is arranged so that any attendant may have immediate communication with an attendant at any other position in the RAPCON Center, even though the called attendant may be talking on a radio channel or a telephone line. Figure 2 is a schematic diagram of the intercommunicating circuits between two positions. Either the duty or assistant controller, who share the equipment at position No. 1, can override the assistant controller at position No. 2 by operating the associated line key. This key connects the controller's telephone circuit at position No. 1- through an induction coil and an operated relay directly to the telephone circuit of the assistant controller at position No. 2. Operation of the key also lights the line lamp at the overridden position to inform the controller that he has been overridden by position No. 1. When the assistant controller's transfer key at position 2 is in the telephone position, a two -way talking circuit is immediately established between the controllers, whether or not the controller at position 2 is connected to a telephone line. When the key is operated for radio transmission, a relay opens the transmitting circuit from the controller at position 2 so that a one -way talking circuit is established from the controller at position 1 to the headset receiver of the controller at position 2. This circuit is not connected to the input of the radio transmitter because the message may be misunderstood by the pilot and confuse him. If the controller at position 2 wishes to talk to position 1 over this circuit, he operates the key to the telephone -lines position. The calling controller may interrupt any conversation in progress. ( Both sides of the called controller's conversation, either radio or wire, are heard by the overriding controller through the sidetone circuit.) Similarly, either the duty or assistant controller at position 2 can override the assistant controller at position 1. The override feature is normally arranged so that override calls can be made to only one (assistant controller) of the two attendants at a position. During light -load periods, however, the position may be operated by only one attendant ( duty controller) For this case, the assistant controller has a third position of his transfer key, as shown in Figure 2. This position is called "single operator." When the duty controller is alone at the position, the assistant controller's transfer key is operated . - DECEMBER, 1957 Fig. 1-G. A. Giddings examines arrangement of modified 102A key unit for RAPCON operation. 491 to this single-operator position. This connects the assistant controller's receiver circuit to the duty controller's receiver circuit and extends the override features to the duty controller. It is very important to record the conversations of the attendants, especially when they talk to pilots. A recorder jack panel is provided so that any one of the Air Force -owned continuously-run ning tape recorders may be connected to record all the radio and telephone conversations concern- ing aircraft control. All radio transmitters use a push -to-talk arrangement and, to permit hands -free operation, this is usually done with a footswitch at the position. However, this footswitch may be bypassed by operating a footswitch- shorting key in the front of the console. When this is done, the radio transmitter is actually keyed with the locking or nonlocking push -to-talk POSITION NO. I POSITION NO. 2 ASSISTANT CONTROLLER TO RADIO CHANNELS i I ,Q rI TRANSFER KEY ice, j RADIO INDUCTION COIL OVERRIDE INDUCTION COIL - RADIO OVERRIDE RELAY DUTY OR ASSISTANT SINGLE OPERATOR TELEPHONE TRANSFER RELAY L CONTROLLER (Th - OVERRIDE LAMP LINE KEY INDUCTION COIL TO TELEPHONE LINES -- DUTY CONTROLLER TRANSFER KEY i TO RADIO o CHANNELS RADIO 1 TELEPHONE C RADIO TRANSFER RELAY Fig. 2-The override feature provides rapid communication between controllers in emergencies. CONTROL TOWER RAPCON CENTER LINE SELECTION KEY LINE 31 'LINE 2 LINE 1 > LINE 3 -! LINE _TO OTHER LOUDSPEAKER SL RELAYS THROUGH INTERMEDIATE SL RELAYS 1 LINE 2 LOUDSPEAKER TO OTHER SL RELAYS S TO r - ii (n) ' -0 L___ J SWITCHHOOK A ',HANDSET - LAMPS AT OTHER POS ASSOC WITH SAME TRANS LINE i LINE LAMP LINE LAMP v á LINE KEY POSITION NO, 1 LINE KEY POSITION NO. 2 During periods of heavy traffic or in an emergency, the "hot line" arrangement makes possible direct communication between the RAPCON Center and the airport control tower. Fig. 3 492 BELL LABORATORIES RECORD switch in the cord of the telephone headset. The 53 -type headsets used have a 12 -foot retractile cord, which permits the attendant to move from his position and still talk on radio channels. One -way "hot" lines ( lines continuously open for transmission ) are installed in the system to provide fast, direct and positive communication between a RAPCON Center and the associated airport control tower. This arrangement is needed in periods of very heavy traffic, and especially so in case of an emergency. Normally one hot line runs from RAPCON to the tower and either two or three from the tower to RAPCON. Figure 3 shows the special termination arrangement for these hot lines. The tower controller can talk immediately to a position in the center by lifting his telephone handset and operating a line -selection key to the line associated with the called position. The switchhook contacts in the handset transfer the "receive" line from the loudspeaker to the handset receiver and connect the talking battery to the handset transmitter. The controller at position 1 in the RAPCON center can talk immediately to the tower by operating a line key which activates a relay associated with that position. The relay performs four functions. (1) It lights the line lamps associated with the transmit line at that and all other positions to indicate that the line is in use. (2) It transfers the recei