L SEP '8A IBAY{ Gy August 1961 Electronic Memory Devices (4B aQ- Bell Laboratories Measurin g Sky'sS 's Electrical Noise `4 Surface -to -Air Data Communications The Art of Counting Calls A Statistical Ammeter www.americanradiohistory.com F. J. Singer, Chairman W. M. Bacon J. A. Burton Editorial Board J. W. Fitzwilliam E. T. Mottram R. J. Nossaman W. E. Reichle H. W. Mattson, Editor A. G. Tressler, Associate Editor Editorial Staff J. N. Kessler, Assistant Editor, Murray Hill M. W. Nabut, Assistant Editor T. N. Pope, Circulation Manager THE BELL LABORATORIES RECORD is published monthly by Bell Telephone Labora- tories, Incorporated, 463 West Street, New York 14, N. Y., J. B. FIsK, President; K. PRINCE, Secretary; and T. J. MONTIGEL, Treasurer. Sub- scription : $2.00 per year; Foreign, $2.95 per year. Checks should be made payable to Bell Laboratories Record and addressed to the Cir- culation Manager. Printed in U. S. A. © Bell Telephone Laboratories, Incorporated, 1961. www.americanradiohistory.com Bell Laboratories RECORD Volume 39 Number 8 August 1961 Contents PAGE 270 Electronic Memory Devices D. H. Looney 276 Measuring the Sky's Electrical Noise D. C. Hogg and H. E. D. Scovil 280 Portable Ruby Optical Maser Demonstrated 281 A New Surface -to -Air Data Communication System J. B. Bishop 285 The Art of Counting Calls W. B. Callaway 289 A Statistical Ammeter J. W. Osmun 292 Ferrite Isolator: New Kind of Pad for TJ Radio -Relay A. F. Pomeroy This ferrite sheet, with its printed wiring contacts, is part of a memory Cover module -one of the many types of memory devices discussed in a review article which begins on page 270. www.americanradiohistory.com Latest development in twistor memories for large wImt, demonstrated by U. F. (; ?Iola (left) and J.A. capacity storage is this card -changeable arrange- Ruff, Solid State Device Derel- )pment Department. 270 Bell Laboratories Record www.americanradiohistory.com Automatic communications systems, with their emphasis on speed and reliability, need fast and dependable memory devices. ¡"or this purpose, communications technology/ has presented the systems designer with several interesting choices. D. H. Looney Electronic Memory Devices Telephone switching systems, and other com- that identifies the physical location of the incom- munications systems of many types, are now be- ing line on a particular frame of equipment. This ing developed to operate at extremely high speeds, number will exist unchanged until the customer measured in millionths of a second. The reason moves or requests a different class of service, or for this emphasis on speed is partly a desire to until the offices revises this equipment number perform communications services faster, but it is for some reason. also a desire to process a given amount of infor- Another form of information that may be stored mation with less equipment. Thus, if the prices for a considerable period is an "instruction." An of high -speed circuits can be held to reasonable ordered set of instructions is a "program" which levels, significant savings may result. prescribes the decisions and sequence of opera- "Memory," or storage of information, is only tions necessary to accomplish the designed pur- one part of a communications switching system, pose of the system-processing telephone calls. but it is a very important part. It ranges from Present -day telephone offices are programmed storing an item of data for only a very short entirely by wiring patterns interconnecting period -maybe only a fraction of a second -to switches such as relay contacts. Programs for storing another item for perhaps days or months. future telephone offices will be stored in electronic For example, when a customer dials a telephone memories as lists of instructions and changed number, memory circuits in the local central office whenever new features or services are added. may be required to store all or parts of this num- These concepts are important to an appreciation ber for only a few seconds. On the other hand, of the many new methods of high- speed, electronic other data may be stored indefinitely. The line memory. They explain, for example, the classifi- coming into the office from a telephone, for in- cation into "temporary" and "permanent" mem- stance, has an "equipment number " -a number ories, and they introduce the important role that August 1961 271 www.americanradiohistory.com memory plays in directing a stored program electronic switching system. In this discussion, it would be impossible to include every new device and idea being consid- ered for memory applications. Also, in such a rapidly changing field, it is difficult to gain enough perspective to separate the significant develop- ments from the many interesting proposals that may never become economically practicable. The few examples here, however, should give some of the flavor of contemporary work on memory sys- tems, and should emphasize the significance of modern electronics for future communications. Aside from the general distinction between temporary and permanent memories, storage sys- tems can be categorized in many ways. One of the most basic is to think of them according to the phenomena responsible for the storage. We can imagine almost any discrete effect which exists in two states -for example, a liquid solution can be either electrically conducting or nonconduct- on these photo- ing. As long as the effect can swing or be Bits of information are recorded tiny clear or opaque dots. "switched" from one state to another, it can be graphic plates as used to design a memory. Two effects used in electronic memories now altered. In this manner, the spot stores a small under field trial in the Bell System are the pres- quantity of electrostatic charge. Its presence or ence or absence of an electrostatic charge on a absence specifies one binary digit, or "bit" of sheet of an insulating material, and the presence information. or absence of an exposed area on a photographic In barrier -grid tubes now being studied, a film. The electrostatic phenomenon is the basis for typical storge array consists of a square pattern the design of a barrier -grid temporary memory, of some 16,284 spots (128 by 128) . Each spot is and the exposed film is the basis of the flying -spot only about 5 thousandths of an inch in diameter. store or permanent memory-both used in the Because of such small dimensions, the electron electronic central office currently under field trial beam must be positioned and deflected very pre- at Morris, Illinois (RECORD, December, 1960). cisely. After a charge has been stored on a spot, it Barrier -Grid Store can be removed ( "read out ") and a new charge The barrier -grid store is a system having a inserted ( "written in ") in a cycle time of 21/2 the vacuum tube in which a beam of electrons is di- millionths of a second. The beam can detect rected to the surface of a sheet of mica. Im- presence of charges ( "interrogate ") on only one mediately in front of this sheet is the "barrier area at a time, and can thus read out only one bit -grid tube is grid " fine screen or mesh of wires that at a time. Consequently, the barrier -a bits controls the secondary electrons falling back on particularly useful in systems which handle tubes are the mica sheet when the state of the memory is serially. If a number of barrier -grid changed. The homogeneous mica surface is di- operated in parallel, however, the output is a read vided into a large number of discrete storage "word" of several bits of information, all areas or "spots." When the beam impinges on out simultaneously. a particular area, the average number of elec- In the permanent memory of the experimental trons arriving at the surface would ordinarily Morris Central Office, the data bits appear as equal the number of secondary electrons leaving. transparent or opaque areas on a photographic However, a metal plate is mounted in back of the film. Like the mica sheet of the barrier -grid tube, mica sheet, and when the voltage on this backing a single section of film is divided into a square plate is properly adjusted, the number of elec- array consisting of a large number of storage trons which stick to the mica can be temporarily areas. In this case, however, the information is Record 272 Bell Laboratories www.americanradiohistory.com FORWARD LOSS REVERSE LOSS RETURN LOSS 40 1.2 35 1.0 30 0.8 25 0.6 20 0.4 11.8 11.0 11.4 11.8 10.6 11.0 11.4 11.8 10.6 11.0 11.4 FREQUENCY IN KIIOMEGACYCLES PER SECOND changes Loss curves for a typical lA Isolator. The curves most of the frequency band. Temperature the performance. reveal a "front -to- back" ratio of 2,690,000:1 over do not appreciably degrade A permanent mag- zinc alloy. These covers support the pole pieces or by ac or dc magnetic fields. it is partially demag- and keep dust from gathering on the magnets. net is said to be stabilized if fully magnetized. The hys- The pole pieces, made from cold -rolled steel, are netized after being a stabilized permanent aligned with the ferrite bars to achieve the de- teresis characteristic of as that of a sired magnetic field configuration. magnet has the same general shape The operation of a field displacement isolator fully magnetized magnet. stabilization is previously (RECORD, November, A simple way of thinking about has been explained magnet 1957). In this rapidly expanding art, theoretical as follows. To magnetize a permanent is applied. This considerations are subject to continuing evolu- fully, a very strong magnetic field overcoming the resist - tion as experimental observations add to the fund field can be thought of as most stubborn single do- of knowledge.