Characteristics of Flying Spot Store

Transcription

1 High-Speed High-Capacity Photographic Memory C. A. LOVELL THIS PAPER concerns an information store which represents a considerable achievement in the development of photographic storage. Some suggestions will be made for the use of such a store in digital computers. Credit for the achievement belongs to those who developed the store. Stored program control of digital computers is about 15 years old. Its advantages are so numerous and important that all general-purpose computers today use it to the limits of the capabilities of the storage systems available. There is a strong movement toward greater use of fixed programs of many types to reduce human effort in programming computers. Every computation laboratory has a much-used library of such programs. \Vhen in actual use, these programs are stored in high-speed erasable memory because no other type of memory in existence has the required access speed. This has created demands for enormous memory capacities and the demands continue to grow. Although the erasable memory costs are usually a large fraction of the total cost of a computer, the memory capacity is frequently found to be grossly inadequate. The erasable property is not essential for fixed pr0gram storage. High speed readout, high capacity and lower costs are the requirements for a satisfactory store. The subject of this paper is a word-organized permanent information store having these properties. This store is expected to be suitable for program storage and other digital computer uses. Fig. 1. ACRT r / l. J!' r..., :" Flying spot store circuit organization The Flying Spot Store The Bell Telephone Laboratories has developed a photographic information store! which, with presently known techniques, can be extended to have the characteristics shown in Table 1. It is a 10- megabit word-organized permanent information store with random access time of 5 microseconds per word, which will cost on the order of 1/2 cent per bit. The present state of the development of the store will be described here so that the reader can see how much of the predicted characteristics have been achieved to date and how much is promised through extrapolation of the techniques. The stores which will be described as the "present form" of the flying spot store has a larger capacity and is somewhat slower than the store which was designed for control of telephone switching systems. Capabilities of the components are such, however, that the present form of the store can be constructed from them. I t was chosen for discussion in the belief that it is better for digital computer use than the smaller store designed for control of telephone switching systems. CIRCUIT SCHEMATIC Fig. 1 shows a diagram of the store. Except for minor detail, this diagram represents the present and predicted store equally well. The storage medium is a set of photographic plates each of which has a number of separate storage areas which will be called channels. A cathode-ray tube is used as a light source for writing and reading the stored information. A number of lenses, equal to the number of channels, are used respectively to focus the spot of light on the storage areas of the photographic plates. Random-access de- Table I. Characteristics of Flying Spot Store Bit Capacity , bit-word or 10 7 bits Random Access Readout Time... 5 JLs/word Writing Time for 10 7 Bits minutes Time to Fill Memory from a Library of Plates... I to 2 minutes Shop Costs... Of the order of II::! cent per bit capacity Fig. 2. Flying spot store physical organization flection circuitry to position the cathode ray beam is provided. There is a photodetector to read information stored in each channel. A light intensity stabilizer, input or address registers, and output information registers, not shown on the schematic, complete the list of major functional components of the store. PHYSICAL ORGANIZATION OF THE STORE Fig. 2 shows the physical arrangements of the cathode-ray tube, lens array, and photographic plate. There are four information plates, each containing 19 separate storage areas which provide a total of 76 storage areas or information channels. The lens for each channel focuses the used part of the cathode-ray tube face on the storage area and sets up a one-toone reciprocal correspondence between differet!tial elements of these, surfaces. The capacity of the store is the product of the number of bits per channel and the number of channels. A word contains one bit from each channel. The address of the word is the binary address in rectangular co-ordinates of the spot of light on the cathode ray face which illuminates the word. Photo detectors used in reading are not shown on the slide. The array of channels seen in the form of a cross in Fig. 3 is used as a part of the means for getting accurate control of the beam deflection. SERVO CONTROL OF BEAM DEFLECTION The resolving power of the photographic film is greatly in excess of that of ordinary cathode-ray tube and deflection circuitry. Since the optical system merely maps continuously the cathoderay tube face on the photographic plates the channel capacity is limited by the resolution obtainable in the cathode-ray tube, lens system and deflection circuitry. The most significant advances made in the development of the flying spot store are C. A. LOVELL is with Bell Telephone Laboratories Whippany, N. J. 34 Lovell-High-Speed High-Capacity Photographic Jfemory

2 Fig. 3 (left). Servo beam encoder prates in the development of high-speed highresolution cathode-ray tubes and deflection means. The beam deflection is controlled by a servomechanism, the construction and operation of which will now be described. A number of channels contain code plates from which can be read at all times the horizontal and vertical positions of the light spot on the cathode-ray tube. There is a servo channel for each binary bit required to define the beam position. Thus 8 channels are required for 256 positions in one co-ordinate and 16 channels will define (256)2 positions on the cathode-ray tube face. In operation the input registers are set to indicate the address of the desired cathode-ray beam position. This information and that of the beam position, read from the servo plates, are compared by digital logic circuitry which gives as an output an error signal suitable for driving the beam to the desired address, where the error signal vanishes and the beam comes to rest. Such a servo exists with a resolution of 256 positions per co-ordinate and an effective bandwidth of about a half Fig. 4 (right). Optical assembly of Hying spot store megacycle. The servo differs from the one described in the referenced Bell System Technical Journal article in two respects. I t has 8 servo channels per coordinate instead of one in the beam position encoder. There is a moderately accurate digital-to-analog converter superposed on the servo to speed the action of the beam while moving between widely separated spots. These improvements result in better resolution and faster random access to stored information. The lo-megabit store will require a 9-bit address per co-ordinate. LIGHT INTENSITY STABILIZER There are 17 channels in the servo plate but 16 are sufficient to define (256) 2 beam positions. The deflection servo acts on digitally quantized error signals while the beam is more than one cell from the desired address. However, when the beam nears the balance point, the error signal becomes a linear function of the error giving a more conventional analog servo. This servo is in balance when an edge of a code plate intercepts half the light from the cathode-ray tube. Absolute uniformity of light over the cathode-ray tube face is difficult to achieve with a given beam current and more difficult to maintain under nonuniform use conditions. A central channel is furnished to measure the light intensity in every beam position. The intensity is compared to a standard to produce a signal which is fed back to modulate the beam and hold the light intensity constant. In this way it is possible to recognize the half-light balance point of the analog servo. The light reference channel occupies the 17th position in the plate and is of course transparent over its entire area. Fig. 5 (left). Side-front 3! 4 view of Hying spot store optics Fig.r6 (right). A 19-channel information plate Lovell-High-Speed High-Capacity Photographic Afemory 3f>

3 Fig. 9. Cathode-ray tube for Flying spot store Fig. 7. An information channel Fig. 11. Close-up of monitor scope showing beam traces Table II. Comparison of Flying Spot Sto re Characteristics Present Store Predicted Extensi:>n Fig. 8. Enlarged section of a test plate THE WRITING PROCESS The store is used to write information in its own storage channels. Servo deflection control is used in writing as well as reading. In the writing operation all channels except one are closed by shutters and the information is written into that channel. Since the beam must be left on for servo operation the writing is done by scanning the entire raster, stopping for a timed interval on each of the cells into which ones are to be written. The beam passes over the cells which are to contain zeros so rapidly that it does not expose the cell. The time required to expose a one varies with film speed and can be of the order of 250 microseconds (tlsec). Since the information stored in a channel contains a bit from each word, some data processing is required to assemble the information in a form suitable for writing by channels. This is done automatically by a digital computer and stored on magnetic tape for use by a plate exposure unit. PHYSICAL ASPECTS OF PRESENT STORE Fig. 4 shows the optical assembly of the store as seen from the photo detector positions. In the foreground are mounted condensing lenses which are placed be- Fig. 10. Monitor scope in parallel with flying spot store tube tween the information plates and the photodetectors. The condenser lenses have been removed from a part of the assembly so that the lenses which focus the light spot on the information plates may be seen. The information plates are not shown in this photograph. They will be mounted on the frame which is seen behind the condenser lenses. The horizontal bars are shutters used in writing. Fig. 5 shows another view of the assembly. Fig. 6 shows one of the four I9-channeJ information plates. Fig. '( shows a photograph of a 256 X 256-bit channel with a one written in every cell. Fig. 8 shows an enlarged section of an information plate with a pattern of spots recorded for test purposes. A one is represented by a transparent spot in an opaque background. This is achieved by a reversal which changes the negative resulting from the writing operation to a positive during development and fixing processes. This reversal gives a large improvement in signal-to-noise ratio. Fig. 9 is a special cathode-ray tube developed in the Laboratories for use in the store. This tube has sufficient resolution over the phosphor face to read and write reliably channels of (256) 2 bits each. It has a small high-intensity light spot and a fast phosphor. The light spot is less than 2.5 mills in diameter over a 6 1 / 4 inch X 6 1 / 4 inch raster. No deflection de-focus corrections are required. Bit Capacity... (256) 2 76-bit , bitwords words Random Access Read... 4 J1.S per word.. 5 J1.S per word Writing Time min. per.. 45 min. per 10 7.') X 10 6 bits bits Time to Fill Memory from Library... 1 to 2 minutes.. 1 to 2 minutes Shop Costs... Order of 1.. Order of l/z cent per bit cent per bit Fig. 10 shows a monitor scope with plates connected in parallel with those of a flying-spot store tube. The pattern of spots shown on the face represents part of the lllain program for controlling a telephone switching system. This program is repetitive so that a static pattern is shown in the photograph. Some spots are used more often than others as may be seen from variations in exposure in the photograph. Fig. 11 shows a close-up of the face of the monitor scope. Traces of the paths of the beam in going from spot to spot seen on both slides emphasize the random nature of the access to information stored on the photographic plates. This completes a brief description of the store. Comparisons of its characteristics and those of the predicted extension are shown in Table II. It is seen that the extension simply assumes that the information per channel can be doubled and that the larger capacity store will be a little slower. Cost per store would not be increased appreciably. There are under consideration a number of variations of the flying spot store which promise considerable increases in capacity, more convenient writing processes. and other improvements. Some of these advantages will be realized but characteristics of the resulting stores cannot be described at this time. Today these pos ;36 LoveU-High-Speed High-Capacity Photographic }.femory

4 sibilities will be ignored and attention will be centered on the lo-megabit store which has been described. Digital Computer Applications COMPARISON WITH EXISTING STORES The flying spot store differs materially from any store now used in digital computers. It is a fixed information store with a capacity about one third that of an average reel of magnetic tape. However, the fast random access on readout is comparable with that of fast erasable memories. While the write time is considerably slower, this fact is relatively unimportant for fixed information. Such a store should have a great many uses in digital computers. What are some of these uses? MACHINE LANGUAGE TRANSLATION The store described is proposed for use in machine language translations. This subject has received considerable attention during the last 12 years. In 1956, Wa112 described some of the engineering aspects of the problem. He estimated that 40 megabits of permanent information storage is required and that a machine must translate about 10 words per second to compete, costwise, with human translators. He postulated a photographic store with cathode-ray tube scanning for reading stored information and a repetition rate of 100 kc. Four photographic stores such as the flying spot store described will provide the required capacity. The search routine is much more efficient than that assumed by "Vall. If the storage is alphabetized and every word is found through searching the memory an average of 384 beam positions would be required per word. The time required is 5 J.1.sec per beam position. Hence the average search time per word is less than 2 milliseconds. Thus between 10 and 500 words per second could be translated depending on the number of multiple searches required' 011 the average. In estimating the storage capacity required, Wal1 assumed it would be necessary to store more information than is 10gicaJ1y necessary in order to speed up the search operations. The speed of the store described is such that a better balance might be achieved by reducing the information stored to approximates more closely than logically required. In this case a number of stores less than four may be sufficient. It is apparent that the store described is a practicable permanent store for machine language translation. SOLUTION OF LOGICAL PROBLEMS The flying spot store was developed to solve problems in this class 3 and its usefulness in this field is established. It is improbable that stored program control of a large telephone switching system is the only problem in the class worth solving by this technique. The author suggests as an application the problem of air traffic control in the neighborhood of a large airport such as may be expected to exist 10 or 20 years from now. The problem of control of all the traffic lights in a large city under the changing conditions which exist from hour to hour, day to day, and variations of longer periods appears to be another suitable application. Acute problems, created by special events, can be handled effectively by stored programs in both proposed applications. These examples of logical problems are characterized by having a large number of more or less acceptable solutions. They are also problems in real time involving large number of variables about which the programmer has certain statistical information. His problem is to write a program which gives solutions that are optimum in some sense. The method of attack on these problems is indicative of the flexibility and power of stored program control. There must be an instruction for every combination of the input variables which requires action. Each particular combination of inputs is caused to generate an address suitable as an input to the flying spot store. At this address is stored either the instruction specifying the action to be taken or a part of the instruction and the address of the next part, and so on until the entire instruction is given. When more than one event requires action at the same time, a main program specifies an order for acting on them sequentially. The real-time aspect of the problem is to achieve a speed such that the maximum delay is inconsequential or tolerable. It is readily seen that an instruction of almost any length and degree of complication can be given as a result of a single input address, so long as sufficient information capacity and speed are available. :Y.I:ore will be said about this. STORED PROGRAM CONTROL OF COMPUTATION Consider an application of more immediate interest; i.e., use of programs stored in a flying spot store to control computations. An application of this sort which is easy to describe is the storage of the library of programs, routines and subroutines used in computation labora- tories to reduce human programming effort. Every computation laboratory has such a library stored usualjy in a form suitable for their input units so that the desired programs may be read into the erasable memory as needed. The availability of a low-cost fast-access permanent store as an internal computer component for this purpose offers advantages which may be taken in the following ways. The temporary memory can be reserved almost entirely for variable data relating to the problem, thus making it possible to do much larger computations in one step_ The low cost per bit of capacity makes it feasible to store permanently an extensive library of routines of all sorts. The routines most frequently used can be assembled on one set of plates, while others can be made available merely by changing plates. Among the routines stored might be rather extensive diagnostic computer test routines which should lead to lower maintenance costs and less down time for the computer. The entire library of programs which have been assembled at the Murray Hill Laboratory for use with an International Business Machines Corporation 704 computer would occupy only about 15 per cent of the capacity of one set of plates of the flying spot store described. Many computation laboratories have such extensive libraries that they may exceed the capacity of one set of plates. The lo-megabit capacity provided in this store certainly makes feasible a much more extensive development in fixed computer programs than has been possible heretofore. So far nothing has been said about possible changes in program structure which might be advantageous if a flying spot store is available. In a manner analogous to the use of a stored program to solve logical problems, a routine of any length or degree of complexity can be started by sending an 18-bit address to such a store. The resulting 76-bit readout makes it possible to use the computer as a 2- or 3-address computer or to operate on variable word length instructions. The nature and speed of access to the information stored allows the use of routines with many branches, re-entrant points and other interrelations. More and more of the programming work is being done by computer manufacturers. With the types of store described the manufacturers could not only furnish efficient and convenient programs with their computers, they could write the programs permanently on information plates ready for insertion in the store. The programs written for such memories could be problem Lovell-High-Speed High-Capacity Photographic Memory 37

5 oriented a!1d each customer supplied with a number of programs best suited to his particular class of problems. This in effect makes a general purpose computer into a number of special purpose computers for ease and convenience in solving each customer's particular problems. Summary A photographic store has been described and an extension proposed for use in digital computers. Specific applications suggested include: 1. a difficult translation problem; 2. solutions of a class of logical problems; 3. storage of programs of several sorts to reduce human efforts in programming or gain other advantages in digital computer operations. In closing, the question is asked, "If you had a store such as that described, how would you use it?" References 1. FUNDAMENTAL CONCEPTS IN THE DESIGN OF THE FLYING SPOT STORE, C. W. Hoover, R. E. Staehler, R. W. Ketchledge. Bell System Technical Journal, New York, N. Y., Sept. 1958, p SOME OF THE ENGINEERING ASPECTS OF THE MACHINE TRANSLATION OF LANGUAGES, Robert E. Wall, Jr. AlEE Transactions, vol. 75, pt. I, Nov. 1956, pp AN EXPERIMENTAL SWITCHING SYSTEM USING ELECTRONIC TECHNIQUES, Amos E. Joel, Jr. Bell System Technical Journal, New York, N. Y., Sept. 1958, p Discussion G. R. Tiannaca (Rome Air Development Center): Did you have a problem of phosphor burning on the cathode-ray tube? If so, how was this problem overcome? Mr. Lovell: There was a phosphor burning problem which was solved; in part by moving the tube physically and continuously so that the entire phosphor surface is used In low-speed data transmission applications (approximately up to 200 bits per second) the Frequency Modulation (FM) systems can tolerate about 10 db (decibels) more white noise and maintain the same systems performance as Amplitude Modulation (AM) double-sideband systems, in respect to speed or bandwidth utilization. The immunity to level variation is one of the crucial factors which paved the way for FM into telegraph carrier systems. 1-3 The same arguments do not apply entirely for high-speed data transmission, where other factors have to be considered. In low-speed applications, with many carmore or less uniformly; in part by changing the phosphor and its method of deposition. C. A. R. Kagan (Western Electric Company): To what extent would you consider this photomemory suitable, technically as well as economically, in the performance of arithmetic computation using logic table look up in memory in lieu of chains of toggles or other forms of serial or parallel adders? Mr. Lovell: I have not thought much about the use of such a memory in arithmetic computation in the manner described. It certainly could be used in this manner. However, for such simple and well defined operations I would guess that the usual forms of logic circuitry would furnish strong competition. The table look up method might win in cases where such a store is available and its capacity not fully used otherwise. A small amount of consideration has been given to this problem in the Laboratories. I am not familiar with the results and conclusions. An Experimental Modulation Demodulation Scheme for High-Speed THE primary motivation of the group in this experimental development was to find a suitably economic system in terms of reliability and equipment simplicity for transmitting binary data from point to point. To this end solutions, primarily with respect to private telephone lines were considered. At the heart of this problem is the right choice of modulation and demodulation schemes, so that before describing the system, one should survey the limitations of various approaches which were faced in the light of boundary conditions of the present telephone network. On-Off Modulation or Double Sideband Amplitude Modulation Systems The on-off modulation schemes are historically the first modulation schemes used for binary information transmission. The two binary states are characterized by the presence or absence of an information carrier frequency (subcarrier). This subcarrier is usually located in the Data Transmission E. HOPNER middle of the available frequency band. The advantage of such schemes is their simplicity. They are, however, sensitive to sudden amplitude variations of the line and are relatively vulnerable to noise. Since both sidebands are transmitted, better frequency spectrum utilization seems possible. Frequency Modulation Systems rier cycles per bit of information, the impulse noise does not represent a problem, since the bit is so much longer in duration than the impulse noise disturbances. At higher speeds where there is often only one cycle of the F.:M subcarrier per bit, the problem of vulnerability to impulse noise and to noise in general becomes severe. The FM capture effect loses effectiveness at higher data transmission speeds. The FM systems remain insensitive to amplitude vadations, even at higher speeds, and that makes them attractive, as long as the noise does not become a paramount problem. Since FlYI systems require both sidebands, they are like double-sideband AM systems in that they are not efficient in bandwidth utilization. Vestigial Sideband On-Off Schemes The vestigial sideband on-off modulation schemes operate with greater effi- E. HOPNER is with International Business Machines Corporation, San Jose, Calif. The work described in this paper is a result of a joint effort between H. G. Markey and the writer. The equipment was assembled and tested by O. F. Meyer, whose patience during many hours of testing is greatly appreciated. Dr. E. S. Kuh of the University of California gave substantial support in network synthesis problems. Pulse shapipg networks and phase equalization of experimental lines, which made the tests possible, were his contributions. The contribution of Dr. N. M. Abramson of Stanford University is in theoretical considerations of error probability and interbit interference. T. C. Kelly and C. M. Melas of the International Business Machines Corporation Research Laboratory of San Jose were helpful in materializing the phase correction and pulse shaping networks. 38 IIopner-Experimental AI odulation-demodulation