Patent GB-913971-A (GB)
Title: Improvements in thermoplastic film data storage equipment
IPC: G11B7/12
Filed: 1959-08-10
Abstract: 913,971. Digital data-storage apparatus. GENERAL ELECTRIC CO. Aug. 10, 1959 [Aug. 25, 1958], No. 27279/59. Class 106 (1). Digital data is stored on a low-melting and normally electrically non-conductive thermoplastic surface in the form of a diffraction grating for each bit, the data being read out by optical means. The grating is made by modulating an electron beam which scans the plastic surface so that the deposited electrons in an area about 5 microns square form a grating pattern of about 5 lines, and then heating the plastic to allow it to flow under the electrostatic forces set up between the deposited electrons and induced charges on a conductive backing layer, so that corrugations are set up which when the plastic film is cooled form a semipermanent grating. Erasure of information is by heating the plastic to a higher temperature so that it becomes electrically conductive, the electron pattern disappears and the plastic smooths itself by surface tension. Two embodiments are described in detail, one in which data is stored on the surface of a rotatable drum, and the other in which it is stored on the surfaces of flat plates which may be selectively withdrawn from a stack ; in both cases circuits for writing, reading and address selecting under control of signals from an associated computer (not described) are described in detail. Drum embodiment.-Figure 1 shows schematically the arrangement for transferring data between drum 11 and a magnetic core register 29 associated with computer 21, at an address on the drum ordered by the computer and set up in register 19. The drum is movable axially by motor 16 and angularly by motor 13 by two servo-systems of which one only is shown, that of motor 13, inside the broken line. This converts the digital address in 19 to an analogue voltage applied to selsyn 14, the rotor of which moves with the drum shaft to develop an angular error voltage amplified at 22 and applied to motor 13 to adjust the position of the drum. When the error voltage then vanishes, a brake 25 is brought into operation and a signal sent to the programmer 28 to indicate that data transfer may begin. Data transfer takes place with the drum stationary, the block of data read or written comprising 32 x 32 binary digits. Writing is effected by control unit 27 (shown in more detail in Fig. 2) which receives successively the bits read out of register 29 by the computer and controls the deflection voltages applied to electron gun 26 accordingly. Referring to Fig. 2, the deflection voltages for the gun are obtained from units 126 and 135, and the unblanking voltage for switching on the beam during writing is provided by driver 146. The timing is effected by a 320 kc/s. oscillator (Fig. 3) which shifts the data out of register 29 to a switch 129 to gate either 1.6 mc/s (binary " 0 ") or 1.136 mc/s. (binary " 1 ") oscillations to unit 127, and a 10 kc/s. signal obtained from it by a frequency divider which controls the sawtooth generators 128 and 137. A start pulse from the programmer 28 enables the next 10 kc/s. signal to switch flip-flop 138 to start vertical sweep generator 137, and generator 128 which is synchronized with the 10 kc/s. signal also starts a scan. A delay unit 141 then opens gate 142 for the next 10 kc/s. signal to start operation of a divider unit 144 which opens gate 139 to allow read-out from 29 to begin (gate 145) and unblank the tube. 32 bits are read out in the next horizontal sweep, and in each bit time the output of one of the oscillators 131, 132 is added to the output sweep voltage of 128 to produce a step-like voltage (having four or five steps) causing the electron beam to pause four or five times in its travel over the plastic surface. This action is repeated for 31 subsequent spaced horizontal scans, and the writing is terminated by counter 144 which switches 138 to stop read-out from 29, cut off the tube 26, and send a " finish " pulse to the program unit 28. This initiates the " curing " action described above, by R.F. heaters which heat the plastic film to 100-150 C. so that the areas of high electron concentration, where the beam had been caused to dwell on the plastic surface, are converted into depressions. The spacing of these depressions, depending on the frequency of the signals superimposed on the horizontal trace at the point, thus correspond to the binary digit read out from 29, and are suitable for refraction of blue and yellow light by the 1's and 0's gratings, respectively. Read-out is effected by blue and yellow light sources situated at 33 inside the drum (see Fig. 1) and optical system 34 with colour splitter 35-37, and videcons 38 and 39 receive colour images of the data block and scan these images to provide data output signals. The optical system is shown in Fig. 5, the two light beams from sources 375, 376 are arranged at 23.6 degrees to the normal at the thermoplastic surface 125, and illuminate the block of data being read out. A lens assembly 379 produces images of the " 1's " and " 0's " at videcons 38 and 39 (half-silvered mirror 37 and colour filters 35, 36 separating the diffracted coloured light) which images are scanned by the means shown in Fig. 3. Fig. 3 also shows schematically the output logic unit 41 of Fig. 1, and the master oscillator 301 with its frequency divider 304 which delivers the 10 kc/s. signal to the horizontal sweep generator 305 so that horizontal sweep voltages at 10 kc/s. are delivered to both videcons. At a " start of read-out " pulse from the programmer on line 314 a vertical sweep is commenced and a flipflop 321 switches on the videcons. Also flipflops 339, 341 and 342 which are coupled to form a scale-of-five counter are set to 000. The videcons start to scan the block in which data is recorded, but will not at first receive any light signals, since the first horizontal scan 356 (Fig. 4) will be outside the area of gratings 357 0  and 357 1  (illustrating 0 and 1 gratings on the film). The vertical sweep generator 313 is arranged to provide 5 horizontal sweeps 358, 359, &c. per horizontal sweep of the writing apparatus, and in order to avoid error the read-out sweep must be centred at 361, 363 in each line of gratings. This is achieved by advancing the count in binary counter 339-342 to 101 by means of flip-flop 338 when a consistent output is first received from " OR " gate 333 to start saw-tooth generator 346, so that two horizontal sweeps later the count reaches 000 (010, 001 and 011 are excluded by the feed-back in the counter) and this opens " and " gate 336 to route the output of the videcons to the driver circuit 351 of magnetic core register 29 and also applied shift clock pulses via 349 to the register. The videcon outputs representing 0's and 1's are amplified at 325 and 328 respectively and applied, with the " 0 " signal inverted at 335, to " and " gate 336. Provided that a " 0 " signal is not simultaneously received, any " 1 " signal from 331 will then pass for registering at 29 ; if " 0 " and " 1 " signals are received simultaneously an error alarm signal is generated by " and " gate 334. Plate storage embodiment (Fig. 6).-Storage in this embodiment is on thermoplastic-coated plates each 7/10 inch square and having 64 x 64 blocks of data recorded on it. Four stacks of 64 such plates are arranged in a movable holder, and servomotors bring a desired plate opposite ejection mechanism for removal from its stack to reading and writing means, its place in the stack being taken by the previouslyselected plate. Further servomotors then adjust the position of the plate relative to the reading or writing means so that a desired block of 32 x 32 bits can be read out, or data written into this block position. The apparatus is otherwise generally similar to that of the first embodiment but with modifications particularly in the optical read-out system and servo-systems. Fig. 6 shows the whole system schematically; units named similarly to units in Fig. 1 perform similar functions (the controller 609 corresponding to programmer 28). Two servomotors 605, 606 move the plate holder 604 to position a plate 601 whose address (from computer 608) is registered in 607 opposite extractor 611, which then ejects plate 601 from its stack on to a plate holder where data can be written on to it by 619 (as in the drum embodiment) at a desired block position by moving the plate by servomotors 612, 613 in accordance with a block address set up in registers such as 618 (for motor 612). Read-out at a similarly selected block address is by a flying spot scanner 625 which scans the bit gratings, the diffracted light being detected by photo-cells 628, 629 and the data being recorded in 621. The array of plates is movable by servos 605 and 606 in accordance with the plate address supplied from the computer to register 607, and when the stack has been correctly positioned extractor 611 ejects the desired plate on to a plate holder, the previously extracted plate being returned to the stack at the thus-vacant position; the new address of the replaced plate is recorded at 607. The plate holder is now moved to position the desired plate for reading or writing by servomotors 612 and 613; these are controlled by the error signal from a selsyn coupled to the plate holder and receiving the analogue address from unit 617 fed with the digital address of data on the plate. Writing is by unit 619 in a similar manner to that in the drum embodiment, data being read out of the matrix store 621, and deflection signals supplied by 608. 609 then cures the plastic through circuits 624, which also are used for erasure (at a higher temperature), and may be used to preheat the medium (before writing) to a temperature of about 50  C. Read out is effected by the optical system shown in Fig. 7, a flying spot scanner controlled by logic circuitry somewhat similar to that shown in Fig. 3 forms a light source which is focused in