Patent GB-806274-A (GB)
Title: Information recording or sensing system
IPC: G11B7/08
Filed: 1956-11-09
Abstract: 806,274. Statistical apparatus. STANDARD TELEPHONES & CABLES Ltd. Nov. 9, 1956 [Nov. 10, 1955], No. 34300/56. Class 106 (1). In apparatus for recording or sensing binary data on photographic film an accurate beam positioning arrangement enables a very high density of data indicating marks to be used, e.g. 90,000 per square inch. The data is recorded on a strip of film which may be divided into frames each consisting of a number of clear and opaque areas as shown in Fig. 2. The data indicating pulses and synchronizing pulses are recorded as clear areas 5, 6 while the remainder of the film is opaque. A reversal film emulsion is used which gives a clear area 5 or 6 as the result of exposure to light. There are five data code positions for each digit recorded arranged in vertical lines with the synchronizing pulses between them. Recording.-The recording system is shown in Fig. 3 the information to be recorded being transmitted through an encoding circuit 7 which converts it to a plurality of code pulses suitable for recording. The output from this circuit passes to an intensity modulator to switch the beam of a cathode-ray tube 10 on and off and to a triggering pulse generator 8 controlling the horizontal and vertical scan control plates. Each data code position causes the vertical control to step the beam to the next recording position of a column and the horizontal control steps the beam to the next column. Beam registration.-The accurate registration of the beam necessary is achieved as follows. The cathode-ray tube 10 has a lens 11 in front which directs the light from the spot on to another lens 21. For the purposes of explaining the way the optical system works the lens 11 is assumed to have a mask 12 placed over it having four equal holes in it, this mask not in fact being used. The four beams converge at a point on the other side of lens 21 and just beyond this point is placed a graticule 13 which is shown in Fig. 5 to consist of a ruled pattern of lines there being 300 lines in the vertical and horizontal directions with clear spaces between the lines. The four beams passing through the lens 21 pass partly through a clear space as shown at 13a each being half obscured when the beam positioning is correct. The light passing through the graticule is partly passed and partly reflected by a partial mirror 14, the reflected part being focused by lens 15 on to the film 16 to effect the recording and the part passed falling on a group of four photo-cells 17-20, Fig. 4, which control the deflection and positioning of the beam. The movement of the spot over the face of the cathode-ray tube does not affect the position of the illuminated areas on the photocells since this illumination is confined to stationary regions having an extent equal to the image size at this place of the holes in the mask 12 as modified by passage through one of the clear spaces of the graticule. If the beam is centrally positioned with respect to the clear space of the graticule the output signals from each pair of photo-cells 17, 18 and 19, 20 are equal but misalignment in a vertical direction would cause a difference to appear between the outputs of cells 17, 18 and horizontal misalignment would cause output differences in cells 19, 20. These differences are found in subtracting circuits 22, 23 and are applied to the vertical and horizontal beam deflection control circuits 9b, 9a to correct the beam position. Synchronization pulses 6 are applied to the film in a regular sequence after each group of five code pulses so that the beam deflection control signals shall be available at frequent intervals to prevent the accumulation of large errors. Sensing.-The apparatus used for sensing the record film is similar in many respects to that used for recording. It comprises a cathode-ray tube 24, Fig. 6, the scanning spot being directed on to the film 25 by lenses 26 and 28. Light passing through the clear spaces of the film falls on to photo-cells 29, 30, &c., which are used as before for beam deflection control. The sum of all four photo-cells is the required code pulse and is obtained by connecting them to an adder circuit which may control some sort of read-out device, e.g. a standard teleprinter machine.