Patent GB-1418718-A (GB)
Title: Display apparatus
IPC: H04N5/74
Filed: 1973-08-22
Abstract: 1418718 Cathode-ray tube displays INTERNATIONAL BUSINESS MACHINES CORP 22 Aug 1973 [28 Sept 1972] 39706/73 Heading H4T [Also in Division G5] Display apparatus displaying two separate items of information is viewed through at least one set of parallel spaced apart louvres so arranged that each item can only be viewed from a respective viewing position. As described, the apparatus employs a cathode-ray display tube which, in one embodiment, is positioned vertically as shown in Figs. 1 and 10. A set of louvres comprising a commercially available film 14 (see also Fig. 3) formed of plastics and consisting of a central core 15 incorporating black or coloured louvres 16 sandwiched between cover sheets 17, 18 is supported adjacent the tube screen 11 in a housing 20 which carries mirrors 25, 26 to enable "viewers" A and B to view respective halves of the film 14 through respective apertures 21, 22. Six lines of data information are produced on the tube screen in two separate groups of three lines each at positions 12a, 12b the characters of group being laterally inverted and those of group 12b vertically inverted (see Fig. 2) so that when viewed via mirrors 25, 26 the characters are correctly oriented. Each character is produced by intensity modulation of the tube beam as it is scanned horizontally through 42 character positions, at each of which a "wiggle" trace consisting of 10 vertical scans is produced extending vertically over 6 horizontal lines. The "wiggle" trace is produced by a ramp generator 71 (Fig. 4) controlled by the "16" output of a clock-pulse driven counter 58 via a decoder 70 eleven clock periods being required for the "up" stroke (see Fig. 5) and five for the "down" stroke (i.e. the retrace). Thus each character position, in effect, consists of a 10 Î 11 matrix and each character is produced during seven of the "up" strokes, the remaining three providing intercharacter spacing. In the production of each character the first clock period is employed for the production of a cursor (which underscores each character) in the case of the characters of group 12a (i.e. those, such as the charactor B in Fig. 5, which are not to be vertically inverted) whereas, in the case of the characters of group 12b, which must be vertically inverted, the cursor is produced during the eleventh clock period, and in order to leave a space between the cursor and the character, the clock period following that in which the cursor is produced is not utilized. This operation is controlled by selective energization of lines 40, 41 which gate either the "1" or the "11" count output of counter 58 via gates 51, or 52 and gate 53 to a further gate 54 which passes the cursor bits (seven bits) from buffer 65 to the video amplifier 56 via gate 55. The control lines 40, 41 are also employed in a decoding circuit 59 supplied with the "16" output of counter 58 to provide the binary outputs having bit values 1, 2, 4 or 4, 2, 1 in accordance with energization of line 40 or 41 respectively such outputs controlling the character generator 66 to produce the nine character forming bits, in one order "0" to "8", when line 40 is energized and in the reverse order "8" to "0" when line 41 is energized since the characters of display group 12b must be vertically inverted. The main deflection is produced under the control of a character counter 69 responding to outputs from "wiggle" scan counter 68 to produce outputs at the 1st, 3rd, 42nd and 56th character periods, the 1st and 42nd being employed via a decoder 72 to control the generation of a horizontal ramp having a trace duration of 42 such periods and a retrace duration of 14 such periods at the termination of which the 56th output controls, via line counter 74, the vertical deflection of the beam such that a total of six horizontal character line scans each of 42 character periods is produced. Additionally, in order to allow sufficient time for the circuitry to settle down after character buffer 65 has been loaded, the 3rd and 42nd outputs of character counter 69 are employed via OR gate 62 to condition AND gate 63 to cause buffer 65 to transfer the character bits (six bits) to character generator 66 only between the 3rd and 42nd character periods so that the first two character positions in each line are blank. Thus only forty characters may be displayed on each line.  In a second embodiment, the display tube 10 (Fig. 6) is mounted horizontally and separate sets of louvres 14a, 14b (Fig. 7) having the louvres set at different angles as shown in Figs. 8 and 9 are positioned in front of the tube screen so that the top half of the screen can be viewed only from position A and the lower half only from position B. In this embodiment all the characters on the screen are upright and read from left to right and the modifications of the circuitry of Fig. 4 to achieve this are discussed.