Patent GB-1350480-A (GB)
Title: Character display systems
IPC: G09G1/10
Filed: 1971-04-19
Abstract: 1350480 Cathode-ray tube displays PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 19 April 1971 [5 March 1970] 23033/71 Heading H4T In an arrangement for tracing characters on the screen of a cathode-ray display tube the characters being formed by straight line segments each of which may have a selected one of the eight directions shown in Fig. 2, each character is specified by n groups of bits, one group comprising either four or five bits per segment, in a permanent store 7 (Fig. 1b), from which the groups are supplied, successively, through a transfer register 8 to a decoder 9 which decodes the four or five bit segment information to produce five bit words of which three bits determine one of the eight directions, one bit determines a single or double unit length for the segment and one bit determines the blankunblank condition of the tube beam. The most complicated character (e.g. B), requires 14 segments (i.e. n = 14 = 70 bits) for display and in order to allow a standard 64 bit store to be employed for store 7 the characters are coded such that the first segment can comprise only a selected one of the directions 1, 2 or 3 (Fig. 2) the second of the direction specifying bits of which are each = " 0 ", so that this bit need not be stored in store 7 since it can be inserted " after " the store, and by omitting the " end-of-character " word (five " 0 " bits) for characters of 14 segments and also inserting these bits " after " the store 7. Thus, as shown in Fig. 1b the store 7 is connected to stage 8 2  (the second bit position) of transfer register 8 by means of a twelve core cable 72 and the missing " 0 " bit inserted by means of terminal B (connected, e.g. to a fixed potential) the other stages of register 8 being supplied from store 7 over thirteen core cables 71, 73, 74, 75 and the missing " end-of-character " bits inserted via terminal E (also connected to a fixed potential) except when the character requires 13 or fewer segments in which case the " end-of-character " bits are included in store 7. In the general operation of the system a sequence of phase-shifted pulses T 1 -T 7  and gating pulses R 1 , R 2  from a pulse generator 5 scans the transfer register 8 which supplies its output to the decoder 9. The latter, under control of clock pulses from generator 2 having a duration of either t 1  or t 2  (t 2  = 2 x t 1 ) determined by the fourth (segment length) bit then supplies outputs to X and Y D/A converters 10a, 10b and to pulse generator 100 connected to the X, Y and Z amplifiers, Fig. la. (In this figure GS depicts the character generator of the present invention and GP depicts circuitry not described for controlling the position of each character on the tube screen). Fig. 3 shows the sequence in tracing the character G together with the start of a succeeding character H. The size of the characters is determined by the number of unit lengths (Fig. 2) employed to trace each segment and this is partly determined by the duration of the output from decoder 9 (t 1  for one unit length and t 2  for two unit lengths) in accordance with the fourth bit. The arrangement also provides an additional enlargement facility by means of a gate 6 controlled over line 18 by commands from the computer 1 to condition generator 5 to increase the length of the phase-shifted pulses T 1 -T 7  so that multiples of the t 1  or t 2  clock periods occur during each of which one or two segments respectively are traced.