Patent GB-1423716-A (GB)
Title: Character display system
IPC: G09G1/00
Filed: 1973-04-17
Abstract: 1423716 Cathode-ray tube displays SANDERS ASSOCIATES Inc 17 April 1973 [18 April 1972] 18559/73 Heading H4T In a cathode-ray tube (or equivalent) character display system of the stroke writing type in which each stroke is defined by a digital data word stored in a Read Only memory (R.O.M.) each character is permitted a maximum of a first predetermined number of strokes and a second predetermined number of word storage spaces is allotted for storing the strokes for. two characters, and by suitably selecting the pairs of characters to be stored in the R.O.M. and by eliminating redundancies (e.g. omitting the storage of the second of successive identical words) the second predetermined number of strokes may be substantially less than twice the first predetermined number. Additionally, the stroke words of one character are stored at adjacent addresses counted from one end of the allotted storage space and the stroke words of the second character are stored at adjacent addresses counted from the opposite end of the allotted space. In the described embodiment the latter comprises thirty two (32) word spaces. Fig. 1, shows the general arrangement in which a computer 21 controls the read-out of successive stroke words stored in a R.O.M. 52 and supplies such words to a function generator 82 controlling the beam deflection and blankunblank of the display (CRT) 26 to display the corresponding characters. The R.O.M. 52 comprises sixteen (16) commercially available units 52a ... 52h (Figs. 4 and 5) each unit having a storage capacity of two hundred and fifty six (256) for (4) bit words. As shown schematically in Fig. 4, these are organized in pairs of rows, the latter as shown in Fig. 6 in respect rows 0 and 1 of the pairs 52g, 52h, containing eight (8) bit data words (read vertically) specifying the beam deflection and blank-unblank requirements for the character @ (see also Fig. 7) stored from the left hand side and eight (8) bit data words specifying the same requirements for the character A (see also Fig. 8) stored from the righthand side. The X and Y bits specify (in a given code) various beam deflection magnitudes and directions, such as shown in Fig. 9 in respect of one quadrant, the bit UB specifies beam blank-unblank and the bit SK specifies skip-no skip, skip being specified (by a logical "1") when a particular stroke direction and magnitude is to be repeated (e.g. the second half of the trace (the second stroke) from the bottom left hand edge of the character "A", Fig. 8, which terminates at the "cross-bar"). This avoids the necessity for using a storage space to store the repeated word. However in order to avoid complication of the apparatus consecutive skips are not employed and thus the portion of the trace from the "cross-bar" to the apex, which is identical to the first two strokes is specified by a second identical word including a skip bit for the second portion (the fourth stroke). Additionally, at the apex and in other characters having acute apex angle portions, in order to prevent the beam direction from being reversed before it has actually reached such apex (due to the finite bandwidth of amplifiers) so that the character A would be shorter than, e.g. the character X (which has no acute apex) the word comprising eight binary "0"s indicates that the beam dwells whilst blanked for one clock period. Details of the timing and other control circuitry is given with reference to block circuit and waveform diagrams (not shown).