Patent GB-1244084-A (GB)
Title: Character display systems
IPC: G09G1/10
Filed: 1969-03-18
Abstract: 1,244,084. Cathode-ray tube displays. INTERNATIONAL BUSINESS MACHINES CORP. 18 March, 1969 [15 April, 1968], No. 14011/69. Heading H4T. Characters are traced on the screen of a cathode-ray display tube by applying to the beam deflection means a succession of stroke signals appropriate to the character, successive signals being applied within intervals shorter than the time constant of the deflection means. The beam is thus moved continuously at a uniform velocity during the tracting of a complete character all the elements of which therefore are of uniform brightness. As shown in Fig. 1, coded signals indicative of each of 64 characters to be traced are derived from a data processor or a storage unit such as a recirculating delay line and supplied to a decoder 21 which provides an output signal indicative of a selected character on a corresponding one of 64 output lines which are connected to individual capacity type matrix circuits. Fig. 2 shows one such matrix to terminal 41 of which the output line corresponding to a selected character is connected and to terminals TP1 ... TP40 of which (a maximum of 40 strokes being provided for the most complex character) successive timing pulses of 125 nanosecond repetition rate are supplied from a timing pulse distributer 27. On the occurrence of each pulse an output is produced on lines 81-85 specifying the deflection and beam blanking requirements for each stroke. Thus as shown output signals +x, -x, +y, -y and z specifying left-right, up-down and beam blanking are produced and fed to individual sense amplifiers 87 which increment or decrement counters 91, 93 controlling the flow of beam deflection current in the scanning yoke 125 and control the beam blanking via intensity control 95. Each character is composed within a 10 x 6 stroke matrix (Fig. 3A) positioned within a 12 x 8 stroke character box the point 31 of which is the origin (0, 0). Initially the beam is positioned at this point by a main deflection yoke (not shown) and the tracing of the characters is then affected by yoke 125 (Fig. 1). Because the individual stroke signals are applied to the latter in a time which is shorter than its time constant the position of the beam of any instant lags behind the position to which it is directed by the actual stroke signal by a distance which may be as much as one unit step (= matrix step) so that to reach the point 33 (y = 10; x = 0) (Fig. 3A) 11 units of deflection current must be supplied to the yoke. Due however to the " settling " of the yoke from the earlier steps and from step 11 the beam would be deflected to point 35 and to prevent this a negative vertical deflection signal is applied at point 33 at the same time that the first horizontal deflection signal is applied to the yoke. The time constant of the latter is 175 nanoseconds and this is in the ratio of #/2À1 to the timing pulse period of 125 nanoseconds and this is stated to produce a nearly perfect rightangle turn and by not allowing the yoke to " settle " a variety of angles and fine line curves may be produced. Electrostatic deflection may be employed instead of electromagnetic.