Patent GB-1241871-A (GB)
Title: Cathode ray tube display system
IPC: G09G1/10
Filed: 1969-01-06
Abstract: 1,241,871. Cathode-ray tube displays. INTERNATIONAL BUSINESS MACHINES CORP. 6 Jan., 1969 [12 Jan., 1968], No. 765/69. Heading H4T. In an electromagnetically deflected cathoderay tube display system utilizing beam deflection coils each having the same non-linear. deflection-versus-time characteristic, curved line traces are displayed by applying deflection signals of similar shape to each deflection evil at different instants so that the resulting deflecting field is; produced by operation of the respective coils at different parts, of the characteristic. The invention is described in connection with an Alpha-Numeric display system (Fig. 1) in which data, signals 10A, 10B defining the X and Y end-points of successive strokes which form a character &c. together with data signals 10C indicative of the time in which a particular stroke is to be traced, data signals 10D indicative of the beam blanking requirements and data signals 10E indicative of the last stroke forming a character are derived from a stroke storage Register 10 and after conversion to analogue form utilized to trace characters &c. on the screen of a display tube; 50 at positions determined by control circuitry 35 which supplies the main deflection yokes 40. This tracing is effected by supplying the outputs of the D/A converters 24, 26 to the X, Y character deflection yokes 30 and to maintain uniformity of stroke brightness irrespective of the tracing velocity of the beam the beam intensity electrode is supplied with suitable control signals, (in addition to the blanking signals) from a circuit 20. Fig. 2 shows the deflection versus time characteristic of each of the X, Y yokes 30, the time to reach steady-state current being designated as 5TC (where TC is the time constant of the yoke). Each stoke is allotted a particular time by data signals 10C which, when decoded in 32, select an appropriate delay DI ... DN the output of which is fed via AND gate 62 to the register 10 to initiate the supply of data signals appertaining to the next stroke. When curves are to be traced the end-point data supplied from register 10 in respect of one of the X, Y co-ordinates of the previous (straight line) stroke is greater than required to reach the end-point defining such stroke for which a time of one TC unit is allocated and when the tracing of the curve commences register 10 supplies only the new end-point data signal of the other of the X or Y co-ordinates so that the deflection due to this data is produced at one end of the yoke characteristic whilst that due to the other co-ordinate data has not yet reached this position on the characteristic. Thus the deflection is non-linear and a curved trace results. Figs. 5 and 6 show the deflection current supplied to the X and Y coils of yoke 30 and the actual deflection yoke currents respectively, plotted against. deflection yoke time constant in respect of the tracing of the character "R" (Fig. 4). The first stroke, stroke 1, is traced by the application of 10 units of deflection current to the Y deflection coil whilst the X deflection coil has zero input so that the. stroke has the end-point co-ordinates X = 0, Y = 10. Five units of time (5 x TC) are allowed for the tracing of this stroke after which the output of the 5 x TC delay unit (Fig. 11) supplies a signal via OR gate 60 and AND gate 62 to the register 10 to cause the data appertaining to the next stroke, stroke 2, to be produced. This stroke which has end-point co-ordinates X=3, Y=10 preceeds curved stroke 3 and the data from register 10 is such as to "aim" the beam at the co-ordinate X = 4, whilst utilizing the existing Y co-ordinate value which at the end of the 5 TC time period will have reached the steady state. The signal 10C for this stroke specifies a time period of one TC, during which the X deflection will reach the point X = 3 whilst the deflection current is still below the steady state portion of the yoke characteristic so that when stroke 3 is to be traced register 10 supplies only the Y co-ordinate data Y=7À5 which produces a change in the T steady-state Y deflection current. Five units of time (5 x TC) are allowed for tracing stroke 3 and since the X deflection current is below the steady-state condition, i.e. the X and Y deflection coils are operating on different parts of the characteristic, a curved trace having the endpoint co-ordinates X=4, Y=7À5 results. Stroke 4 is then produced but this stroke, which does not form part of the character, is blanked. Strokes 5 and 6 are produced in similar manner to strokes 2 and 3 and after a blanked stroke 7 stroke 8 and then blanked stroke 9 leading to the origin X=0, Y=0 0 are produced. Since stroke 9 is the last stroke a signal 10E is produced which, when the stroke has been completed operates a flip-flop 22 to provide via AND gate 64 a character complete signal to the data processor 12 to cause it to select the next character to. be traced.