Patent GB-1035047-A (GB)
Title: Improvements relating to character display systems
Inventor: HUGHES GORDON
IPC: G09G1/12
Filed: 1963-08-07
Abstract: 1,035,047. Cathode-ray tube circuits. FERRANTI Ltd. Aug. 5, 1964 [Aug. 7, 1963], No. 31095/63. Heading H4T. In a system for displaying alpha-numerical characters on a cathode-ray tube, each character is formed of a combination of separately and successively written portions, said portions being chosen from within a first plurality of available basic traces, when the character is a large capital or number, as defined by the size of the area enclosing the character, and from within a second plurality of available traces, obtained by modifying some of the basic traces, when the character is not a large capital or number, as defined by the smaller size of the enclosing area. Large capitals and numbers are displayed within the area P, Fig. 1 (not shown), defined by lines 11, 12, 13 and 14, lower-case letters are displayed within the area SQR, defined by lines 14, 15, 16 and the bottom line, and small capitals and numbers are displayed in area Q, defined by lines 15, 16, 17 and 13. Areas S and R 1  are also respectively used for suffixes and subscripts. The production of a capital B and P, from a plurality of basic traces 21/26, is shown in Fig. 2 (not shown), whilst the production of various lower-case characters is shown in Fig. 3 (not shown), traces 27 and 28 being modifications of basic trace 21, and trace 32 being a modification of basic trace 25. 44 basic traces are provided in the first plurality and 20 of these are modified to provide the second plurality of traces. These traces are displayed in four possible modes: (a) the basic 44 in area P; (b) the same 44, but modified by being reduced in size and displayed in area Q; (c) a selection of 20 of those used in mode (b) but transferred to area R; (d) the same 20 as in mode (c) but in area S. The traces are selected by a seven-bit code, A to G, 44 of the 64 combinations of bits B to G defining the basic traces, and the remaining 20 combinations defining the modified 20 traces, the state of bit A defining whether mode (c) or (d) is to be used. In the system of Fig. 4 (not shown), bit A is fed to an area selector 45, and bits B/G are fed to a decoder 42 and converter 43. The decoder 42 detects the codes corresponding to the 20 modified traces and gives a Z output of 1 when they are detected, and 0 when they are not present. The codes corresponding to the basic 44 traces pass through converter 43 without alteration to cause a part generator 47 to produce X and Y energizing signals to produce the required trace. When the detector detects one of the 20 modified trace codes, a signal is fed to the converter 43 which causes it to convert the modified trace code to the code of the corresponding basic trace. The Z output of decoder and bit A are fed to area selector A, which sends signals to the part selector 46 in dependance on the states of the output and bit as follows: when A is 1 and Z is 0, the signal causes the selection of mode (a); when A is 0 and Z is 0, the signal causes the selection of mode (b); when A is 0 and Z is 1, the signal causes the selection of mode (c), and when A is 1 and Z is 1, the signal causes the selection of mode (d). Bits B, C and D control the X energizing signals, and bits E, F and G control the Y energizing signals. When B is 1, the X scan is the full width of area P on Q, depending on the states of A and Z, and when B is 0, the X scan is half the full width (see Fig. 5, not shown). When C is 1, the X scan is from left to right, and when C is 0, the X scan is from right to left. When D is 1, the X scan starts from the right-hand side of centre line 18, and when D is 0, the X scan starts from the left-hand side of centre line 18. Three combinations of bits B, C and D are used to define fixed positions of X, on the centre line 18, and on the lines 11 and 12 for area P, or lines 15 and 16 for areas Q, R, S. Each trace is drawn in seven successive segments, such that curved traces can be drawn by the combination of seven straight line segments. The timing of the seven successive segments is controlled by a timer 52. The part generator 47, comprises a plurality of resistors, a group of which is selected by the part selector to correspond to a chosen trace and introduced into the energizing circuit of the cathode-ray tube one resistor at a time, under the control of timer 52. A section of the part generator 47 is shown in Fig. 7 (not shown), for a full width X scan in area P. The part selector 46 produces a signal on line 62, indicating that a large capital in area P is intended, and a signal on line 63, indicating that a full width scan is intended. AND gates 72, 72<SP>1</SP>, 75, 75<SP>1</SP> each have one input energized, and AND gate 71<SP>1</SP> has two inputs energized. The counter 52 has three outputs K1, K2, K3, which are energized in a binary succession manner. A signal first appears on output K1, opening gate 75, and, via OR gate 73, also opening gate 72. This places a resistance 4R in series with the X deflection coils. A signal now appears on output K2, opening gate 75<SP>1</SP>, and, via OR gate 73<SP>1</SP>, also opening gate 72<SP>1</SP>. This places a resistance 2R in series with the X deflection coils. A signal now appears on outputs K1 and K2, such that resistances 4R and 2R are applied in parallel in series with the X deflection coils to give an equivalent series resistance of 4/3R. The sequence is continued, such that the current flowing through the X deflection coils and the resistors, increases in seven equal steps. If the X and Y deflection currents both increase in equal steps, the trace will be a straight line, if, however, the resistances R &c. are chosen such that the steps are not equal for one or both of the deflection currents, or the currents first increase and then decrease or vice versa, the resultant trace will be a curved line, Fig. 9 (not shown). Bias deflection currents are used to position the trace in the correct area. In an alternative embodiment, Fig. 12 (not shown), each trace is formed in one step by the Lissajous combination of two simultaneous single cycle sinusoidal scans in the X and Y directions. The scans are of the same frequency, but of either full, half or zero amplitude, and a bright up signal is applied to the beam over the portion of the combined scan to be displayed. Elliptical or circular traces are produced by phase differences between the scans, and the brightness of the beam is varied in a sinusoidal manner when the speed of writing varies in a sinusoidal manner. 48 basic traces are provided, 16 of which are modified. Bit A serves the same function as in the first embodiment, but bits B and C control the duration of the bright-up period, e.g. full cycle, first halfcycle or second half-cycle, bits D and E control the amplitudes of the scans, but F controls the relative phase of the scans, and bit G controls the starting point of each scan.