Patent GB-1212080-A (GB)
Title: Improvements in or relating to systems for the display of characters and symbols on the screen of a television tube
IPC: G09G1/04
Filed: 1968-09-26
Abstract: 1,212,080. Cathode-ray tube displays. SIEMENS A.G. 26 Sept., 1968 [27 Sept., 1967], No. 45765/68. Heading H4T. In a system for displaying symbols &c. on the screen of a cathode-ray tube the symbols are produced by deflecting the beam along selected elements only of a basic matrix pattern composed of straight line elements under the control of step voltages derived from digital information and supplied, together with line and frame scan sawtooth waves) to deflection coils associated with the tube. The step voltages, which comprise portions controlling the positions of parts of individual characters and the position of each character in a line of the display together with portions producing the actual deflections required to trace the elements of characters in terms of co-ordinate differences and portions controlling the production of a conventional television line raster are combined to form composite voltage waveforms which are supplied via push-pull transistor drive amplifiers to the X and Y deflection coils of the tube. The step voltages also include portions for blanking undesired traces (e.g. start and finish and link-lines). Fig. 3 shows the manner in which typical symbols are traced and Fig. 4a shows the basic matrix pattern of which only the elements appropriate to a particular symbol are actually traced. Fig. 7 shows the composite deflection voltage waveform applied to the line deflection coils when a character is being traced (upper curves) and the current which flows in the deflection coil when a character is not being traced (lower curves), whilst Fig. 8 shows the voltage (upper curve) and current (lower curve) associated with the frame deflection coil when a character is being traced. Fig. 9 shows the form of the deflection amplifiers. VA which comprises an input transistor T5 (receiving the composite step voltage and television scan sawtooth wave from a digitalto-analogue converter U) the output of which is fed to a driver pair T2, T4 driving the output power stage T1, T3 to which the deflection coil LA is connected. Silicon diodes Gr1 . . . Gr4 supply the base bias for the driver transistors T2, T4 and provide a large degree of temperature compensation and temperature drift of the input transistor T5 is compensated by providing an emitter voltage via R4, T6 and Zener diode Gr6 which voltage is made temperature dependent by means of a silicon diode Gr5. To prevent thermal overloading of the output transistors T1, T3 due to excessive environmental temperature or due to excessive input amplitude a positive temperature coefficient resistor RK in heat-transferring relationship to the heat sinks of T1, T3 is connected in series with the input to T5. The adjustable resistor P3 enables the vertical (or horizontal) dimension to be adjusted, P1 enables the quiescent current to be adjusted and P2 in the negative feedback path enables the output quiescent voltage to be adjusted. The display tube and its associated deflection amplifiers may be connected over coaxial lines to a remote control station (Fig. 10, not shown).