Patent GB-1066643-A (GB)
Title: Correction of distortion in the trace on a cathode ray tube
IPC: G06G7/28
Filed: 1964-06-15
Abstract: 1,066,643. Cathode-ray tube circuits; transistor amplifying; switching and sawtooth circuits. INTERNATIONAL BUSINESS MACHINES CORPORATION. June 15, 1964 [June 28, 1963], No. 24711/64. Headings H3T and H4T. [Also in Division G4] A cathode-ray tube beam deflection system in which x and y deflection co-ordinate input signals are changed into beam deflection signals of respective magnitudes x(1 + kr<SP>2</SP>) and y(1 + kr<SP>2</SP>), where r<SP>2</SP> = x<SP>2</SP> + y<SP>2</SP> and k is a constant, includes analogue circuits which generate correction signals representing the moduli of xkr<SP>2</SP> and ykr<SP>2</SP> by determining log (xkr<SP>2</SP>) and log (ykr<SP>2</SP>) and taking their antilogarithms, and means responsive to the input signals to add or subtract each correction signal to or from the associated input signal depending on the sign of the latter. Pin-cushion distortion is thereby corrected. The embodiment relates to orthogonal deflection yokes; the invention may be applied to electric deflection systems. General arrangements. Fig. 2.-The computation process for each channel is similar, a summation generator (X<SP>2</SP> + Y<SP>2</SP>) and a circuit generating log (X<SP>2</SP> + Y<SP>2</SP>) being common to both channels. The circuits are transistorized and described in detail for the X channel; the X deflection yoke comprises a pair of coils which afford outputs in push-pull form which differ from a constant potential by a potential representing x/2, the magnitude (irrespective of sign) of x being denoted #x. Amplifiers.-A differential amplifier (Fig. 4) forms outputs (lines 38, 37) representing (K plus or minus #x/2) and (K minus or plus #x/2), where K is a constant; the circuit incorporates a constant current source comprising a transistor (28) and a Zener diode (32). A resistor (27) in the transistor output circuit is  adjusted such that when #x is zero a pair of further transistors (21, 21) carry equal currents. Output voltage swing is set by a resistor (35) bridging the further transistors and a resistor (24) in the H.T. common feed to those transistors is adjusted to give the correct input level for the ensuing stage when the input potentials to the differential amplifier are equal. The ensuing stage (Fig. 5) produces a voltage representing #x, the inputs from the differential amplifier being fed via respective diodes (50, 51) to a common input to the base of a transistor (52) the emitter of which is connected to the base of a further transistor (53); a third transistor (59) isolates any interaction between the other transistors on later stages and adjusts the potential on the output line (67) which varies from just below + 50 volts to lesser values in direct proportion to #x. Analogue calculating circuits derive the functions shown in Fig. 2 (see Division G4). A summing circuit (Fig. 9) has respective (#x)<SP>2</SP> and (#y)<SP>2</SP> inputs (lines 170, 171) and incorporates the subtraction of a constant value by the action of a pair of transistors (176, 177) which with their associated networks constitute a constantcurrent source which subtract current from that in a pair of resistors (186, 187); two D.C. sources (181, 185) are in opposition to afford a stable potential, it being assumed that changes in ambient conditions will vary each source potential by the same amount. As increased inputs occur the output voltage (line 189) drops linearly, from a constant potential, in relation to the input sum. The final correction signal, from the antilogarithmic function generator, is fed to a selection switching circuit (Fig. 11) to route the correction signal to the appropriate coil of the yoke depending on the outputs from the differential amplifier. In the switching circuit high input impedances for the latter are afforded by transistor pairs (230, 231 and 236, 237). Circuit balance is adjusted by a resistor (235), a resistor (241) in the H.T. feed to the former being of relatively high value to increase resolution, only a small difference in potential between the inputs from the differential amplifier causing one or other of the transistor pairs to become back-biased. Deflection voltage input for one of the pair of X coils is applied to the base of a transistor (271, Fig. 12) the emitter of which is connected to the coil (1) and the collector feeding into a current amplifier (272); current through the coil responds linearly to the input voltage and correction is effected by depriving the coil of current.