Patent GB-661264-A (GB)
Title: Improvements in or relating to scanning circuits for cathode ray tubes
Inventor: WHITE ERIC LAWRENCE CASLING
IPC: H04N3/233
Filed: 1946-12-19
Abstract: 661,264. Analogue computing systems. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Oct. 24, 1947 [Dec. 19, 1946], No. 37351/46. Class 37. [Also in Groups XL (b) and XL (c)] A circuit arrangement for scanning a cathoderay beam over a target in a cathode-ray tube in which an image on said target suffers from spiral distortion comprises individual means for deflecting said beam in two separate directions and in which the deflecting wave forms applied to said means are such that the cathode beam is caused to trace a spirally distorted raster corresponding closely to the spiral distortion in said image. In the case in which the scanning beam suffers from spiral distortion as a result of passing through a non-uniform electric retarding field, the waveforms applied to the deflecting means are such as to produce a compensating spiral distortion so that the raster produced by the scanning beam is truly rectangular. It is shown that the deflections applied to the X and Y plates to generate a spiral distortion are approximately of the form x - by (x<SP>2</SP>-f-y<SP>2</SP>) and y+bx (x<SP>2</SP> + y<SP>2</SP>), where x, y are the normal saw-tooth variations applied to the plates to produce a rectangular scanning field. In the circuit arrangement, Fig. 2, devised to produce such waveforms, the frame synchronizing pulses applied to a valve 10 effect discharge of a condenser 11 and a voltage y, of saw-tooth waveform at the anode of valve 10, is fed to a valve 13 whose cathode is coupled through a condenser 14 to the charging resistance 12 in the anode circuit of the valve 10. A voltage proportional to y<SP>2</SP> is derived from a pentode valve 40 whose anode is coupled to its control electrode through a condenser 41 providing a low impedance in the operating range of frequencies and whose control grid is also connected through a relatively high resistance 42 to the cathode of the valve 13. Similarly a voltage proportional to x<SP>2</SP> is derived from the anode of the pentode 50. The sum of the voltages from the pentodes 40 and 50 is applied to the control grid of a valve 70 whose anode is connected to the cathodes of valves 71, 72, which have a voltage y applied to their control grids so that a voltage proportional to - y (x<SP>2</SP>+y<SP>2</SP>) is generated at the anode of the valve 72. This is combined with a voltage x from the cathode of the valve 23 so that a voltage of the form x - by (x<SP>2</SP> + y<SP>2</SP>) is produced at a terminal 74. In a similar manner a voltage y + bx(x<SP>2</SP> + y<SP>2</SP>) is produced at a terminal 84. The valve 73 is controlled by the common cathode coupling with the valve 70 and its anode circuit is connected with the anode circuit of the valve 72 so that variations of the form x<SP>2</SP>+y<SP>2</SP> are prevented from being set up at the anode of the valve 72. In order to produce the modulation of opposite sign to be added to the voltage y the valve 80 is cathode controlled from the valve 83, to the control grid of which a voltage proportional to x<SP>2</SP>+y<SP>2</SP> is applied. The valve 83 operates to prevent variations of the form x<SP>2</SP>+y<SP>2</SP> from appearing at the anode of the valve 82. The circuit may be utilized to obtain approximate deflecting voltages of the form x - by<SP>3</SP> and y+bx<SP>3</SP>. The integrating circuit may be of the form described in Specification 641,599, [Group XL (c)], and the required deflecting waveform may be obtained from circuits such as those described in Specification 661,266, [Group XL (b)]. The modulator circuits 70 ... 73 and 80 ... 83 may be replaced by the circuits described in Specification 661,267, [Group XL (c)].