Patent GB-705197-A (GB)
Title: Improvements relating to cathode ray tube arrangements
Inventor: HADFIELD LLOYD DYSON
IPC: H04N3/38
Filed: 1951-06-28
Abstract: 705,197. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd. June 10, 1952 [June 28, 1951], No. 15314/51. Class 40 (3). [Also in Group XL(c)] In a cathode-ray tube arrangement for scanning a continuously moving film wherein the scanned patch is displaced in the direction of film movement, the arrangement being such that the frame scanning motion tends to become distorted with displacement of the beam from one patch to another means are provided for superimposing upon the frame scanning motion a sawtooth component tending to equalize the amplitude of frame scan as between relatively displaced patches and a parabolic component for rendering the separation between lines in one patch substantially the same as the separation in a displaced patch. The invention is described as applied to the apparatus described in Specification 673,752 in which in order to effect'an interlaced scan at 60 frames per second of a film moved at 24 frames per second the scanned patch is moved in a sequence at frame frequency to five different positions by superimposing a stepped wave, Fig. 2, on the frame scan wave. The stepped wave is generated by means of a potential divider 30-33, Fig. 5, from which tappings 41-45 are taken off through relay contacts 51-55 to a common terminal 56, the relays being operated in the required sequence by a chain of multivibrators triggering at frame frequency. The stepped wave at terminal 56 is then superimposed on the frame scan wave in a mixing circuit, Fig. 6 (not shown), and the combined wave fed to the frame scan coils. Fig. 4 at 116-120 indicates the actual nature of the frame deflections produced at the five different positions 111-115 of the scanned patch, and at 121- 125 indicates the desired linear and identical deflections. Lines 126-130 indicate the sawtooth components of the distortions and lines 131, 132, 134, 135 the parabolic components of the distortions. In accordance with the invention the sawtooth components are compensated by adding sawtooth voltages of the correct amplitude to the stepped wave produced at terminal 56. A sawtooth wave is applied from terminal 57 through resistors 47-49 to tapping points 42, 43 and 44 respectively the values of the resistors being chosen so that the amplitude of the sawtooth variations at tappings 42 and 44 are equal and smaller than the variation at tapping 43. Simultaneously there is applied a sawtooth at terminal 56 of such magnitude and sense that when terminal 56 connects to tapping 43 no resultant sawtooth variation appears on terminal 56. By this means there is set up at terminal 56 the correct amplitude of sawtooth component for each level in the stepped waveform as indicated in Fig. 4. In practice the sawtooth wave applied to terminal 56 may be dispensed with and the amplitude of the normal frame scan sawtooth increased. The parabolic components of correction are applied in the correct sense from terminals 140 and 141 to the upper and lower two tappings respectively. The normal frame scan sawtooth is preferably generated by the circuit described in Specification 682,864, [Group XL (c)], in which a correcting waveform is generated by means of a circuit including an integrating circuit, and the parabolic components are derived from the integrated sawtooth variations. In accordance with a further feature of the invention, Fig. 8 (not shown), part of the current through the potential divider of Fig. 5 is derived from the E H.T. supply for the cathode ray tube so as to render the displaced positions of the scanned patch independent of variations of the E.H.T. supply.