Patent GB-1045468-A (GB)
Title: Improvements in and relating to television scanning circuit arrangements
IPC: H04N3/22
Filed: 1964-08-19
Abstract: 1,045,468. Cathode-ray tube circuits; transistor circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Aug. 19, 1964 [Aug. 22, 1963], No. 33874/64. Headings H3T and H4T. Relates to television camera or picture tubes having circular screens and being scanned by a line and frame rectangular raster. In order to utilize the whole of the screen without damage to the tube, the tube beam must be suppressed when it is not directed to the screen and in accordance with the present invention this is accomplished by adding parabolic waveforms, Fig. 2e (not shown), derived from the line and from the line and frame scanning waveforms, Figs. 2a, 2c (not shown), and applying them to a clipping circuit to afford a pulse train, Fig. 2f (not shown), for beam brightening. In the embodiment a circuit 9, Fig. 3 (not shown), incorporates an integrating capacitor 23 affording a parabolic waveform, Fig. 2b (not shown), from the line sawtooth waveform input, a connection from the capacitor to a potentiometer 17 removing any sawtooth components from the circuit output. In circuit 11 this parabolic waveform is added to one synchronized with the frame sawtooth waveform, Fig. 2c, 2d, the adding circuit output being fed to cascated emitter-followers in circuit 37 each of which feeds into a corresponding clipping circuit e.g. 52. In clipping circuit 52, the clipping level is set by the emitter resistor such that the circle size ensuing is that of image screen 16 of camera tube 1; the capacitor across the emitter resistor decouples the A.C. component, the D.C. component passing through the resistor. The clipping circuits afford pulse train outputs, e.g. Fig. 2f, 2g (not shown), corresponding to circles of differing diameters. It is arranged that the clipping circuit which is conducting for the shortest periods is connected to the first emitter-follower &c. The amplified video output from screen 16 is applied to a circuit 58 together with the pulse train from transistor 51 of such an amplitude and polarity that during beam suppression the same video level as during the scanning of dark areas of the screen is obtained. In unit 60 there is added to the video signal the pulse train from transistor 50 to afford beam suppression at the receiving television picture tube; that pulse train corresponds to a slightly smaller circle than the trains from transistors 51, 52 and determines the picture dimensions of the reproduced scene. An amplifier 59 includes an automatic gain control arrangement utilizing a rectifier 61 preceded by a gating circuit 62 itself controlled by pulses from transistor 46 which correspond to a small circle at the centre of the picture; this is useful in arrangements for recording X-ray pictures, the light content of the central portion of which is most important. By adding sawtooth components to the parabolic waveforms the circle may be shifted to any desired part of the screen, Figs. 2h, 2i. The ratio between the amplitudes of the parabolic waveform derived from the line and frame scans is equal to the square of the aspect ratio of the rectangular raster to afford circularity.