Improvements in and relating to television standards conversion


993,191. Television standards conversion apparatus. BRITISH BROADCASTING CORPORATION. Feb. 20, 1964 [Feb. 22, 1963], No. 7341/63. Heading H4F. In an apparatus for television standards conversion in which input signals whose line standard is to be changed are applied to produce a visual display upon the screen of a cathode-ray tube and the display is viewed by a camera tube operating at the same field frequency but at the desired different line frequency, the field scan, curve (b), Fig. 2, in the camera tube is delayed relatively to the field scan, curve (c), in the display tube by less than the field-blanking interval and means are provided, operating during the times of overlap OT between the camera and display tube blanking intervals, to reduce the charge difference, curve (a), over the target of the camera tube. The effect of the invention is to eliminate spurious images due to failure in the camera tube to erase completely in one field scan the charge image which is formed on the target. A pulse, curve (d), is derived during the overlap period and is applied to control the camera operation. When the camera is an " Orthicon "or" Emitron " (Registered Trade Mark), Fig. 3, the pulse is used (a) to produce a pulse of light which is caused to flood the side of the target 11 on which the display image is focused and (b) to drive the stabilizing mesh negative. Photo-electrons emitted from the target by the pulse of light are impelled back to the target and redistributed so as to reduce any charge differences. The net change of target charge is zero if all photo-electrons are returned to the target, any tendency of the electrons to be collected by other electrodes, such as the wall anode being prevented by also pulsing such electrodes negatively. Where the camera is on Image-Orthicon, Fig. 4, a pulse of light is impinged on the photo-cathode and the field mesh 18 is driven positively. Secondary electrons released from the target 16 by the photo-electrons are returned to the target by the retarding field due to mesh 18 and are redistributed so as to reduce charge differences. The target acquires a net negative charge however which must be overcome before a useful positive charge can be stored. This may be overcome by biasing the display tube to produce a fixed minimum brightness as shown in Fig. 5. The picture information will then be displayed on a substantially linear part of the tube characteristic, thereby eliminating or reducing the need for gamma correction. In another way of operating with an Image- Orthicon camera, Fig. 6 (not shown), the photo-cathode is pulsed with light and is also raised from its normal negative potential to zero potential. The accelerator electrode in the image section is also raised to a low positive voltage below the first cross-over potential. The overall effect is that the target is stabilized at the zero potential of the cathode.




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