GB1444915A - Television camera arrangements - Google Patents

Abstract

1444915 Cathode-ray tube circuits EMI Ltd 14 Sept 1972 [12 Oct 1972] 47061/72 Heading H4T In an arrangement for reducing the effect of "lag" in the output signal of a television pick-up tube 3, i.e. failure of the scanning beam to discharge completely all the elemental charges on the tube target 2, the latter is scanned with a raster in which each line is scanned twice in successive line periods of a two field interlace the video signals generated during the first scan of each pair of scans (switch 6 closed, switch 13 on conductor 14 and switch 10 open) being supplied to the input of a one line-period delay 11 and to the camera output terminal 12 whilst during the second scan of each pair (the switches being in the position shown) the delayed signals are supplied to output terminal 12 and to an inverter 9 the output of which intensity modulates the tube beam via cathode 8. Thus any residual charges remaining on the target at the end of the first scan of each pair of scans is removed during the second scan of that pair so that, in the subsequent field, the elemental target areas will charge-up proportionally to the incident light. The video signal output at terminal 12 provides signals indicative of a complete scanning raster but of half the normal vertical resolution since each line is repeated once. Instead of line sequential correction the tube output signal may be sampled at a highfrequency (dot-frequency) and the correction applied before the beam moves to the next dot by means of a spot wobble technique. Bias lighting may also be employed. The scanning is effected in a "two field interlace" raster of the kind described in Specification 1,426,434 the scanning being in the line sequence 1, 1; 5, 5; 9, 9 &c. during the first field and in the line sequence 3, 3; 7, 7; 11, 11; during the second interlaced field.