GB979370A - Improvements in and relating to the conversion of television signal standards - Google Patents

Abstract

979, 370. Recording television signals; standards conversion. BRITISH BROADCASTING CORPORATION. July 16 1963 [July 31,1962] No. 29450/62. Heading G5R. [Also in Division H4] A television field-frequency standards converter, Fig. 1, comprises a magnetizable body 11 (e.g.a drum disc or endless tape thereon) which is driven by a motor 10 in the direction of arrows 12 at a speed corresponding to the desired output field frequency, a motor 13 for driving a pair of recording heads 14,15 in the direction 16 at a speed corresponding to the difference between input and output field frequencies, the heads 14, 15 being fed in parallel and recording along separate tracks 17,18, a pair of replay heads 20, 21 which are in a fixed position peripherally of the drum but which are movable in the directions 22, 23 parallel to the axis of the drum (e.g. by cam means driven by motor 13) so that one is operative when the others are inoperative and collision with heads 14,15 are avoided, and switching means whereby output signals are derived from the operative one of heads 20, 21. The invention is described as applied to produce a reduction in field frequency in the ratio 4:3. Heads 14 and 15 are disposed 180 degrees apart and the speeds employed for recording are such that the heads each record on their tracks exactly two fields. Operation then preceeds as shown in Fig. 2 where the position of the replay heads with reference to fixed point on body 11 is shown by solid lines andthe positions of the recording heads are shown by broken lines. At times A on track 17 the positions of the recording and replay heads are coincident. The corresponding conditions on track 18 recur at times A<SP>1</SP>. The operations are timed so that heads 20 and 21 are moved axially out of the paths of heads 14 and 15 at these times and the output is switched to the operative head. The horizontal shading denotes the transference of input fields to output field and curve (c) shows how the output signal is formed alternately of three fields from each record track. One in every four of the input fields is not utilised, the remaining three being replayed in the time of the original four. The result of this is that two like fields (odd or even) become adjacent in the output signal once in every "beat" period (i.e. the period of the difference between input and output field frequencies). Thus in curve (c) output field 10 and 30 (corresponding to input fields 1 and 3) become adjacent with field 2 not utilized. This gives rise to a half line mistiming in the output signal (assuming two fields to a picture with odd line interlacing). The mistiming may be corrected by switching in and out a one-half-line delay during alternate beat periods. Alternatively one recording or replay head may be displaced with respect to its position by the track distance corresponding to one half a line of signal. This operation however results in vertical displacement, equal to picture line spacing, of picture detail with respect to the line structure of the raster during alternate "beat" periods. The effect of such displacement may be alleviated by interpolation between adjacent lines using apparatus as described in Specification 966,318. According to a modification (not described in detail) the apparatus may be set up so that a whole picture (i.e. one odd field plus one even field) is omitted once in every two "beat" periods. The effect of this is to eliminate the mistiming difficulty described above but to introduce a discontinuity in the portrayal of moving picture detail. The effect can be alleviated by using interpolation apparatus as described in Specification 966,318 to combine suitable proportions of adjacent input fields. According to modifications the replay heads may be spaced apart and the recording heads may be aligned. Drum 11 or its equivalent can be moved in step with the input field frequency. The direction or rotation of heads 14,15 is reversed when fields are to be added so as to increase field frequency. Specification 928,730, relating to change of line standards, is referred to.