British Kinematography (1952)

August, 1952 39 A NEW TELEVISION RECORDING CAMERA W. D. Kemp, B.Sc, A.M.I.E.E. * Read to a joint meeting of the British Kinematograph Society and the Television Society on April 2, 1952. TELEVISION systems designed for broadcasting have very short intervals between successive pictures in order to economise on bandwidth. The present intervals used by the British Broadcasting Corporation are about 1.4 milliseconds, which represents about 12° if the complete picture period is taken as 360°. At present it has not been found practicable to advance the film in such a short period. To overcome this difficulty the first B.B.C. system of television recording used continuous motion.1 A new version of the original equipment is shortly to be installed at the Lime Grove Television Studios.2 These equipments use 35 mm. film, and the running cost is high (about £120 an hour). With the growth of recording here and in America cheaper methods became desirable, and development was concentrated on 16 mm. Continuous motion recording did not seem attractive in the case of 16 mm. film for the following reasons : — (a) The velocity stability required to obtain correct interlacing would be higher than that necessary for 35 mm., because of the smaller line pitch. (b) The linear velocity of the film is under half that of 35 mm., and this renders stabilization more difficult. (c) There is only one sprocket hole per frame in 16 mm. film and thus the sprocket hole flutter frequency is 25 cycles. Flutter at this frequency is particularly liable to cause bad interlacing. (d) The small size of the film necessitates a halving of mechanical tolerances generally for equivalent results. (e) No continuous motion mechanism had been developed for 16 mm. film which gives the required performance. For the above reasons possible mittent methods were considered. inter Some Intermittent Methods of Television Recording Fig. I illustrates some exposure sequences which give more time than the interval between television pictures for advancing the film. The film camera must be phase locked to the television signals. It will be realised that any such exposure sequence must loose information of some sort, and although each sequence gives an interlaced picture of the full number of lines on each film frame this is only done by repeating some television lines on successive frames of the film. (i) 16 2/3 Method3 The period is three television scans, two of which are recorded, the third being lost while the film is advanced one frame. The pull down required is thus 120° of the cycle. Since for every three complete television pictures only two film frames are produced, the film must run at 16 2/3 f.p.s. It is possible to step print the negative to obtain a 25 f.p.s. film by repeating every other frame. (ii) Step Printing in the Camera The step printing process can be done in the camera itself by the use of a double optical system, and a film traction mechanism which advances the film alternatively one frame and then two frames. With the phasing shown in Fig. I, for instance, television picture 1 (scans 1 and 2) is recorded in both top and bottom apertures of the double gate, and then the film is advanced two frames during scan 3 after which scans 4 and 5 are recorded in the top aperture. The film is then advanced one frame which leaves two unexposed frames in the gate for the cycle to repeat. In the period of 3 complete television pictures, three film frames are produced, and therefore the Late B.B.C. Planning and Installation Department, now with High-Definition Films, Ltd.