British Kinematography (1952)

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*6 BRITISH KINEMATOGRAPHY Vol. 21, No. 2 Conclusions The intermittent system employed is a satisfactory method of recording television signals which have very short intervals between television pictures. Excellent interlacing can be achieved with good general definition. Movement is satisfactory, although jerkiness can be detected, particularly on camera pans. Further work remains to be done to improve the optical system geometry. REFERENCES Baker, H. W., and Kemp, W. D. The Recording of Television Programmes, B.B.C. Quarterly, 4, No. 4, Winter 1949, 1. Kemp, W. D. Television Recording, Proc. I.E.E., 99, Part 3A, No. 17, 1952. Smith, D. A. Television Recording, Wireless World, 55, 305. B.B.C, Duddington, C, and Kemp, W. D. Improvements in and relating to the recording of television pictures upon cinematograph film. Brit. Pat. 23,216 Appl. 21 Sept. 1950. DISCUSSION Mr. R. H. Cric ks : Why cannot the " flying spot " system, as used in the Telecine film transmission apparatus be used for recording ? The Author : I suppose the principal reason against that is the afterglow on the C.R. tube. In the 35 mm. film scanners at Alexandra Palace there is relative movement between the raster and the film. If there is afterglow, white objects tend to smear in the vertical direction. That is one of the difficulties, but I am not saying that it is unsurmountable. A Visitor : I wonder whether the author would indicate the order of the time of desirable afterglow. Would it be equal to or less than one line period ? The Author : 1 think very much more than one line period would be acceptable. It should be down to 1/100 of its value in a few lines. Mr. S. N. Doherty : The horizontal resolution of a telefilm does seem to be somewhat below that of ordinary film from Alexandra Palace. Would you say that the loss was in the optical system or in the actual television tube with which the picture is recorded ? The Author : It is true that in photographing television you do seem to lose definition. It is a very big subject, and there are a number of things which cause loss, such as : ( 1 ) The Electron beam scanning over the phospor, which is losing something by dispersion in the phospor ; and (2) an imperfect optical system. In these double optical systems you tend to get lower resolution. You could not resolve a 400 line raster on 16 mm. Plus X. We know there are very fine grain stocks, but when this film was shot we hadn't light to expose on them. Theoretically it is quite possible to produce television recordings which are identical with the original. Dr. F. S. Hawkins : If you were to photograph two cathode-ray tubes, each showing a half of the interlaced scan instead of photographing the same tube twice to secure the complete scan, would this produce other difficulties which are even worse ? The Author : I am afraid it would. It would be very difficult to get television tubes with linearity accurate enough to superimpose them. Mr. R. L. Hoult : The reflecting shutter which you showed in the diagram (Fig. 2) was, I take it, included in the Moy camera. I would have thought that the form of shutter would give some relative keystoning between the two images owing to the different angles of the two mirrors. Is that so ? The Author : The keystoning which came out is undoubtedly due to the mirrors. The focal length of the second lens was chosen at 75 mm., which is very long for 16 mm. film, entirely to make the keystoning negligible. Mr. B. J. Stocks : Was any spot wobble used in the system by which these films were recorded ? The Author : No. Mr. B. Honri : Is the two frame pull-down related to interlacing ? If you had sequential scanning would it be necessary ? The Author : The camera was designed around the transmission standards of the B.B.C, and it only works on interlace signals. The whole reason for making this rather specialised camera was to try and record television standards as radiated. If you can make your television standards anything you like, this kind of camera has no application at all. Mr. Lucas : Does the shutter cause any air turbulance, which will throw dust onto the lower surfaces ? The Author : No. The main lens comes in front of the reflecting surfaces, and is situated with the optical system in a metal housing, which is practically airtight.