Journal of the Society of Motion Picture and Television Engineers (1950-1954)

can then be raised from about 0.7 to over unity, with consequent increase in negative sensitivity. Additional sensitivity is also obtained from the change of contrast gradient, since if the highlights are kept at the same brightness, any particular tone in the picture will be brighter. (6.3) Direct Positive Recording If the vision signals are reversed in polarity before being applied to the recording cathode-ray tube, a negative picture is produced. Unfortunately, however, this picture no longer bears a simple-power-law brightness relation to the original scene. If a reciprocal law is introduced into the vision chain, the simple-power-law relationship can be re-established and a single photographic process will yield a positive. Provision for this type of recording is made in the Lime Grove installation. Such "direct positives" have been widely used in the United States, mainly for economic reasons. In most cases, a simple polarity reversal is employed, and the tonal distortion tolerated. (7) CONCLUSIONS Since the overall resolution of television on present standards is low compared with optical systems and 35mm motion picture film, it might be thought that the I resolution of these items would not be a limiting factor in the overall definition of a television recording. Experience has shown that this is not the case and that it is essential to have first-class optical systems and fine-grain film stock for best results. When 16mm film is used this is even more essential. Interlaced recording by continuous motion implies a very high degree of velocity stability and usually necessitates complex optical systems. For these reasons it would appear that intermittentmotion methods are more likely to provide the ultimate solution. The development of a suitable quick-pull-down method is therefore of primary importance, since a simple optical system can be used and difficulties due to cathode-raytube afterglow do not arise. There is room for improvement in the cathode-ray tube used for recording. It is felt that existing designs are based largely on normal television-receiver practice, where cost is of primary importance. It may be that by reducing the life of the cathode-ray tube, or some other such factor, considerable improvement could be obtained. Much work remains to be done on contrast-gradient correction and general photographic technique. Further gains can be expected in this direction in the future. The recording of television programmes is now a standard operational procedure both here and in America. Since most of the development has been done since the war, this must be considered the youngest branch of television. There seems to be no reason why further development should not yield results of a standard comparable with that of sound recording. If this is so it may be expected that the scale of television recording will be increased to provide a comparable number of programme hours. For countries starting television systems, it seems that cost will necessitate a large percentage of recorded programmes, and the further development of television-recording technique for the interchange of programmes therefore seems of international importance. Acknowledgments The author is indebted to the Chief Engineer of the British Broadcasting Corporation for permission to publish the paper, and to W. H. Cheevers and B. R. Greenhead for help in its preparation. He also wishes to thank others who have W. D. Kemp: Television Recording — Abstract 383