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

(pull-down period 72°), separate registration pins are not provided and the elimination of jump is a major difficulty. The position of the film in the focus plane is also liable to vary, particularly in continuous-motion systems, and this gives rise to focus float. The effect is most serious where long-focus lenses are used, owing to the small depth of focus in the image plane. It is found necessary to focus the B.B.C. continuousmotion camera by actually exposing film at various focus settings. This is not required in the intermittent KempDuddington camera, where the film plane is absolutely fixed by the shuttle gate and aperture plate. Emulsion pile-up is the main cause of focus variations over long periods of time. (4.6) Emulsion Pile-up The emulsion on raw stock is quite soft, and during the exposure of a 1000-ft roll of film, small particles are apt to stick to any portion of the gate surface which is rough, or where there is local pressure. Friction and local heating follow, and the particles build up to solid flakes which hold the emulsion surface away from the image plane. Great trouble from this was experienced in the early days of B.B.C. recording. It was completely overcome by obtaining hard chromeplating on all gate surfaces and oiling the film. The oil used was pure sperm oil, which was applied by means of two small felt pads spring-loaded so that they made contact with the film adjacent to the sprocket holes on the emulsion side, prior to the film entering the gate; the picture area on the film was not oiled. A single drop of oil applied to the felt pads when each reel of film was loaded was found to be sufficient. Sperm oil forms an emulsion with film developer, and, provided the quantities of oil used are very small, no ill effects result. Intermittent-motion cameras are particularly susceptible to emulsion pile-up, since there is time for the emulsion to stick to the gate surface and the high initial acceleration of the film then de jj taches particles. In America use has been made of nylon gates which can be ij finished with a high polish. Another J method used is pneumatic holding of the J film in the gate aperture so that the nor | mal pressure pad can be dispensed with.11 j The shuttle gate used in the KempDuddington camera is good in this respect as there is no pressure on the film ; while it is being moved, the film being quite loose in the gate slide. (4.7) Spot-Position Modulation Providing the resolution of the film stock used is adequate, a correctly interlaced television recording contains 377 j lines in the picture height. When this is retransmitted on the same standards, the second scanning raster will not necessarily fall over the recorded raster, as has been explained in Sec. 4.2. Moreover, for transcription purposes, the i second scanning raster may have different standards, so that registration of the t\vo rasters is impossible. The effect of this lack of registration is i to produce patterns of light and shade over the transmitted picture, the dark portions corresponding to the places where the scanning spot is traversing the unexposed spaces between the recorded scanning lines. The patterns may change in a random manner owing to the random nature of the errors, or may be constant if the accuracy of scanning is sufficient. The patterns can be completely eliminated by eliminating the line structure of the image by a technique known as spot wobbling.14 This consists of applying a small highfrequency sinusoidal deflection to the electron beam of the recording cathoderay tube in order to increase the effective height of the scanning spot without increasing its width. The sinusoidal nature of the deflection tends to produce the effect of a double scanning line because of the low velocity at the maximum and minimum limits of deflection and the high velocity in the mean posi 378 April 1953 Journal of the SMPTE Vol. 60