The Cine Technician (1938-1939)

Nov. -Dee., 1938 THE CINE-TECHNICIAN 133 Bell & Howell NON-SLIP 35mm. SOUND PRINTER IN the early part of 193G, the B.C. A. laboratories advocated a printer for 3.") mm. sound track, the main feature of which was a departure from the customary film control by means of a precision sprocket. The design of the printer was based on the principle that two strips of film of different lengths (shrunk negative and unshrunk positive) can be made to travel past a contact point without slippage between the two, provided that they are made to travel between two drums in such manner that the shrunk negative film maintains a uniform speed and forms an arc of constant chord-height while the unshrunk positive, though travelling in close contact through pressure with the negative, is left free to assume a concave or convex curvature, the radius of which will vary according to the differences in length of the two films. This is possible because of the fact that by forcing the negative film to travel over a drum of a certain predetermined diameter, the film will maintain its mean length, while its convex side is stretched in proportion to the arc that it subtends. BELL & HOWELL NON-SLI? F2l."STEe 9O0O CYCLES RECORDED ON EASTMAN 15 57 NEGATIVE (u V ) 9000 CYCLES BELL& HOWELL PRINT FROM EASTMAN 1357 NEGATIVE (u.v) 9000 CYCLES RECORDED ON EASTMAN I34>0 NEGATIVE (WHITE LIGHT) 9000 CYCLES BELLS. HOWELL PRINT FROM EASTMAN 134,0 NEGATIVE (WHITE LIGHT) Figure i The longer positive film, if held in close contact with the negative, with sufficient pressure to produce traction, will follow a concave, or convex path, the result being that the outer convex surface of the negative is stretched while the surface of the positive automatically assumes a form which will compensate for the differences in length of the two films. The simple test of running an unshrunk positive film through the non-slip printer in contact with a negative made up of unequally shrunk sections, therefore varying in length, will clearly demonstrate that changes in the size of the feed loop of the positive film will cause a considerable change in the angle at which the film approaches the printing contact point, thus forcing the positive film to make contact w ith the shorter negative film only along the line of tangency which varies automatically according to the differences in the lengths of the two films. Film shrinkages are thus accommodated within relatively wide margins. HORIZONTAL OPERATION The first departure from the conventional design of non-slip printers was to construct the Bell & Howell model to operate horizontally instead of vertically. In this position, the heavy rolls of film are fully supported and will unwind with a minimum of effort and a minimum of friction between the convolutions of the film roll. Another important factor is that both films are thus kept in such a position with reference to the moving parts (it tin' machine which need lubrication that the danger i'i nil or other lubricants coining in contact w ith the film is entirely eliminated. Equally efficient operation of the printer in either the forward or the reverse direction, which has been made possible by the horizontal construction of the machine, results in a considerable saving of time by eliminating the necessity of re-threading the negative after each print. Besides the time saving feature, the forward and reverse operation introduces a safety factor by reducing the handling of the negative to a minimum. DIAGRAMMATIC EXPLANATION As shown in the accompanying photograph (Fig. J), the negative hhn "A," supported by flanges of 1,200 foot capacity, is led to a sprocket over the rollers "B." The negative sprockets "C" are precision bobbed 32 tooth sprockets. The form and pitch of the teeth are calculated to ensure smooth driving of the film and proper stripping action at the point where the film leaves the teeth. The wrapping of the Him around the sprocket and therefore the number of teeth in mesh is controlled by the roller " B" and the film guard "D." From the sprocket, the film is led over the fixed roller "E" and the floating roller "F," both of which are mounted on hall bearings and flanged with spring guides to guide the film positively around the printing drum "G." Neither the negative rollers "F" nor the positive rollers "I" are set in a fixed position. The switching of the direction of operation of the printer from forward to reverse, or vice versa, is controlled by means of the handle "K," which, by means of a yoke and adjustable stops, also causes the rollers to assume the proper position according to their function, the function being determined by the direction of opera