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68 PRINCIPLES OF CINEMATOGRAPHY
lucent screen is erected behind the actors, and onto which the moving picture of some inaccessible background is projected by a machine at the rear. A camera, mounted in front of the screen, as seen in Figure 25, will then record a complete picture consisting of the natural foreground action, plus the background as seen on the translucent screen.
For such an arrangement to succeed it is essential that both camera and projector mechanisms should be absolutely synchronised throughout the shot. Several systems are now in use which make this possible, and it is therefore also usual to run the sound recording camera in synchronism and from the same system. One method used to obtain this result is known as the 'selsyn' A.C. Interlock System in which the primary and secondary winding of each motor used to drive the various instruments is connected in multiple with the respective windings of all the other motors of the system the primaries being excited by alternating current. The secondaries of all motors are thereby electrically interlocked, and mechanically driving one motor of the system, called the distributor, will cause all the others to rotate at the same speed.
The earliest method used to obtain synchronising marks on both picture and sound negatives was by employing a 'clapper boy', as explained in Chapter 1. This was soon succeeded by the following method. The necessary identification to the scene was mounted on a board, and held in front of the camera as before, but this was done some time before the scene was photographed and with the camera running 'wild', that is, not electrically interlocked with any other apparatus. After sufficient footage had been exposed on the identification title the camera was stopped. When all was ready to take the scene the camera was interlocked with the sound recorder so that, on starting up, both camera and recorder would rotate in synchronism. A further refinement, located within the sound recorder, caused a short flash exposure to be automatically made as the mechanism commenced to rotate. It was therefore only necessary to line up the last frame showing the scene identification board, on the picture negative, with the last clear section before the fogged area on the sound negative, to obtain perfect synchronism. It is important to realise that both the camera and recorder start in synchronism from rest and gradually gather speed while still maintaining perfect synchronism.
Whilst this system had the advantage of completing all the identification work long before the actors were keyed up to perform, it still suffered from three main disadvantages. Firstly, it used a considerable amount of film which was useless once the