International projectionist (Jan-Dec 1937)

20 INTERNATIONAL PROJECTIONIST April 1937 The Erpi (W.E.) Mirrophonic push-pull sound head to give the proper and desired results. This type of transformer connection is known in radio as a "push-pull" connection— hence the name. Inexpensive Modification Possible Some soundheads that have been installed for the past few years can be inexpensively modified for reproducing a push-pull sound track. In the case of most of the heads, however, this is not so easy. Both RCA and ERPI are today in a position to furnish soundheads already equipped for reproducing either standard or push-pull tracks. In many respects they are similar. Instead of utilizing valuable space by employing two separate photo-cells, a new type of double photo-cell has been designed which contains two separate cathodes in one envelope, each individually excited. This photo-cell is placed in the position regularly occupied by the photo-cell, with an optical system placed between the film and the cell so that each of the light beams is properly directed to its cathode. The electrical connections from the photo-cell are wired to a switch and to a balancing potentiometer of the screw-type located in the film compartment of the soundhead, and then to the special transformer located in its ordinary position. It is possible to connect this apparatus so that with the switch in one position, standard track may be reproduced; in which case both photo-cell cathodes are paralleled, since half of the track leaches each cathode, and are connected regularly to the opposite ends of the transformer primary winding. With the switch in the other position, push-pull recording may be reproduced by connecting each cathode in a push-pull connection as described previously. The problem of properly directing the light beams to their respective cathodes is dependent on space conditions and the location of the photo-cell. In the RCA soundhead a photo-cell is employed, mounted horizontally, which has two cathodes rectangular in shape, one above the other. Since the two sound tracks are side by side, with the result that the two light beams are alongside of each other in a horizontal plane, it is necessary to twist the beams so that they will be one above the other as well as properly directed to their proper cathode. This requires some intricate prisms, particularly considering the small space available. The ERPI soundhead uses a photocell, mounted vertically, whose two cathodes are alongside each other in a horizontal plane. Thus it is not necessary to twist the beams. Double photocells are available with the ordinary cathode split in half, so that even if it is mounted horizontally it would not be necessary to twist the light beams, but it would be necessary to separate them a bit so that each would strike the middle of its cathode. Principal Adjustment Required The principal adjustment that is required of this equipment for satisfactory results, in addition to placing the switch in the proper position, is to be sure that the output of the cathodes are balanced. LENS LATERAL BARREL GUIDE ROLLER ASSEMBLY This is accomplished by balancing the excitation voltage with the aforementioned potentiometer. To do this one runs some standard sound track recording (any type except unilateral variable area) through the projector, placing the switch in the "Push Pull" position. In this way you will be directing exactly similar light beams of one-half of the track, which for symmetrical variable area or for variable density give equal beams. When the outputs of the cathodes are properly balanced the sound output will be as near zero as possible, as the beams will be cancelling each other, since they will be mixed, 180 degrees out of phase with each other. It is at all times necessary, of course, that the beams be properly directed to their respective cathodes; but once set in the factory or by the serviceman, no further adjustment of the optical system should be required. The push-pull method of recording sound on film has made possible the most extended volume range yet achieved. This is due to the elimination of the highly critical ground noise reduction equipment in the recording process which the recordist finds difficult to always keep properly adjusted, particularly when temperamental directors, actors, and stage crews are anxious to make a "take." Furthermore, this type of recording reduces to a theoretical minimum the amount of sound track exposed. Since only the useful portion is exposed, the opportunity for dirt or oil to intercept the light beam in the reproducer is almost entirely eliminated. Less critical processing of the film is required with push-pull recording, because with the halves of the waves on (Continued on page 29) GUIDE LENS RCA 920 CONSTANT ROLLER AND PHOTOCELL SPEED ADJUSTING PRISM / SPROCKET NUT ASSEMBLY. This view of the RCA High Fidelity soundhead affords an excellent comparison with the Erpi head pictured above. Note particularly the lens and prism assembly used in this head BALANCING POTENTIOMETER PAD ROLLS