International photographer (Jan-Dec 1934)

November. 1934 The INTERNATIONAL P H ( ) T ( ) ( i R A 1 ' 1 1 I k Five Left — C. Roy Hunter I left i, head of the sound department of Universal Studios, explaining to Robert Sibley, U. S. delegate to the World Power Conference, at BerJin, the intricacies of the new sound-on film device for motion pictures, compared with the old disc method of sound reproduction as seen in the background. This photograph gives an idea of the size and delicateness of the light valve. A wax recording machine is in the background. Courtesy Universal Pictures Corporation. Chapter XIV. Center — The light valve tuning panel on which the natural period of vibration of the ribbons on a light valve is raised to 8500 cycles per second. Courtesy Universal Pictures Corporation. Both Chapters XIV and XV. Right — Mechanical and Ornamental. Jean Arthur, who plays the leading feminine role, enjoys the cool seclusion of the sound truck while on location during the filming of Universal's comedy, "Bad Boy." The film recording machine mounted in a sound truck. Both feed and take-up magazines are at the top. The footage counters and the large PEC amplifier box, just below the exposure chamber, may be located easily. Courtesy Universal Pictures Corporation. Chapter XIV. sposed between the eye and a source of light, the slot appears to be only one mil wide and 256 mils long. The flat side of the portion of the ribbon that passes over the slot is parallel with the face of the light valve frame ; but the remainder of the ribbon that forms the loop around the pulley is turned so that the flat side of the ribbon is perpendicular to the frame. This necessitates a quarter turn being made in the ribbon in each leg of the loop between the bridge and the pulley, because the ribbon must rest flat against the surface of the pulley at the end of the loop. When the light valve is mounted on the core of the electromagnet in position for recording, the ribbon of the loop is approximately centered in the air gap between the wedge-shaped elevations on the electromagnet core and on the light valve frame. And, as has been explained, when the light valve is strung with the ribbon loop, the sides of the loop contract the width of the aperture through which the light beam from the lamp can pass to a width of just one mil. Size of the Optical Image The objective lens in the end of the exposure chamber has a two-to-one ratio, and so it reduces the optical image of the slit at the point where it is focused on the film to one-half the actual slit dimensions. This means that the image of the slit on the film is one-half mil wide and 128 mils long when the light valve strings are in their normal positions of rest. In making a print from the film after it has been developed, a part of the sound track is matted off in the print, however, leaving a track just 100 mils wide on the final positive print that is released to theatres. (In £■ Shape o elec tgomagmet core Light val ve frame jhe insulatep brloges Pecoboino La ip IN THIS DIRECTION Slot LlGHT VAL VE &IBBO/V /Cnualeokhob A1A6NETIZ//VG WI/V0IIVG A — Side view of light valve mounted on the electromagnet. cidentally, positive motion picture film stock is used as the negative in film recording machines, principally as an economy measure.) It is necessary to limit the width of the sound track in this manner in order to prevent it crowding into the picture portion of the print. Stringing the Light Valve The stringing of a light valve, which is the term used WfOGE-SHAPEO ELEVATIONS O/v CORE^^ ON LIGHT VALVE Light Valve frame. T*per//vg slot LlCHT VALVE RIBBON Protecting Sioe wall KagNETiZINIG WINDING B — End view of light valve mounted on the electromagnet. for the operation of placing the loop of ribbon in position on the valve frame and adjusting it over the slot, is a very delicate operation that requires extreme care on the part of the recording engineer. The duralumin ribbon comes already cut in strips of the proper length, which are fastened to a sheet of aluminum for support. The ribbon must be handled very delicately with tweezers, because any kinks or twists or nicks in it will make it unsuitable for use. The strip of ribbon is placed carefully in position on the light valve frame with the center of the strip looped around the insulated pulley. Then the two ends are threaded through the slots in the windlasses; and the windlasses are turned slowly by means of screwdrivers. This serves to twist the ends of the ribbon around the windlasses, securing the ribbon in place and drawing the loop up tight. After the ribbon has been pulled firmly into position, the light valve is placed in a special frame under the lens of a microscope and the four pincers adjusted until the two sides of the ribbon loop are exactly centered over the sides of the slot in the projecting wedge on the light valve frame and are spaced precisely one mil apart over the entire length of the slot. From the above description, light valve stringing may (Turn to Page 22) Please mention The International Photographer when corresponding with advertisers.