Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, Octooer, 1929 Page 15 Reproducing Machine for Picture and Sound New Projector for Reproducing Sound from Disc or Film By H. Pfannenstiehl* ANEW projecting machine for reproducing sound from either discs or films has recently been developed by the Laboratories for manufacture by the Western Electric Company and is already extensively used in theatres. To be as nearly universal in its application as possible, the frame is designed to mount practically all of the commercial projector heads in use at the present time. Instead of being arranged in separate units as has been done previously, all the equipment — consisting of a disc sound-reproducing unit, an optical sound-reproducing unit, a film take-up mechanism and a motor drive — is mounted on a single base, as shown in the accompanying photographs. The motor, which may be for either d-c. or a-c, has a double ended shaft connecting at one end with the disc drive and at the other with a main drive gear unit from which three drives — two vertical and one horizontal — are taken. Accurate speed control is obtained by a vacuum tube regulator1. For sound pictures the speed is held constant at 1200 r.p.m., but provision * Special Products Development, Bell Telephone Laboratories. im -f \a i 11 1 \L ;y| zy : :3 ^"^ 1 * t. V''' '■'"■" i "' ■ fm iilBam 1 ' : f*'^" "c*»»5 in !i& vm B *i»':: yj Fig. 2. Part of the main drive gear unit is hidden by the metal guard but the general arrangement is evident. has been made so that, when pictures are to be shown without sound accompaniments, any speed from 900 to 1500 r.p.m. may be obtained. This permits an increase or decrease in speed to accommodate a definite time schedule. Accurate synchronism between the picture and the film or disc record is essential to successful sound pictures and steadiness of motion of the sound film or disc is also required. Vibration must be eliminated. To fulfill these requirements all the gears are made to great accuracy, and the drives for the disc and the sound sprocket have flexible couplings and mechanical filters. The filter for the disc drive, shown in Fig. 3, consists essentially of a flywheel connected by springs to the driving gear. To avoid surges in the filter system, frietional damping of the spring connections is necessary. This is supplied by wrapping each spring with a layer of felt which is kept in contact with the convolutions of the spring by a second and outer spring winding. Over the felt are strips of metal which prevent the outer spring from imbedding itself in the felt and interfering with the proper action of the inner spring. The turntable itself serves as a flywheel. The filter used with the drive for the sound sprocket is similar in function but differs in having hydraulic rather than frietional damping. To prevent mechanical vibration from being transmitted to the disc reproducer, cushioning is provided between the pivot of the reproducer arm and the bracket that supports it. As an additional precaution the entire turntable assembly is mounted on a soft rubber pad. A record clamp is provided also to prevent the record from slipping on the turntable and thus destroying synchronism. The clamp is self -locking but is readily released by lifting a small latch. Of the two vertical drives, geared to the motor shaft in the main drive gear unit, the front one — shown in Fig. 2 — is connected through two universal joints and a telescoping shaft to the Fig. 1. From the operating side of the new machine all the principal elements but the drive may be seen. 1 Bell Laboratories Record, November, 1928, page 101. Fig. 3. The mechanical filter in the disc drive is mounted directly beneath the disc itself.