Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

April 5, 1930 EXHIBITORS HERALD-WORLD 39 REPRODUCTION IN THE THEATRE The addition of sound to motion pictures more than doubled the amount of projection equipment necessary in the theatre. The images on the film are after all just translucent miniatures of what is to be shown on the screen. The sound source on the other hand is either a tiny scratch on a wax disc or an odd looking border along the film. In both cases a delicately elaborate arrangement of electrical machinery must intervene before the sound locked in the film or disc by the recording process can be brought to new life. Three essential elements make up a reproducing system. They are: (1) A pick up or reproducer. (2) An amplifier. (3) A loud speaker or receiver. The function of the reproducer is to transform the sound record into electrical energy. The function of the amplifier is to magnify the infinitesimal electrical energy to the desired value. The function of the loud speaker is to transform this amplified electrical energy into acoustic energy and to distribute the acoustic energy or sound throughout _ the theatre or auditorium wherein it is being produced. At the present time there are two types of sound recording used commercially. These are known as the film and the disc methods. The only essential difference between the systems used for reproducing film and disc records is in the pickup apparatus. Fig. / is a schematic diagram showing *Theatre Acoustics Engineer, Electrical Research Products, Inc. S. K. Wolf* FIGURE 1 the general layout of a sound reproducing system. You will note that provisions are made for reproducing both film and disc records, also that two machines are equipped, making in all four pickup devices. Obviously two machines are necessary to the continuity of the picture and sound reproduction. Provision is also made for selecting either the film or the disc pickup in Machine No. 1 or Machine No. 2. The next piece of apparatus in the circuit is the fader, the function of which is to control the volume or sound energy of this system. Following the fader is a switching panel which permits use of non-synchronous reproduction as well as synchronuus reproduction. After the switching panel are the amplifiers, a more detailed discussion of which will follow. The next element in the circuit is an output control panel, the function of which is to join the amplifying units with the loud speaker or receiver units. The receivers, as has been stated, serve to transform the amplified electric energy into acoustic or sound energy. DISC REPRODUCER In elaborating on the above description, let us discuss first the method known as disc reproduction. In disc reproduction a magnetic type of reproducer is used almost exclusively. This type of reproducer consists of a stylus connected to an armature of high permeability which is located within a small coil. In operation the stylus attached to the armature vibrates as a needle follows the grooves on the sound record. The movement of the armature between the poles of the magnet which surrounds the armature causes a variation in magnetic lines of flux and a voltage with corresponding variations is induced in the coil. This induced voltage is an electrical image of the sound record. A section of this type of reproducer is shown in Fig. 2. This is a simple schematic diagram which will serve to illustrate the fundamental electric principle involved in the transformation of the sound record into an electric image of the record. This device makes use of a fundamental principle of electricity. It is the basic principle of operation of electro-dynamic apparatus. This principle may be stated briefly as follows: When magnetic lines of force are permitted to cut a conductor or coil of wire, a voltage is induced in the conductor or coil of wire. 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