16-mm sound motion pictures : a manual for the professional and the amateur (1953)

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GENEEAL CHAEACTEEISTICS OF AMPLIFIEES 279 All these can be determined only by measurement. Performance claims for apparatus, the performance of which has not been measured, are sheer guesswork and have little meaning. Reputable manufacturers customarily measure every piece of equipment offered for sale for each performance characteristic claimed. Such production test records are accumulated for extended periods by the manufacturer. These data are usually available on the request of the equipment user. Noise Reduction Principles If a piece of clear film is run in a sound projector, noise due to dust, dirt, irregularities, and scratches on the film is heard in the loudspeaker. If film is used regularly in projection, it is found that the noise produced increases with wear, becoming very annoying as the film acquires more scratches and dirt in its many trips through the projector. If the clear film is kept stationary in the projector, and the amplifier turned up to its maximum gain, considerable hiss is heard from the loudspeaker of the projector; if the light beam of the projector is blocked with an opaque card, the hiss previously heard is reduced materially. The hiss that was eliminated was due to the light falling on the photocell and its effect upon the input circuit of the reproducing amplifier; the noise remaining is substantially the noise produced by the amplifier itself. With good amplifier design, it is possible to reduce the amplifier hiss to a negligible amount when reproducing good sound film ; however, to reduce the hiss from the photocell circuit and the noise resulting from the random dust, dirt, irregularities, etc. requires masking of the film. Since the noise produced is a function of the light transmitted to the photocell, it is apparent that this light flux should be reduced to a minimum. Because the variations in light cause the electrical output of the photocell, it is apparent that the problem is to keep the light variations at a maximum while keeping to a minimum the average light transmitted. This is accomplished by a process identified by a number of different names, such as noise reduction, noiseless recording, ground-noise reduction, antiground noise system, etc. The principles of noise reduction in photographic recording were appreciated early. Patents were issued to Adsit, an independent inventor, by the U. S. Patent Office shortly after World War I ; this was several years before sound film was commercially introduced successfully in 1928 by Warner Brothers. Subsequently, patents were issued to Sacia of Western Electric on an early form of control track (predecessor to the control tracks used in Fantasia and in the Bell Telephone Laboratories