International projectionist (Jan-Dec 1945)

New Standard Tests for Motion Picture Sound Quality By C. R. KEITH WESTERN ELECTRIC COMPANY ONE of the important problems of sound engineers is to devise suitable methods for measuring the quality of sound reproduced from records in order that the final result may be as nearly like the original sound as possible. A departure from ideal reproduction is termed distortion and may take various forms, such as wave-form distortion, improper frequency response, flutter, noise, and other less commonly encountered forms of distortion. The measurement of each particular type of distortion requires a special test so that it can be dealt with separately. Because of the problems raised through SPIRAL BEVEL GEARS fob 2uiet Operation and Jlonq&i Jllf^e LaVezzi precision-made Spiral Bevel Gears maintain continuous pitch-line contact. Their oblique, curved teeth engage gradually instead of coming into full contact at once, like straight bevel gears. And with Spiral Bevel Gears there is always more than one pair of teeth in contact to share the load. seRvic€-* LaVezzi Machine Works 18 0 NORTH WACKER DRIVE CHICAGO 6, I It I N O I S the need for such tests, the Armed Forces have requested that suitable standards for the quality of sound be set up to assist them in obtaining satisfactory 16-mm prints for the training films now used so extensively in all branches of the services. Consequently, the American Standards Association War Committee on Photography and Cinematography, Z52, in cooperation with the Society of Motion Picture Engineers, has been endeavoring to set up such standards during the past year. The first step toward providing the ' required tests has now been taken and methods of measurement for two forms of distortion have been agreed upon and issued as American War Standards. These are the American War Standard Method of Making Intermodulation Tests on Variable-Density 16-mm Sound Motion Picture Prints, Z52.15-1944, and the American War Standard Method of Making Cross-Modulation Tests on Variable-Area 16-mm Sound Motion Picture Prints, Z52.39-1944. These are designed to measure the particular kinds of waveform distortion most likely to occur in sound film records of the types specified. In addition, the American War Standard Method of Determining Signal-toNoise Ratio of 16-mm Sound Motion Picture Prints, Z52.38-1945, has also been adopted. This method is important in the quality control of print production since improper handling during film processing will be reflected in an increase of the background noise in the sound records. In the case of each of these forms of distortion it would have been desirable to specify limits of allowable distortion as a basis for acceptance or rejection of individual sound prints. At the present time, however, experience in the use of such tests on 16-mm prints has not been sufficient to warrant setting up definite limits. The best that could be done under the circumstances, therefore, was to agree upon the method of measurement so that the data obtained from various sources could be correlated and suitable limits specified at some later date. In the meantime, sound recording engineers can determine by means of intermodulation and cross-modulation tests the print density which ; produces a minimum of distortion for a particular set of recording and processing conditions. It may be noted that in the fifteen years and more during which sound has been recorded in connection with 35mm motion pictures, the industry has not been able to agree upon any such limits nor have even the methods of measurement been standardized. The present American War Standards in the 16-mm field are, therefore, a step in the 22 INTERN ATIOIV AL PROJECTIONIST