Projection engineering (Jan 1932-Mar 1933)

FEBRUARY, 1932 Page 15 Types of sound records By Charles Felstead* A review of the various methods employed in recording sound for reproduction with synchronized pictures EVEN to motion picture projectionists, the various types of sound records used in motion picture work are confusing. The term sound record is applied here comprehensively to motion picture sound records in general, regardless of the kind of medium on which the sound is recorded. With the present-day recording systems, the two recording media that are used are positive motion picture film stock and the soft wax record blank. In the case of the film, the sound energy is recorded on it as a narrow "sound track," approximately one-tenth of an inch wide, near one edge. The sound track on the wax record is a narrow, wavy spiral groove running uninterruptedly from near the center of the flat disc to almost the outside edge. These are the two types of sound records ; but there is a variation in them that is caused by the method employed in making this permanent record of the sound energy. There are four sound recording systems in general use: the Warner Brothers' Vitaphone, the RCA Photophone, the Fox Movietone, and the Western Electric system. The first mentioned system records on wax records alone ; the next two systems record only on film; and the Western Electric system employs either, or both, film and wax as its recording media. There are two distinctly different types of film sound tracks produced by these systems ; but on the photoelectric cell "eye" of the reproducing system, a light shining through either of these two sound tracks has exactly the same effect. The wax records produced by the two systems that record on wax are identical, be "Sonnd Engineer, Universal Pictures Corporation. cause almost exactly the same recording equipment is used in both systems. The finished wax records turned out by the Vitaphone and Western Electric systems of recording are similar in appearance to the records used on ordinary phonographs, except that they are sixteen inches in diameter instead of twelve inches and that they are somewhat thicker. They are of a hard, black wax composition ; and they are caused to revolve at a speed of 33 1/3 r.p.m. in recording and reproducing. The electro-mechanical recording device operates a sharp stylus that cuts a groove of constant depth and width in the original soft, soapy wax record from which the final hard wax records are later derived by a method called "processing." This cutting stylus moves at a steady rate of speed from a point near the center of the record to the outside, producing a spiral groove. As it moves steadily forward in this spiral path, the stylus also oscillates from side to side under the influence of the speech currents. This causes the stylus to trace in the soft wax a groove that undulates about a fixed spiral. The amplitude of the side to side movement of the cutting Sound track of variable densityfixed area type produced by the Western Electric recording system. stylus is governed by the strength of the speech currents producing the movement, and the rapidity of the movement is regulated by the frequency of the speech currents. A turntable geared to the projection machine and an electrical reproducer are used in reproducing the sound from the records. Types of Film Records The two types of film sound records are known as the variable density-fixed area track, which is produced by the Fox Movietone and Western Electric recording systems, and the fixed densityvariable area track that is created by the RCA Photophone system. Examples of these sound tracks may be seen in the accompanying illustrations. As has been said, these sound tracks have a similar effect on the reproducing system, because in the first case the intensity of the light that passes through the track at any instant is governed by The picture half of, a section of composite film. the density of the track at that point, and in the other case the amount of light that is permitted by the track to pass is regulated at any particular point bj the proportion of the sound track that is transparent. The output voltage of the photoelectric cell in the projection machine is directly proportional to the amount and intensity of the light falling on the sensitive surface ; so it is immaterial to the cell whether the intensity of the light reaching it is governed by the density of the film through which the light passes, or whether the amount of light striking it is regulated by the area of the sound track that is transparent. The overall width of both types of tracks is very nearly the same, and they are recorded in identical positions on the film. The variable density-fixed area film sound track is made up of an infinite number of very fine parallel striations, or bands, of varying exposure but of fixed lengths lying transversely across the sound track. These striations vary in all shades of gray from almost pure white to nearly jet black, but there is no variation of density in them in the transverse direction. They are produced in the Western Electric system by shutting off more or less of the light that falls on the moving film from a light source of constant intensity. This is accomplished by means of a special slit, the width of which is modulated by the speech currents, through which the light reaches the film, and which is called the "light valve." It is made up A strip of composite, or "movietone," film.