The motion picture projectionist (Nov 1931-Jan 1933)

November, 1931 Motion Picture Projectionist 25 Outline of Sound Recordi >ouna r\ecoraing By George DoBSONf Part III of Mr. Dobson's Outline of Sound Recording deals principally with the subject of recording on disc. The author describes in detail the manner in which such recordings are made and discusses the advantages and disadvantages of the disc as a recording medium. — The Editor. Part HE IT will be noted from Figure 5 (see previous installment) that in the studio provision is made for two types of recording, disc and film. The first talking pictures, those made by Edison, which, however, were not a commercial success, used a phonograph record at that time in cylindrical form. The first talking motion picture which recorded sound and musical accompaniment, "Don Juan," with its accompanying shorts, as well as the overwhelmingly successful "Jazz Singer," both made by Warner Brothers, were on discs. Commercial considerations such as ease of cutting the negative, facility in meeting the demands of the censorship boards by changes in the positive and somewhat simpler handling which, of course, is familiar to the projectionist, have placed sound reproduction by film commercially ahead of that on disc. One cannot foresee what may happen, but at present only 20 per cent of the reproduction is by means of the disc. In recording, however, the disc has the outstanding advantage that the sound can be played back immediately, whereas in film recording it is necessary to develop the negative and make a positive print before the sound can be reproduced. When a director is in doubt whether the recording is satisfactory, or when artists new to the motion picture work are being trained, such an immediate reproduction is of great importance since it enables the immediate correction of faults of diction and verbiage, thereby saving much delay and many retakes. The relative quality of the two methods is still a moot point. Disc Recording However, in view of its historical importance and the fact that disc is still being used in a large number of theatres, a discussion of this method of recording may be of interest. It might be noted that while in only a few cases original recording is still being done on disc practically all the producers make their disc records by re-recording from original film records. The method of recording on disc is at present very similar to the method of recording phonograph discs previous to the introduction of talking Fig. 8. Disc Recording Machines motion pictures. However, the stylus or cutting instrument, is electrically driven and an electric drive is also used for rotating the disc under the stylus. To provide a material which can be readily cut, a so-called "wax" disc is used. This is actually a hard soap, somewhat similar to laundry soap in appearance, also containing other materials, such as Canuba Wax to make it easier to cut. The Recording Turntable This disc is rotated at a speed that must be so nearly constant, that it is only by use of special checking methods, such as a stroboscopic flashing light, or flutter tests, that even objectionable diversion from constancy can be detected. While the disc is rotating, the recorder is moved very slowly •{•Commercial Engmeering Dept., Electrical Research Products, Inc. Fig. 9. Record, Magnified in a horizontal plane by means of a fine screw and a half nut so that if no talking currents are used to operate the stylus, a very fine spiral will be cut in the disc; the adjacent turns of the spiral being approximately .010" apart. The machine for accomplishing this is shown on Figure 8. The motor drive is under the bench at the right, and the large pot at the left contains the device for damping out any variations in the speed of rotation of the disc. The wax disc is on the left of the machine on the turntable. The edge of the latter is marked with vertical lines for checking the constancy of speed by stroboscopic methods. The microscope over the disc is for checking the width and cleanness of cut also for checking in case of possible overcuts. The small tube with the bent end in front of the recorder is normally connected by means of a rubber tube to the suction equipment above. It is used for removing the wax thread (cut by the stylus) so that particles of wax may not fall on the engraved portion of the wax and mar it. Action of Stylus When the talking currents are impressed upon the recorder, the stylus moves to and fro in a sideways direction, so that the spiral is no longer even, but takes a snake form. A small portion of the record, much magnified, is shown in Figure 9. The effect of a sound of constant pitch, when impressed upon the recorder, is to produce a regular wavy pattern.