Projection engineering (Jan 1932-Mar 1933)

Page 18 Disc recording for industrial and educa tional motion pictures By L. D. Minkler* IN this paper I shall attempt a brief sketch of the entire art of recording sound-on-disc and then to describe in detail that part of the work which involves the actual practice used in transcribing the sound originating in the studio on to the wax master. First, let us trace the sound in its course from the studio to the wax. The voices, music, and other sounds occurring in the studio are picked up by the microphone, which converts the audible vibrations to a corresponding electrical current. This current passes into the transmitter amplifier, thence through the intermediate amplifier to the mixer and gain control. The mixer brings the current from individual microphones together into one circuit and the overall volume of all the microphones is regulated by the gain control. From the gain control the current passes through the input amplifier to a recording bus and from there is fed into separate amplifiers for each wax cutter used. From the recording bus the current is also fed into the monitor amplifiers, which operate the monitor loudspeaker. Usually two wax cutters and two monitor speakers are in operation simultaneously. The wax cutter consists of a permanent magnet and pole-pieces suitably arranged to create a magnetic field around a voice coil, and a pivoted armature. This armature is mounted vertically above the wax, the lower end carrying a sapphire cutting stylus. The armature and stylus vibrating in exact accordance with the electrical impulses received by the coil, cuts the groove in the wax with minute variations, which are, it might be said, the actual "sound wave" engraved in wax. The wax master, itself, is about eighteen inches in diameter and approxi mate presented at the January 7, 1932, Meeting, Chicago Section, S. M. P. E. "Recording Engineer, Brunswick Recording Laboratories, Chicago. An understandable article describing the making of disc records for educational and industrial purposes mately two inches thick, and is composed of a combination of animal fats and other materials, the compounding of which is considered a trade secret known only to the recording companies. After the wax has been cut, it must then be processed to obtain a metal stamper from which the required number of pressings, or prints, can be quickly made. Briefly, this "processing" is as follows : The wax master is treated to make it conductive to electricity. This is done by either dusting it with fine graphite, or by a treatment of silver nitrate. The silver method is preferred. The wax is now suspended in an electric plating bath in such a way as to be kept in continuous motion, and a deposit of copper is built up on the wax, forming a shell which is called the "copper master." This takes about thirty-six hours, after which time the shell is removed from the wax, the rough edges cut off and the master trimmed down to size. In "rush" cases where pressings must be obtained in minimum time, the shell may be taken off sooner and discs pressed directly from this master. However, to preserve the master from possible damage by accident, the usual practice is to plate a second shell on this master, first treating the master with a potassium solution, called the "strike" solution, to prevent the second shell from sticking to it. This second copper shell is known as the "matrix" or "mother" and is a positive. That is, the grooves are exactly like the original wax, whereas the copper master is the reverse, having ridges in place of grooves. Obviously, records cannot be processed from the "matrix," so a third copper must be plated on to the "matrix," first using the "strike" solution, to get a negative plate for stamping records. This third copper plate thus obtained is known as the "stamper" and after being nickel-plated and cut to size is ready to be fitted to a record press. The press itself might be compared to a printing press where the printing type becomes the "stamper" and the paper becomes the record material. The record material, or stock, is pre PROJECTION ENGINEERING viously prepared in slabs of just the right weight to contain enough material for one record. These slabs are then heated on a steam table next to the press, as rapidly as they are used. When hot. the stock is folded into a ball by hand and placed in the press under the center of the "stamper." As the press closes, with a pressure of several thousands pounds per square inch, the "stamper" and backing plate are heated by steam from behind them, and the hot stock is forced out to the form of the finished record. At this point the steam is cut off and cold water placed around the "stamper" so the record is hardened almost instantaneously. Then the press opens and the record is removed. The entire operation of the record press is automatic and one good operator can turn out about one thousand records a day. In its application to sound motion pictures a number of changes and refinements in the technique of recording became necessary. First, a method of interlocking the motors driving the cameras and the wax recording machine was needed. This problem has been completely solved, through the use of the interlock system which holds all the motors in exact step at all times. Electrically driven wax recording machines have been highly developed for phonograph recording during the past few years, so that speed change or variation in disc recording is almost unknown in a modern recording system. Refinements in wax cutters and machines, and a greater accuracy in recording technique have made possible the use of a finer feed, or pitch, of the recorded groove, permitting a longer playing time, without approaching too closely the center of the disc, where, as it is well known, the quality of the sound falls off very rapidly. The cutter, moving across the wax at a pitch of around 96 lines per inch has been considered the maximum for a record having good volume and long wearing qualities, but it is now practical to use as high as 120 lines per inch, without sacrificing anything in the quality of sound, volume, and long wear. No special needles are needed for re Wax recording machine showing suction pump, recording cutter and microscope. Playback pickup seen in rear of microscope.