Projection engineering (Jan 1932-Mar 1933)

JANUARY. 1932 Improved disc recording AN epochal advance in sound recording and reproducing was demonstrated on December 10 to 1500 engineers at a meeting of the Society of Motion Picture Engineers and the Institute of Radio Engineers, in the Engineering Societies' auditorium, New York, by Halsey A. Frederick. Using disc records cut by the vertical method, a new high power amplifier and the latest types of loudspeakers, Mr. Frederick produced an extraordinarily faithful reproduction of organ, orchestral, and vocal music, which his audience could scarcely distinguish from the original in either quality or volume. Speaking also before the meeting, Leopold Stokowski, director of the Philadelphia orchestra, explained ■ the problems connected with recording music from the standpoint of the musician. The vertical method of recording on wax discs differs from the so-called lateral method which is standard practice in the phonograph and sound picture industries in that the groove instead of wavering back and forth along an otherwise spiral path is a true spiral whose depth varies in a perfect pattern of the sound waves which have been recorded. Such a method was the original conception of Edison but lacking modern electrical technique, it was superseded by the lateral method. With the availability of microphones, amplifiers, and other electrical adjuncts to modern recording the old method now finds itself in the forefront of progress. Among reasons for its excellence is that the needle is no longer thrown from side to side by the vibrations, with the resulting over-travel and wear on the groove, but rather rides smoothly up and down. A close fit of the reproducing point in the groove — a requisite of the old method — was secured by incorporating a certain amount of abrasive material into the disc which would soon wear the steel needle to an approximate fit. This fit is not required in the new method and hence a permanent sapphire point is used in the reproducer with a resultant saving in weight of the steel needle and its clamping device. A further saving in weight is effected by an electrical reproducer whose moving system consists only of the tiny sapphire point, a coil of flat wire about an eighth of an inch in diameter, and a bit of thin metal to hold the whole in alignment. On account of its lightness, the moving element is able to follow vibrations up to 10,000 per second with entire fidelity. Needle Scratch On account of the abrasive material embodied in the older records, the reproducing system was designed to eliminate from the output those frequencies above about 3,500 cycles, where "needle scratch" was especially conspicuous. A new method of preparing the original wax for recording, and the use of a finer grained material for the ultimate commercial records make the new rec A A ▲ Page 23 ord extraordinarily free from this source of noise and thereby brings about a much greater range in volume from the level at which surface noise would intrude on the program to the point at which the moving parts would be overloaded. Further increase in volume is enabled by the use of a vertical groove in which there is no danger, in the louder parts of the program, of the recording stylus overcutting into the adjoining groove. In copying from the original wax record, it has been the commercial practice to dust the wax with very finely powdered graphite. The particles of graphite were still large enough to contribute to the surface noise so that method has been abandoned in favor of depositing a molecular film of gold thrown down by an electrical discharge in a vacuum. On this gold film a layer of copper is electroplated and the whole is then backed up by a lead alloy. Further operations of pressing the discs are carried out much as in the present process but the ultimate records are pressed in cellulose acetate which has a surface texture extremely fine. The power amplifier, which is the last of several stages of amplification, consists of two 1,000 watt vacuum tubes connected in push-pull. Mr. Frederick explained that although its full 2,000 watts could be drawn upon if necessary, the reason for the use of such large tubes was in order that they might be very lightly loaded. With loudspeakers responding to frequencies as high as 12,000 cycles, the spurious tones produced by a heavily loaded vacuum tube are distressingly perceptible and for this reason none of the tubes in the amplifying system are worked at more than a small part of their rated capacity. Answers to Questions Nos. 21-25 CONTINUING the examination begun in the October issue of Projection Engineering, herewith are given answers to questions 21 to 25 presented in the December issue, page 23. 21. a. The lens combination which gathers the light from source and brings same to a point of focus on aperture in gate. b. An appliance for storing up electrical energy, made of a number of thin sheets of tin foil laid on top of each other and separated from each other by an insulator. c. This is generally known as an "A" condenser. The chemical condenser is best suited for low voltage work. 22. The Western Electric system employs a three-stage amplifier mounted directly on the projector with output rated to be approximately equal to the output of the electromagnetic pickup without amplification. The R.C.A. system on the other hand does not employ a special amplifier other than that regularly used in the system and the energy from the photoelectric cell is carried to the regular amplifier rack there to be passed into the amplifiers. With respect to the W. E. photoelectric cell amplifier, one of these amplifying tubes is located in the cell housing and the other two tubes are located beneath the chamber which houses the photoelectric cell. The cell is resistance-coupled to the first amplifying tube, a 239 A. Another stage of resistance-coupling links this tube with the second stage, another 239 A and the third stage is transformer-coupled operating into a third 239 A tube. The output of the amplifier is transformercoupled with a 500 ohm output impedance. 23. Motor starting rheostats or starting boxes are designed to start a motor and bring it gradually from rest to full speed. They are not intended to regulate speed and must not be used for that purpose. Failure to observe this caution will result in burning out the resistance which in a motor starter is sufficient to carry the current for a limited time only, whereas in a speed regulator sufficient resistance is provided to carry the full load continuously. 24. From 91/ to 10 amperes. 25. Because the voltage is higher and the current lower than if the dynamos were in multiple. There is a savins: also of machinery.