Radio Broadcast (May 1928-Apr 1929)

200 JANUARY, 1929 Sound Motion Pictures (Continued from Page 182) But in the talking movies how is the director to know, when the action is recorded, what the audiences are going to do? His spacing is a matter of guesswork. He can, of course, have an audience present and try it on them, but, as every actor knows, different audiences do different things. To return to volume control, however — what is the remedy for some of these defects? My answer is no better than that of anybody else, but 1 will venture a few suggestions. One consists in disagreement with the dogma that volume control is all taken care of in recording and, after the initial gain setting is made in the projection room, the faders should be left alone. This is good theory, but it doesn't always work in the present state of the recording art. Part of the trouble is that the recording engineers, especially when they are working with discs, always have to worry about the ground noise. Their tendency is to bring up the gain on low portions and to iron out the record to one level. When the changes are not too rapid this could be fixed in projection. A wider range in volume can be secured there without excessive background disturbance, since lowering the reproducing gain brings down the ground noise with the signal. And, in general, accurate control of volume in recording, particularly in rush production, awaits future developments in instruments and technique. In short, while the best answer would be to record so well that projection could take place without any change in gain, with the present technique of recording skillful gain adjustment in the theatre could do a lot of good. The projectionist, clearly, cannot be relied on for this. For one thing, he has too many other things to do. Secondly, he is not listening in the house, but in a more or less noisy and uncomfortable projection room. Thirdly, he is seldom fitted by temperament for an audio monitoring job. A POSSIBLE SOLUTION /^\NE solution is to use automatic, electrical, ^-^ or mechanical means of some sort to vary the amplification in the theatre within certain limits. This adds to the complications, but it is a possible future development. In the meantime the best answer may be a limited gain control manipulated from a point in the house by someone who has nothing else to watch. This man may be a theatre musician, whose judgment is likely to be good on such matters as proper volume of speech and music. He will operate a remote gain control permitting a latitude of, say, 15 or 20 tu. The setting in the booth will be such that the house gain control can be brought up to the maximum without causing overloading or any such difficulties. The operator of the house control will preview the film several times with the house manager or someone in authority. In this way he will be able to arrange a cue sheet, which can readily be memorized after the first few trials, enabling him to turn out a much smoother performance than under present conditions in most theatres. For example, if there is an abrupt change in selections, he will at least be able to fade down to low volume during the shift. He will be able to drop an appropriate number of units for dialogue, bring up the gain once more for heavy musical accompaniments, tone down pianissimos which have been recorded with too much amplitude, and in general graduate the performance. 1 suspect that such a man, judiciously chosen, would more than earn his salary, and that his presence would help, in some measure, to preserve the life of the goose which is laying the golden eggs. — Carl Dreher An Inexpensive Audio Oscillator (Continued from Page i8y) condensers used in connection with the circuit. The greater the capacity across Li, the lower the frequency. It is suggested that the oscillator be calibrated from either tuning forks or a reliable frequency standard. If only the middle part of the audio-frequency scale is desired, the oscillator can be calibrated from a piano keyboard by striking a key that corresponds to the frequency generated by the oscillator. An audio-frequency transformer with a fairly good characteristic between 60 and 6,000 cycles should be used in connection with the oscillator, otherwise the higher frequencies will be cut off or will come through so weak that additional amplification will be necessary to step up the voltage of the oscillator. The question of harmonics will probably be brought up by the readers of this article; it is admitted that the percentage of harmonics with this type of oscillator is rather high. However, the results obtained with the oscil lator are sufficiently good for most experimental purposes. WHAT PARTS TO USE THE picture of the oscillator on page 186 shows Western Electric tubes; any type of tube may be used instead of the ones shown; a power-type tube is suggested, such as the 112 or 171A, depending on how much power is desired. The C bias of the amplifier tube should be carefully adjusted so that no distortion may take place in this part of the circuit. The normal bias for the 171 with 90 volts on the plate is about 16.5 volts. In the following list of parts, the writer has indicated few trade names; the reason is that any well-constructed apparatus will work as well as any other. The only special part in the list below is the Ford coil, and even here the name Ford indicates nothing more than that such a coil may be used — any similar spark coil will do as well. The complete list of apparatus follows: Ci — Several small fixed condensers of various capacities. C2 — Two fixed mica condensers, 0.00025 and 0.005-mfd.; C3 — One paper condenser from Ford unit (about 0.01 mfd.); Li L2 — One Ford ignition coil used on Model T cars; Ri — One grid leak, 2 megohms; R2 — Two Federal potentiometers, 600-ohm; R3 — One rheostat, 6-ohm; Swi. Sw2 — Two small multipoint switches (Carter, Frost, Yaxley, etc.) Ti One audio transformer of good characteristics. To place this oscillator in operation, the following will be needed: Two power tubes, 1 12A— or 171A— type; One storage battery, 6-voIt; B batteries or equivalent power supply, 90-volt: One C battery, 16.5-volt. As intensely practical problems as the frequent conflict between optimum development for the picture and optimum development for sound, comparisons between different systems of recording, and other problems which are being widely discussed and some of which will be considered in this department. The movie people, of course, have the same difficulty in the other direction. At present a lot of skillful bluffers from both camps are getting by and drawing their imposing the Broadcaster See (Continued from Page 185) salaries. This will not last. One dark morning the deflation of experts will begin, and the exproperty men and fourth-rate broadcast operators will be propelled back into the rear ranks. As a practical criterion at the present juncture, I should say that no one should be allowed to qualify as an engineer in sound movie work who cannot understand all the articles on broadcast and audio-frequency technique in the Proceedings of the Institute of Radio Engineers and the Journal of the American Institute of Electrical Engineers, as well as the papers on sound movies in the T ransactions of the Society of Motion Picture Engineers. That is a reasonable minimum; of course if he can follow some of the optical material in the latter publication, and the electrical and radio-frequency discussions in the former two, he is better qualified to hold the position and is of much greater value to his employers.