International projectionist (Jan-Dec 1935)

10 INTERNATIONAL PROJECTIONIST May 1935 0-8 * MP westcn^jo/ rfMMETE/Z —y ao AMP. 250 f ~ tp1ple p0le switch expuls/m t/pe SefANTC/^r. #76*2 W.L. 3 ZHEOSTPiT P£& FIGURE 6 IMPORTANT : TH/5 CONTROL CA8/NET /S DES/GA/EO ZO REGULATE THE E/ELD CURPEMT rHZOOGH S55-W RSCE/VEXS iVHEN THE ROtVFR SOURCE /S /8 VOLTS £>.C. switches intended not only to reveal the exact point of trouble in a few seconds, but also to restore the show in an almost equally short period of time. The left-hand side of Figure 5 shows the TA-4008 receiver field control cabinet that is diagrammed in detail in Figure 3. To the left of it is its filter. Above it, installed with the connecting conduit, are two single-pole tumbler switches. One short-circuits the stage line, one the monitor line. Assume a break anywhere in this long and complicated series circuit, reducing sound volume to a faint whisper. A snap of the stage-line short-circuiting switch will reveal whether the break is backstage. If it is, sound will immediately return to full volume in the monitor. (Also the ammeter in Figure 3 will go off-scale, because all the backstage resistance has been removed.) Conversely, restoring the stage lines and short-circuiting the monitor will restore sound to the house, if the break is in the monitor line. Backstage Breaks If the break is backstage, with more than one speaker there, the job of finding out which one has open-circuited is simplified by the switching cabinet shown on the right-hand side of Figure 5. The stage crew (or the projectionist) short-circuit each speaker line by means of the switches in the "B" box. When the faulty line is short-circuited, sound resumes at full volume and the necessary splicing or speaker replacement can be attended to at leisure. All that is needed meanwhile is to readjust the rheostat in Figure 3 to compensate for the loss of 4.6 ohms from the series circuit. trouble then becomes a longer and more complicated job. The 6-volt signal lamp of Figure 3 will, however, go out in any case, since it is not connected across the line but is lit by means of the drop through a 4-ohm resistor; and when the series circuit is opened anywhere there is no current in that resistor and no drop across it. The Circuits of Figure 6 Figure 6 is a circuit similar to Figure 3. but one that has only a single rheostat because it is always used with an 18-volt d. c. generator. The speakers in this case may be wired either in series or in parallel. Two speakers backstage will be wired in series, while four require a series-parallel connection, in which the ammeter will read 3 amperes. o + STAGE HOPN5 o The monitor is in parallel with the line, its current regulated by the W. L. Type "A" 7.4 ohm fixed resistor. This resistor can be fixed because the source is always 18 volts. The 18-volt, 2-candlepower lamp is wired directly across the supply line. The current that lights it does not flow through the ammeter. Tracing this circuit from the positive input terminal at the left-hand side of the drawing, the line runs to the ammeter positive terminal. From this point a branch circuit runs downward and right to Point 2 of the switch, through the switch bar to the right-hand Point 2, thence down and left through the signal lamp, and left and up to the negative side of the line. The main positive line continues through the ammeter, then right, down, right, down and right to Lower Point 1 of the switch. Through the switch bar to Upper Point 1, through the rheostat and out to the stage horns. The return is through the negative stage-horn terminal to Lower Point 3 of the switch, through the switch bar to Upper Point 3, then left, down and left to the negative line fuse. Circuits similar to Figures 3 and 6 are the most common line voltage control arrangements. Figure 1, although used in many theatres, is less frequently encountered. ADVANCES IN RECORDING, REPRODUCTION CITED BY RCA CHIEF ENGINEER A RECITAL of the remarkable progress scored in the complex art of bringing realism to sound motion pictures was contained in a series of papers delivered by RCA Photophone research engineers today, before the S. M. P. E. Convention in Hollywood. Opening the symposium, Max C. Batsel, RCA Photophone engineering chief, traced the history of the development of sound motion pictures in the research laboratories of RCA and its associated companies and pointed out that these pioneers were actuated by fundamental ideals which many studio technicians If the two switches shown at the top of now neglect because of their absorption the left-hand half of Figure 5 do not restore sound, either to the monitor or the stage speakers, then the trouble is in the apparatus of Figure 3, or in the power source. Finding and fixing the with production problems. He told how years of experience in sound picture work had guided the direction of new developments. The net results are today's highly per fected recording and reproducing devices "which more nearly approximate the engineer's goal of reproducing the original sound so as to effectively produce the illusion of being present at the scenes depicted by the camera." Mr. Batsel declared that recently developed recording equipment eliminates practically all of the objectionable distortions produced by the mechanical equipment itself. He added that insufficient attention has thus far been paid to the creation of ideal recording acoustics in the studios and ideal reproducing acoustics in the theatres. Double Theatre System Desirable For the reproduction of speech in the theatre the auditorium should be free of resonant conditions and appreciable reverberation through the use of sound absorbing materials, he said. Yet these (Continued on page 30)