International projectionist (Jan 1943-Dec 1944)

Contest Question on Volume Control Oilers Many Ingenious Solutions THE ANSWERS to I.P.'s fourth contest question were many, indicating an increasing interest in emergency procedures to keep the show going. What to do when the main system volume control (or fader) fails, there being no other volume control in the system, has brought a large variety of answers. Several contestants proposed to control volume by the adjustment of a rheostat connected in the exciter lamp circuit. Others proposed to reduce the illumination on the sound track or the photoelectric cell by cutting down the light mechanically. The introduction of colored gelatine into the light beam from the exciter lamp was also proposed. The substitution of a voltage divider in conjunction with variation of the exciter lamp or PEC voltage, a homemade volume control or a radio volume or tone control were included. The most ingenious proposal is based on the electrical conductivity of a common salt solution. The contestant proposes to place two fixed electrodes, representing the two outside terminals of a potentiometer, in a jar containing the salt solution and suspend a third electrode, the slider of the potentiometer, in the solution. Volume then is to be controlled by raising and lowering the third electrode. A pulley or other suitable means may be employed to permit the volume adjustment being made at a convenient location on the front wall. Since this control is in the voltage (or PEC amplifier) the signals are of comparatively low level and, as such, subject to noise pickup. This is particularly true of the connections to the grid (slider) and the coupling capacitor from the previous stage. Hence careful shielding is necessary. Also long grid leads are potential sources of serious trouble. So while this method might give sound, if the constants were correctly chosen and preliminary tests made to determine its characteristics, other methods having less probability of trouble are preferable. The control of volume by adjustment of exciter lamp brilliancy gives good results within certain limits. As the brilliancy decreases the color spectrum of the light changes, which affects the frequency response. Toward the lower limit of sound audibility, the adjustment becomes quite critical. This method may be used, with a divider circuit, for i datively small volume changes. Operation above its rated current shortens the life of the lamp materially. Varying the light falling on the optical system and hence on the sound track by an adjustable aperture or dissolver rapidly changes the volume. Here again frequency response is affected when any large volume changes are attempted. Such a procedure would be satisfactory in an emergency, but it would be an improvement to use the divider circuit also. Such a device positioned to reduce the light on the photocell is not nearly as sensitive, as the signal is generated by the change in light intensity but still offers a possibility. Colored gelatines, either between the exciter lamp and the sound track or in front of the photoelectric cell, change the color of the light and, therefore, the quality of sound. Both the sound track and photoelectric cell have a very definite light color response. This may, however, be a rapid way of obtaining volume control when colored gelatines are readily available. The voltage divider circuit may be applied whether the system has a main volume control (or fader) common to both machines or whether each machine has its own. In this case a constant impedance pad having a loss corresponding to the full house fader setting isdesirable. This takes care of the average recording. An auxiliary volume control is necessary to give some adjustment for news reels or high level recordings, and possibly for low level prints if experience indicates this to be necessary. This may be a rheostat in the exciter lamp circuit, the photoelectric cell polarizing potential circuit, or a gain attenuator in the main amplifier. The PEC polarizing potential is not too critical below the flashover point so any ad Contest in Wartime Projection A Test of Skill and Wits KNOWLEDGE of projection, skill, and resourcefulness in meeting unusual conditions arising out of the war feature this novel contest, which is open to all practicing projectionists. Fancy writing, skill of presentation, win no prizes; prizes are awarded solely on the basis of how well the contestant has met the problem presented. The editorial staff of LP. are the sole judges, and their decisions are final. The following prizes are offered each month : First Prize $10.00 in War Stamps Second and Third Prizes $5.00 in War Stamps Next Six Best Answers . . One Year's Paid-up Subscription to I.P. Additionally, at the end of the contest, there will be awarded for the most consistent showing a Grand Prize A $25.00 War Bond All answers much reach this office by the tenth day of the month following publication of the question: that is, all answers to February's question, published below, must reach LP. by March 10. Here is the question for February: Your sound-head motor drives the sound head and projector by a device other than a belt. This device fails and is beyond repair. Because of war conditions, you can't get new parts for some weeks. What would you do? Apply this question to your own equipment, your own projection room. It's your problem, you have to solve it; there'll be no show till it's solved. For the most ingenious and practical solution you win $10.00 in war stamps and a running start toward the Grand Prize $25.00 war bond. 14 I1VTER1VATIONAL PROJECTIONIST