Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, October, 1929 Page 19 Mechanical Coupling If a dummy control be used, there must be no appreciable backlash between the dials of the dummy and master units. Early types of faders made use of a rack and pinion drive between the units. This was soon found to be unsatisfactory and was superseded by bevel gears and shafting equipped with universal ' joints to relieve inaccuracies in the mounting alignment. A recent design, introduced, and illustrated in Figs. 1 and 8, eliminates the need for bevel gears and requires only straight shafting and universal joints between the dials of the master fader and the dummy control. Fig. S. Another view of the fader shown in Fig. 1, showing connection panel and selector switch for changing from film to disc track. It has been found that a properly designed step-by-step contact switch is quieter and more reliable in its electrical operation than the ordinary form of continuously-variable slide wire. Care must be taken, however, in the proper choice of materials for the contact studs and brushes in order to obtain a combination which shall be free from oxidation which might introduce erratic contact resistances. Minute current variations produced thereby, after enormous amplification, would introduce, especially at high volume levels, a disagreeable scratchin'r noise in the loudspeakers whenever the fader was manipulated or subjected to jarring. Characteristics of Peck Amplifiers Technical Data on Transformer-, Resistance and Impedance-Coupled, Photoelectric Cell Amplifiers By F. A. Jewell* THERE seems to be an endless debate over the merits of sound-on-film and sound-ondisc, as to which is the best. Both have their advantages as well as their, disadvantages, and to give a resume of both systems would wind you up just about where you started. But this much can be said in favor of sound-on-film ; since it is the youngest of the two, naturally it has not reached the state of perfection that sound-on-disc has. It can be said that sound-on-disc has just about reached as high a state of perfection as possible and when there has been applied to sound-on-film as much engineering ingenuity as there has been on the former, it stands to reason that this system will be by far the most acceptable, unless its progress is retarded for commercial reasons. One of the big drawbacks to soundon-film is that the electrical energy output of the photoelectric cell is so weak that additional amplification is required to bring it up to the same level of the electrical pickup. This additional amplifier is the big bug-aboo of the system, as the greatest care must be used in its construction to overcome such things as microphonic noises, audio feedback or oscillation and the picking up of strays. However, due to good engineering, most of these problems have been overcome and the peck or head amplifier will deliver to the main amplifier, a fair duplicate of the input sound wave, free from most of the microphonic noises, oscillations and strays. Although there is room for considerable improvement in this amplifier, it does not present any great problems, and if a true sound wave is delivered to its input from the output of * General Manager, Projectionist Sound Institute. the photoelectric cell, this system of sound reproduction, even in its present state of development, will be far superior to the sound-on-disc method. Peck Amplifier Coupling But right here is the "nigger in the wood pile," so to speak — the method of coupling that is used to deliver the output of the photoelectric cell to the grid of the first tube in the peck amplifier. The method of converting the sound waves on the film into light waves, that are impinged on the photoelectric cell, may be perfect as well as the peck amplifier and the rest of the amplifying system, but the quality of reproduction in sound from the loudspeakers will be no better than what this coupling is capable of transferring. In the author's estimation, this should be termed "the heart of the entire system," and due to its apparent insignificance, there has not been as much engineering applied to it as it deserves. There are two generally accepted methods of coupling which are used between the output of the photoelectric cell and the peck amplifier, i.e., transformer coupling, as shown in Fig. 1 and resistance coupling, as shown in Fig. 2. Both of these methods have their advantages as well as their disadvantages, which will be cited here; and then another method will be described which has proven by test to be the superior of the two, as it overcomes most of the disadvantages of both these systems. Transformer Coupling The first thing to take into consideration in describing the circuit of transformer coupling, as shown in Fig. 1, is that the impedance of the photoelectric cell is in the vicinity of a million ohms. This involves the problem of designing a transformer with a primary impedance of a tremendously high value. When you design a transformer of this type that will have sufficient reactance to reproduce with true fidelity the lower or base notes, the distributed capacity of the windings, as shown by dotted lines in Fig. 1, will have a very low cut-off on the higher frequencies, short-circuiting these high frequencies to the ground ; so you have your choice of cutting off the higher frequencies in favor of the low or the favoring of the low to the high, as it is almost physically impossible to design a transformer with a straight-line characteristic curve, from 60 to 5,000 cycles, at the impedance of approximately a million ohms. However, this system is not without merits. Its chief advantage is the high ratio step-down of the transformer which lowers the impedance of the output, which is generally located near the photoelectric cell and transferring its energy through a low impedance circuit to the peck amplifier, which is generally located from two to three feet from the head of the machine. This low impedance circuit from the photoelectric cell to the peck Circuit diagram of a transformercoupled photoelectric cell amplifier.