Projection engineering (Sept 1929-Nov 1930)

I*ro}ection Engineering, April, 1930 circuit having resistance and reactance, the current is : E, sin (cot — 61) + Ee Vr2 + x*2 sin (5 Vlt2 + X12 sin (3 cot + &3 — 63) + VR12 + X52 cot + a5 — 6i). Denoting the coefficients by L, la, Is, etc., we have, i = II sin (cot — ©i) + Is sin (3cot + a3 — ©3) + L sin (5<"t+ a5 — 6,). Where Xi, X3, and X5 are the reactances of the circuit for the fundamental, 3rd harmonic, and 5th harmonic respectively, -IV -1Y t9i = tan -=j; 63 = tan -=; It It dt = tan R DISCUSSION J. G. Aceves FROM the interesting presentation of Professor Powers, there seems to be a number of similarities and contrasts between the recordings by means of discs and those by means of films. It will be noted that in the case of film recording, if the phases have not been altered during the process that follows from the air pressure variations at the microphone to the light intensity variations applied to the film, the contour of the film record curve would be exactly of the same shape as that of the air pressure variations. The reproduction of such records by means of a perfect amplifier would result in an electrical pressure variation (e.m.f.) of exactly the same shape as that of the original sound wave. In the case of recording by means of disc, and assuming the same perfection in the process, the amplitudes of the wave in the disc at any time would be proportional to the air pressures which originated them. (This is not mathematically true unless the diameter of the disc is infinite). However, the generated instantaneous voltages due to the motion of the needle and armature of the pickup are proportional to the rate of change of the ordinates in the recorded wave ; or as Professor Powers pointed out, to the derivative of such curve. Hence, for higher frequencies of the same amplitude, the generated e.m.fs are correspondingly higher. It is therefore expected there will be a very "tinny" reproduction of music with very little depth of tone, according to this theory. Yet, the very same amplifier may be thrown either on the pickup from the discs or from the photoelectric cell in many commercial moving picture machines, and the music or speech comes about the same in quality whether a film or a disc is used for the same selection. The answer, it seems to me, may be found in the fact that the track in the disc recordings does not actually represent the instantaneous air pressure variations at the microphone, even ignoring the well-known reduction in relative amplitude of the very low tones. By inspection of the microphotographs shown by Professor Powers, it will be noted that the frequencies above one kilocycle are scarcely visible; in fact, the picture of the one thousand cycle "whistle" is about the only instance where a frequency as high as this is clearly discernible, and let us not forget that a steam whistle does not speak in whispers. Another interesting point to mention is, that in compound sound waves, the ear receives identical impressions from waves of entirely different shape, so long as the component simple harmonic frequencies have the same amplitudes, regardless of original phase displacements with respect to each other. This is very fortunate, since there are innumerable phase retardations, and displacements in the amplifiers, as well as in the "motors" that cut the disc and reproduce it. Only in case of transients of extremely short duration the phases may make some appreciable difference, but even here we are saved by the fact that the human ear does not recognize sounds having less than 10 to 20 complete cycles. Percy Hodge PROFESSOR POWERS' lecture and the discussion which followed it certainly brought out some most interesting facts as to the present status and difficulties of the sound-picture industry. Both the film and disc methods of recording and reproducing have defects which are not easy to overcome, and there is still much research to be done before we can sit in a movie theatre with our eyes shut and imagine we are listening to the real play or opera. As regards the disc records it would seem that one of the first things which should be considered is how to get rid of the enormous pressure at the needle point (amounting to as much as 40,000 pounds per square inch) which causes the needle to begin to wear and change its shape immediately and to produce rapid deterioration of the record. The necessity for such a high pressure is not easily apparent unless it is required in order to make the needle follow the groove accurately and not slide up on one side or the other. Is it impracticable to use any shape of groove except the nearly semi-circular one which has been adopted? Could not a groove be designed which would offer a more positive lateral thrust for a given downward pressure? In sound-picture records it is customary to start the groove at the inside of the record instead of at the outside as in ordinary phonograph records. This is done because the needle wears less at low speeds and the speed increases as the needle Page 13 moves outward. There must be on the other hand more likelihood of imperfect recording of very short waves belonging to high harmonics when the speed is slow, as at the inner rings, than when it is faster, as at the outer ones. It is obviously impossible to so alter the angular speed of the record as to produce a constant linear speed of the needle such as might be found desirable. It occurs to one that the old form of cylinder record would have important advantages over the disc if it could be used. The matter of constant speed with such a record is easily taken care of. Also a cylinder with a diameter equal to that of the average groove of a disc record would only have to be as long as the width of the groove surface of the disc in order to hold as large a record. A cylinder a foot long would hold four times as much recording as a disc with three inches of recording. One of the difficulties in making disc records is to prevent the adjacent grooves from approaching too near together where large amplitudes occur, thus weakening the wall between and causing "cross-overs." This could easily be prevented on a cylinder. The practical difficulty which presents itself at once is that of reproducing a master record in a lot of duplicates, and there are doubtless others. Professor Powers stated that, since the high harmonics possess great energy for very little amplitude, they can afford to lose a part of that by wear of the record groove and will still have plenty of energy left. Is this necessarily a correct inference? In phonograph records the absence of consonant sounds in speech is particularly noticeable, and these sounds involve very high frequencies. Can we afford to have them partly suppressed by wear? As regards the relative merits of film and disc recording and reproduction there are many plausible arguments on both sides. The use of a sound track involves an optical system which, from the point of view of a lecturer on optics, appears rather formidable. Can such a system be made fool-proof enough for the average operator? In view of the probable development in the near future of larger screen pictures economy of film space is an important consideration, and the present sound track occupies about one eighth that of the standard film. It would be desirable to leave it off if possible. It appears to be the ambition of the manufacturers to make sound pictures available for home use. The film generally adopted for this class of pictures is the 16 mm. On such a film the sound track must necessarily be reduced in size, and there enters the possibility that the grain of the photographic emulsion may cause trouble