Projection engineering (Sept 1929-Nov 1930)

Fete .2 Projection Engineering, April, 1930 Fia. 24. Whistle aDDrox. 1000 cycles oer sec. accurately scaled) 154 and 770 cycles per second respectively.2 Fig. 12B shows a fundamental of approximately 330 cycles, with the corresponding eighth harmonic. (Note the peculiar contour due possibly to the presence of an even harmonic.) This is a piano recording, and it is interesting to note that in general the shape of piano recording is not so complex as voice recording. Fig. 13 is a view (magnification 12% diameters) of a 16 inch record showing difficult recording. The weakened condition of the groove wall at the points of high lateral cuts is plainly visible. Great care is necessary when these records are reproduced to prevent the formation of a cross-over. The exceedingly high amplitude cuts employed in recording low fre 2 The inclusion of the sprocket holes in the illustrations affords a means of determining the magnification, inasmuch as the sprocket hole pitch is 3/16 inch. quencies are shown in Fig. 14. Here the recorded frequency (inside groove) is approximately 50 cycles. The wavelength at this frequency is readily appreciated when comparison is made with the millimeter scale along the top of the picture. Ten diameters is the magnification at which tbis picture was taken. A cross-section view (magnification 57 diameters) of record grooves is shown in Fig. 15. This view accurately shows the dimensional relations between the groove width, the groove depth, and the wall thickness. It might be well at this time to refer to Fig. 5 which shows all the above dimensions. Fig. 16 shows a needle tracing a groove. From this figure we obtain some idea of the relative dimensions in the vicinity of the needle tip. A constant frequency recording at 2000 cycles is shown in Fig. 17. A Fia. 25. Whistle freauencv with other frequencies. Fig. 26. Periodic low frequency groups. millimeter scale is included for purposes of comparison. The photomicrograph was taken at a radius of approximately 6.5 inches and a magnification of 10 diameters. It can be seen that at this radius tbere are approximately four wavelengths per millimeter, which corresponds to the designated frequency marked on the record. A pronounced 1000 cycle note corresponding to a steam whistle as recorded on the trailer of "The Toilers" appears in grooves eight and nine of Fig. 18. Fig. 19 is a typical example of the resulting waveform obtained by the simidtaneous recording of voice and piano. Here the waveform repeats approximately 4.6 times in every 3/16 inch indicating that the fundamental frequency is roughly 450 cycles. Another interesting uniformly repeating wave is shown in Fig. 20. The fundamental frequency here is approximately 115 cycles. A very good example of high-fre quency recording (5000 cycles) is shown in Fig. 21. The wavelength corresponding to this frequency is .0036 inches. Fig. 22 shows the same 5000 cycle recording but at a much higher magnification (30 diameters). This view clearly brings out the relative size of the emulsion grain. It is apparent that the granular structure of the emulsion is one of the serious limiting factors in high-frequency recording. Fig. 23 shows a variety of wave shapes. Attention is called to the recording on the seventh groove where a series of wave groups is found. The frequency can be scaled as 333 cycles per second, (one-third of the whistle frequency shown in groove nine) and checks closely with the corresponding portion of film recording shown in Fig. 26. (3.4 wavelengths per sprocket hole.) Referring again to Fig. 23, grooves nine and ten show a 1000 cycle frequency recording previously referred to and shown in Fig. 18. This whistle frequency checks closely with the corresponding film impression shown in Fig. 24. (10.5 wavelengths per sprocket hole — 1008 cycles per second.) It is interesting to note that this whistle frequency occurs again on the twenty-ninth groove and is easily identified by inspection. The thirtyfourth groove shows this whistle frequency superimposed on a lower frequency. The corresponding film recording is shown in Fig. 25. Intensive experimental work on pickups is being conducted by the Pacent Electric Company at the present time. Highly interesting facts have been revealed as a result of this research, but unfortunately they were not available in time to include them in this paper. Undoubtedly they will be presented to the Radio Club at some future time. In closing, we wish to acknowledge the cooperation of the following : Max C. Batsel, Chief Engineer, RCA Photophone, Inc., who furnished film and disc records for a study. Dr. Percy Hodge, Professor of Physics, Stevens Institute of Technology, who took many photographs and conducted the study microscopically. P. H. Evans, Chief Engineer, The Vitaphone Corp. and J. G. Aceves of Amy, Aceves & King, for many valuable suggestions. Adney Wyeth, Educational Director, Pacent Reproducer Corp., for valuable assistance in preparing the paper. Appendix Consider a voltage wave composed of a fundamental, 3rd harmonic, and a 5th harmonic (constant term equal to zero) then: e = Ei sin <"t + E3 sin (3<"t + a3) + E5 sin (5wt4-a5). When this voltage is applied to a