International projectionist (Oct 1931-Sept 1933)

October 1932 INTERNATIONAL PROJECTIONIST 27 I I I I I I I I I I I I I I I I I I I ' M I 1 I I I I I I III (doo e,ooo ^4,000 eftoo /ctooo Fig. 5. Response-frequency characteristic of ribbon microphone I 1 I I 1 .1 11 I I I I I I I I I I I I I I 11 I I I I I I I 1 1 "«' io sa Too zoo '*oo eoo /.ooo e,ooo 4000 epoo /ctooo Fig. 6. Response-frequency characteristic of reproducing amplifier able width track. The average transmission of such a track is 50 per cent, regardless of the amplitude of the recorded wave. Figure 4 (B) illustrates the kind of variable width track now being used, which utilizes a separator shutter for blocking a portion of the light beam when the recording level is low. The black rectangle in this figure represents the shutter vane. The current that actuates this vane is obtained by rectifying and filtering a portion of the signal. Figure 4 (C) shows how the doubleedged symmetrical track is made. The triangular beam of light moves at right angles to the axis of the slit, so that as it vibrates, the length of the illuminated portion of the slit varies. As in Figure 4 {A), the average transmission of this track is 50 per cent. Figure 4 (D) illustrates a sound track made by this method, utilizing the biasing system of eliminating ground noise. The mean position of the triangle depends upon the current that is supplied to coil d (Figure 2) by the biasing amplifier, which is controlled by the incoming signal. When there is no signal the triangular beam of light is biased to the position shown in Figure 4 {D), producing a very narrow transparent line down the center of the track. When a signal is impressed upon the system, the biasing current is reduced, and the triangular beam of light is raised until the track is sufficiently wide to accommodate the signal. The symmetrical track may be reproduced by standard theatrical equipment without change. Film Speed Vital Factor The extension of the frequency range has made more urgent the necessity of maintaining accurate constancy of film speed in both recording and reproducing machines. Variations of the speed of either the recorder or the reproducer produce the so-called "wows." When the variation of speed occurs at a high frequency, due, perhaps, to gears or sprocket teeth, a more harmful kind of distortion is introduced. Due to speed modulation the high frequencies are distributed into seVeral side-bands, and the reproduced notes sound rough or "wheezy." Variations of speed have been reduced to a minimum in the new recorder and in the reproducer. Another important improvement made in the system lies in using a microphone of the ribbon type, which furnishes uniform response at frequencies ranging from 40 to 10,000 cycles per second. The directional characteristics of this microphone are independent of frequency. The new high-quality recording and reproducing equipment was demonstrated at the Spring, 1932, meeting of the S. M. P. E. Two loud speaker units similar to the one described were employed, being placed behind a perforated screen. The reproducing amplifier had an undistorted power output of 40 watts. It was operated completely by alternating current, and was assembled on a single rack that contained all the necessary electrical equipment. Figure 6 is the frequencyresponse curve of this amplifier. The demonstration records were reproduced on a film phonograph that employed a magnetic drive similar in principle to the one previously described by E. W. Kellogg.' 1 Kellogg, E. W. : "A New Recorder for Variable Area Recording," /. Soc. Mot. Pict. Bug. 15 (Nov., 1930), No. 5, p. 653. RANDOM NOTES ON EXTENDED FREQUENCY SYSTEMS James J. Finn Fig. 4. Four types of variable amplitude sound-on film track ANOTHER milestone in the progress of the sound picture art has been passed with the introduction of recording and reproducing apparatus which extends materially the frequency range. Epoch-making is the term which correctly describes this new development, but it is to be feared that the general moviegoing public will not appreciate its significance in the same measure as do the technical workers in this field. Highquality sound reproduction, in which marvelous tonal quality plays so important a part, undoubtedly will "sound good" to Mr. Average Man, and with this measure of praise will the hardworking technicians have to be content. But here we are talking about public quality-consciousness instead of technique and equipments. This writer has heard extended frequency range systems developed by both major sound companies in this field: the High Fidelity system (to give it its formal title), of the Photophone Division of RCA-Victor Co.. Inc., and the Wide Range system of Electrical Research Products, subsidiary of Western Electric. Both systems evidence a difficult problem intelligently approached and smartly executed and both represent a job so well done as to offer little, if any, basis for direct comparison. Magnificent sound, comparatively speaking, is the net result in both cases. One who knows even the least bit about the art cannot help but be tremendously impressed by the full rich tone, the marvelous sweep and surge and the unerring control demonstrated by these new sound systems. In the last analysis, reproduction will prove the merit of both these systems. In this connection a word of warning might be sounded. No sound company, as such; no engineer, no commercial