International projectionist (Jan-Dec 1957)

IS MAGNETIC REPRODUCTION WORTHWHILE? (Continued from page 17) nance, and impart a characteristically "tinny" tone to the reproduced sound. In Fig. 2 the shaded portion of the chart represents the available soundcurrent output of a modern theatre amplifier. Note that sufficient leeway is provided in both the low and highfrequency regions of the sound spectrum for considerable latitude in adjusting the response in these regions quires special correction of the frequency response. Figure 3 shows that the "signal" recordable on uncompensated optical soundtracks (curve A) is fairly level from 0 to about 2,000 cycles, the point where progressive high-frequency attenuation, due to the width of the 0.5-mil recording slit, results in a loss of about 5 decibels at 5,000 cycles, the "peak region" of 4,000 cycles. This peak is produced by the recording characteristics of the magnetic stripe and by the width of the magnetic gap. Obtaining Level Response A fairly level signal strength in the main 100-8,000 cycle band is obtained in both types of track by special preamplifiers which raise the weak regions and flatten the peaks. Different types of optical recording require different frequency corrections; and if it is de 50 IOO 200 500 1000 2000 5000 10,000 1 FREQUENCY IN CYCLES PER. SECOND 15,000* FIGURE 2 20 50 100 200 500 1000 2000 5000 10,000 "FRECtUENCY IN CYCLES PER, SECOND FIGURE 3 to suit the characteristics of the soundhead (optical or magnetic), the peculiarities of the speakers, and the acoustics of the auditorium. Although this amplifier permits a level output from 30 to 15,000 cycles (see the horizontal line marked "0 db" on the right-hand margin), a perfectly level output is seldom desirable in practice. A slight "boost" of the bass tones with attentuation of the high frequencies beyond the 5,000 — 8,000 cycle range gives more pleasing, if actually less natural, sound. Then too, the acoustics of each auditorium re 10 db at 8,000 cycles, 12 db at 10,000 cycles, and 20 db at 15,000 cycles. Such tracks, however, are usually played with wider scanning slits (1 and l1/^ mils), producing even greater high-frequency attenuation in reproduction, so far as photocell output is concerned. The CinemaScope magnetic track (curve B) is extremely irregular when uncompensated by the recording amplifier. It is relatively only half "normal signal strength" at 60 cycles, and again slightly above 20,000 cycles; but it is nearly twice normal strength in sired to print the track in all three emulsion layers of dye-coupler film, the high-frequencies are given an extra boost to make up for the slight loss of scanning-beam focus. Note, however, that track noises and various types of sound distortion are increased by boosting a weak signal too much. In Fig. 4 we can examine the level response obtainable from optical tracks, as recorded with modern highfidelity, wide-range apparatus. From the practical point of view, optical sound may be considered level (if we want it that way) from 30 to 10,000 FIGURE 4 FIGURE 5 \ (,0 { \ 50 1 40 ■ 1 30 / " \ / \ w / \ 1 Mj». 10 4 * ( s O # '--', .: % at '""»""»i """"' ma a tfa it ■(awuuk .,..._ w. y £ !■§ to / 'A B so / 40 \ . 30 ' / \\i f \ \i 20 B hi 10 m>. '% A ijjfll O ^ i i %'>/,'< ;«f flB 20 50 100 200 500 1000 2000 5000 10,000 Trequency in Cycles per Second 10 20 50 IOO 200 J00 1000 2000 5000 10,000 "Frequency in Cycles per. Second 28 INTERNATIONAL PROJECTIONIST • JANUARY 1957