16-mm sound motion pictures : a manual for the professional and the amateur (1953)

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246 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT ated with variable-area recording. Recently, however, both Western Electric and RCA make either form of recording available. Measuring the distortion of a light modulator is a difficult matter, since the modulator would have to be removed from its associated optical system. Ordinarily, the single-frequency distortion at full amplitude at the frequency of maximum distortion can be kept under 2% in good commercial designs. So far little data has been published by manufacturers concerning distortion and its measurement in their light modulators, or, for that matter, to be expected in their complete recording systems. Intermodulation tests for studying the distortion characteristics of variable-density recording systems (American Standard Z22.511946) and cross-modulation tests for variable-area systems (American War Standard Z22. 52-1946) have no doubt already been applied to the + 10 I 1 1 1 1 II ! 1 ! 1 OQ + 5 A o UJ Q 0 " *s \ ■ ~^ \ -5 l I 8 CVi O O ro O O O O <j if) O o o o o o o o o ro O o o o o o in ooooo oo o oooo o o ooooo o o <x> r-oocno cm ^ CYCLES PER SECOND Fig. 58. Response-frequency characteristic of galvanometer used in Maurer sound recording machine of Figure 52. study of light modulators and to their testing just as they have been applied in the study of distortion arising from film processing. This subject will no doubt receive much attention in the future. Ordinarily it is not practicable for a user to remove a light modulator from its machine for the checking of its distortion; the difficulties encountered in readjusting the machine for correct optical and mechanical alignment in reassembly are almost insurmountable for the ordinary user because of the need for highly specialized alignment test techniques and equipment. The most practicable procedure is to test the sound-recording machine as a complete unit by recording on film whatever tests are necessary ; the film should be of the same emulsion number and lot number as that used for production recording. A great, deal can be learned by the inspection of such developed film as viewed under a microscope. The trend in light-modulator design is toward higher primary-resonance frequencies. The new Maurer machine has a tuning point for its