American cinematographer (May 1933)

K American Cinematographer • May 1933 This graph shows a gamma of .45 obtained by using a .003" and .0006" lines of light in combination width. Opt ical Twin Fidelity With Glow Lamp by Hartley Harrison N OT since 1880, when Edgar Fritts patented a means of recording and reproducing sound vibrations, has there been anything new in the principle of the light beams for the recording or reproducing of sound. The accepted practice has been to create as fine a line of light as possible not only for both recording and reproduc- ing, but also for both variable density and variable area recording. The theory being that the narrower the width of the beam the higher the frequency response, and of course, the better the quality. It should therefore be of considerable interest in the realm of sound when some different principle, namely the use of two lines of light, is employed in the recording of sound. But, before going into a description of the two line principle let us re-familiarize ourselves with some of the optical conditions of sound recording. Although theoretically the width of the line of light determines the frequency response of the optical system, in actual practice, the character of the width of the line, such as sharpness of edge, uniformity of light across the width is so powerful a factor in controlling the frequency response that the width of the line is of secondary im- portance. This has been demonstrated by using a .0006" line of light whose edges were very sharp and the intensity of the light across the width very uniform compared with a .0006" line of light whose edges were not sharp and a varying in- tensity across width. With the sharp uniform line the fre- quency response was 25,000 cycles contrasted with a fre- quency response of only 9,000 cycles for the line whose edges were not sharp and the intensity varying across its width. This difference is unquestionably due to the fact that ordinarily when the line of light is not uniform across its width the variation is a falling off of intensity from the center to the edges which gives a one half sine wave con- dition to the line of light, this half wave subtracts from the sine wave which is being recorded on the film to the amount of its magnitude, which, of course, depends upon how fast the intensity falls off toward the edges. This condition of falling off towards the edges increases as the line of light is made finer, particularly as the mechan- ical stops are brought closer together increasing the ratio of penumbra or edge shadow to the illumination area so that in attempting to create an extremely narrow line of light you run the risk of not only losing exposure, but also of not gaining the expected high frequency. And, of course, in the case of the two .0006" lines of light the one falling off at 9,000 cycles can be duplicated by using a line of light of good character that is .002" in width. Therefore when greater exposure is desired for glow lamp recording it would naturally follow that the line of light should be as wide as possible, consistent with its character as one means of increasing the exposure. Experiments with a .003" width line of light having excellent character showed almost enough exposure for straight line recording burning the glow lamp at a static current of 5 millamps. connected from a noise reduction amplifier, and the quality of the lower register was far superior to the quality obtained with narrower lines of light. Although the frequency response cut off at 6000 cycles the quality of the lower register can be accounted for by the fact that the increased width increased the film motion tolerances by about three times over a normal line of light. (The theory of film motion relative to aperture size is I think too well understood to merit discussion in this article.) Of course, for good recording it is necessary to record much higher frequencies than 6000 cycles and further ex- periments were conducted by using a .0006" line of light in combination with the .003" line of light. The result of this test proved more than gratifying, the frequency range was extended to 17,000 cycles although maintaining the same quality in the lower register and the exposure as shown in the accompanying graph gave a gamma of .45 with fifty per cent transmission with the lamp current still at 5 mill- amps. The performance of this dual optical system may be lik- ened to the dual horn combinations where one horn responds from 25 to 5000 cycles and the second horn responds from 4,000 to 16,000 cycles, except that the narrow line of light which records the high frequencies is recording all of the time, adding to the exposure of the wide line of light, although not changing the quality of the lower register. While at this writing there has been only a few thousand feet of commercial recording completed with this system, the first picture was composed of solid string orchestration and violin solos, the quality of which can only be appreciated by actually hearing the sound.