I truly appreciate the poster who has taken on this rather technical subject but I must point out the creation of a variable area recording is not explained correctly. The optical slit is not what moves but rather the real image being projected of a solid triangle moves across the fine slit to generate anywhere from no line of light to a wide line of light. The method of moving the light is from a galvanometer device (moving mirror) driven by audio plus a direct current bias plus a low frequency AC noise reduction signal to cause the width of the recording line to reduce in size during quiet or no sound passages of the recording. It you are into this marvelous technology and it captured your fancy like me then find a copy of "Elements of Sound Recording" by John G. Frayne & Halley Wolfe, 1947 (circa), Interscience pub. Also 16mm Manual by William Offenhauser, 1949 also Interscience pub and learn the secrets to this art.
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