2 Optical Sound Technology

In initial reports on photographic attempts at recording sound in the early nineteenth century one can read how Daguerre plates were exposed using a mirror affixed to a recording membrane. The discovery of the photoelectric characteristics of selenium^[4] in 1873 led to the development of the photo cell, which was then used in sound films to transform variations in light intensity into sound. Ernst Ruhmer and Eugène Augustin Lauste laid the foundations for the optical sound process. In 1916, Dénes von Mihály presented the first optical sound screenings, followed by Sven Berglund in 1921, by Joseph Tykociński-Tykociner in 1922, and also Hans Vogt, Joseph Massolle, and Joseph Engl, who made a name for themselves as the Tri-Ergon society.

Sound film constitutes the earliest recording medium for sound and image: celluloid strips contain both the images of events unfolding in a timeline and optical sound: audio events that have been visually recorded. In the course of the development of sound film, several optical sound recording processes emerged that, while they differ in terms of their media-technical details, are comparable with regard to the principles of audiovisual transmittability. The recording of oscillographic peaks^[5] has established itself as a standard practice.

The recorded sound is transformed into electrical voltage fluctuations using a microphone. The signal is transferred to a light image by a mirror moved electromagnetically, which vibrates in accordance with the voltage fluctuations generated by the sound. This oscillating mirror reflects a ray of light that creates an image of the vibrations in the form of an audio track set between the picture frame and the perforation on the moving celluloid strip. In this way, the sound is recorded as an oscillographic curve, quasi photographed.^[6] The exposed audio track has a level of transparency proportional to the sound pressure level: when the amplitude is higher, the level of transparency increases. When the film is projected, the curve is scanned accordingly, following the reverse schema. An electronic light source shines through the audio track as it passes and hits the photo cell behind, which generates an alternating voltage level in relation to the amount and intensity of light. These alternating voltages are made audible by means of amplifier tubes and loudspeakers.