The methods of recording on disc are practically the same for all producers and they differ only in details.

65. Variable Density Recording on Film. -There are two different methods of variable density recording. One method, used by Fox Movietone, utilizes a variable intensity light called an "Aeolight." This light varies in intensity with the amplified sound currents and shines through a narrow slit onto the moving film, which is kept running at a constant speed of 90 feet per minute. A quartz block is coated with silver to make it opaque, and the slit is engraved in the metal to the desired size. The width of the slit is about 0.0008 of an inch. When the film is developed after being exposed to the variable intensity light, the sound tract will be made up of lines of varying density extending across the sound track as shown in Figure 47. If the frequency of the recorded sound is low, the width of the bands will be comparatively large, but, if the frequency is high, the bands of light and dark will be very close together and may be hardly distinguishable. The variation of density between successive dark and light bands determines the amplitude of the recorded sound.

The other method of making variable density film recordings is by the use of a light valve. This method is used in the Western Electric system. The light valve in this case varies the amount of light by the opening and closing of a slit. This slit is the space between two taut sides of a loop of wire suspended in a magnetic field. As the sound current passes through the loop, the loop opens and closes passing varying amounts of light through it. This light is then focussed with lenses on the moving film so as to form lines of varying density when the film is developed.

Section 66 was erroneously omitted from the original printing.

67. Variable Area Recording on Film. -The RCA Photophone employs the variable area method of recording on film. In this system the intensity of the light is kept at a constant value, but the area of the sensitized film which is effected by the light is varied. The system consists essentially of a source of light, a mirror which is vibrated by the amplified sound currents, and a suitable optical system for concentrating the light into a very fine beam.

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