CHAPTER VIII

SOUND HEADS

(A) SOUND REPRODUCTION FROM FILM RECORDINGS

81. General Discussion.-We have seen that the aim, in all types of recording on film, is to create a photographic record in the form of a narrow sound track which would vary the amount of light through it (from a steady source of illumination) in proportion to the sound pressures on the diaphragm of the microphone.

Sound reproduction from film recordings requires that the variations of the amount of light transmitted through the photographic record (from a steady source of illumination) be accurately translated into sound. To accomplish this a thin beam of intense light, the width of which is equal to the width of the sound track, is focused on the sound track. The varying light which passes through the sound track affects a sensitive photo-electric cell so as to cause a varying electric current to pass through it. In the RCA Photophone system, this varying current is passed through a transformer primary. The voltage generated in the secondary of the transformer is amplified in the vacuum tube amplifier, the output of which is used to operate loud speakers.

82. General Requirements for Good Reproduction from Film Recordings.-The source of the light which shines through the film must be steady (that is, there must be no fluctuations in the amount of light). The beam of light must be as thin as the beam used in recording, and exactly as wide as the sound track. Stated as dimensions, the beam should not be more than 0.00075 of an inch thick and be exactly 0.070 of an inch wide.

The translating device (photo-electric cell) must be capable of accurately translating the light variations into electrical variations.

The speed of the film passing the light beam must be the same as the film speed of recording and must be absolutely constant. Variations in speed would cause variations of pitch which would be recognized as "wows."

The sound track of the film must be clean. A dirty sound track would cause extraneous variations of the transmitted light and produce a grating noise in the loud speakers.

83. Reproduction from Variable Density and Variable Area Recordings.-Although the sound tracks of the variable area and variable density recordings do not look alike, the variations of the light transmitted through them are the same. Therefore, reproducing equipment which is suitable for reproducing from one type of recording is equally capable of reproducing from the other. All producers of standard sound recordings on film use the same width of sound track, and use a light beam of approximately the same thickness.

84. Exciter Lamps.-Five ampere exciter lamps are used in most of the RCA Photophone equipments but 7½ ampere lamps are sometimes used. These lamps should be operated at a current slightly less than that for which they are rated. They should be kept clean. Any good grade of lens fluid is satisfactory for this purpose. As the lamp becomes old a dark coating inside the lamp materially decreases its efficiency. For this reason exciter lamps should not be used until burned out, but should be replaced when the coating reduces the efficiency of the lamp to a point where satisfactory results cannot be obtained when the lamp is drawing its rated current.

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