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April, 1935] SOUND RECORDING 301
will increase three more decibels even if the utmost care has been taken to preserve the film-track from extraneous dirt. This increase in the noise level is due to the grain structure of the photographic deposit.
Fig. 12 shows the relation between ground-noise and the specular density of the photographic deposit. It shows also that the noise increases with the increase hi the emulsion grain size. The groundnoise, however, increases when the film is run through the projector, as it accumulates microscopic scratches and specks of dust, and although cleaning will reduce the ground-noise somewhat, a film in active service in the theater which has been run some fifty times (on the whole, a favorable condition for a first-run film) has its average value of noise level raised by about 6 decibels. The apparent increase in ground-noise is much greater than is indicated by this figure because of its character.
In view of this great interest in the ground-noise, due to unavoidable handling of the film, any attempt to reduce the ground-noise due to the granularity of the silver deposit does not seem to be worth while, in view of existing theater conditions ; although if some way of avoiding the ground-noises due to other causes could be found, a reduction of the granular structure of the film is not impossible and might in that case be of value.
It is obvious that ground-noise is most noticeable when the signal level is low and is of negligible importance when the recorded sound is loud. Since the ground-noise is due chiefly to dirt, dust, scratches, oil spots, and finger prints on the film surface, and since these surface imperfections produce a larger percentage change in the photoelectric cell illumination through a film which has a high transmission than through one which has a low transmission, it is evident that the ground-noise could be diminished if the average transmission of the film decreased with a decrease in the sound level. In the case of variable-density recording, this could be accomplished if the positive sound-track were denser when the modulation was low and became lighter as the sound increased. This method is utilized in the Western Electric system of "noiseless" recording.
In this system of recording, the exposure on the negative film is made through a light-valve whose ribbons are normally spaced 0.001 inch apart, giving a certain fixed unmodulated density in the soundtrack of the negative and, in turn, of its print. This ribbon spacing is sufficient to permit the movement of the ribbons required by the