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Regulating Density of Sound Track
By Automatically Doing That at the Recorder Technicians Have Reduced Materially Effects of Ground Noises
By H. C. SILENT
Development Engineer Electrical Research Products, Inc., in Paper on “Noiseless Recording Western Electric System"
Read Before Academy Technicians
Copyright. 1930, by Academy of Motion Picture Arts and Sciences.
It is common knowledge that, when a sound print of the variable density type is played in a reproduction machine, the volume of the reproduction is low if the print is dark and if a compensating adjustment is not made by turning up the fader. In addition, the ground noise of the film is also low.
It has been a problem to take advantage of this latter fact with the former methods of recording, because the mere act of printing the sound track dark, while it reduced the ground noise, also reduced the volume of sound from the film.
This, of course, was undesirable. In the method of recording which is now being employed, these undesirable effects are overcome by regulating the density of the sound track at the recorder automatically.
It is well known that there .is a particular value of density or transmission of the photographic emulsion which permits of the loudest volume from the film without exceeding the photographic limits of good quality. Deviation from this point is possible without distortion if the volume or percentage modulation applied to the film is reduced. This can be taken advantage of by causing the film to be dark on low volume modulation, and as modulation becomes higher we lighten the film to the point where it has the greatest possible carrying capacity.
Noise Worst in Quiet
If this can be done without distorting the volume of sound reproduced by the film, then we shall have a con
dition where the ground noise from the film is low during periods of low sound. Thus quiet intervals in the sound will be quiet, and the ground noise, even though it rises with the sound, will always be more or less drowned out by the increased sound, so that there is an effect of considerably reduced ground noise.
In other words, there is produced a constant signal to noise ratio in which the signal is always very predominant over the noise, and since the noise is most noticeable in the quiet intervals, there is a very real reduction in the amount of the ground noise.
There are a number of methods by means of which this variation in the transmission of the film can be effected. If we examine for a moment the light-valve employed in the Western Electric system of recording we shall see how one of these methods can be applied. In the past this system has employed a light-valve in which two ribbons were normally spaced .001" apart.
These ribbons were vibrated by the sound currents, moving but a slight distance on weak currents and a considerable distance on loud currents. The strongest currents would just bring the ribbons into contact as they vibrated. The space between them was therefore greater than necessary to permit the free vibration of the ribbons on weak currents.
Vibrating Ribbons
A sound track recorded under this method had a constant density corresponding to the one mil spacing
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