Exhibitors Herald World (Oct-Dec 1930)

30 EXHIBITORS HERALD-WORLD December 27, 1930 •NOISELESS RECORDING" Western Electric System IT is common knowledge that, when a sound print of the valuable density type is played in a reproducing 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 \ 5 \B ■ . i DEN i ITTY 1 1 i > Ft 1 l . 9 3 D900O TO eo 50 PCS CENT TRAMSWSSCN FIGURE 1 Approximate Variation of Reproduced Noise vs. Density of Sound Track. 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 'Development Engineer, Electrical Research Prod»ci.s. Inc. By H. C. Silent* 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. If this can be done without distorting the volume of sound reproduced by the film, then we shall have a condition 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 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 in. 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 be tween them was therefore greater than necessary to permit the free vibration of the ribbons on weak currents. A sound track recorded under this method had a constant density corresponding to the one mil spacing between the ribbons and this / / / / / / / f LYFULL OPCN t 7 **< 2 MR. t L. V MIN OPENNG 1 / / M -MARQN V / / N RNOISE REDUCTION 5 / / / / / FIGURE 3 New Reduction Amplifier. INPUT FIGURE 2 Light Valve Carrying Capacity vs. Input. density was caused to vary with the voict currents but maintained always its constant average. Under the new system of recording aii auxiliary electrical circuit is associated with the light-valve, so that when the sound currents are small the ribbons need vibrate over but a very small amplitude, they are brought close together and this small vi bration almost entirely fills the space between them. Then, as the sound increase:in loudness, so that the ribbons are re quired to vibrate with a greater amplitude, the spacing is automatically increased by the electrical circuit, so that it is always just a little more than sufficient to permit this vibration of the ribbons. This i.' equivalent to altering the average spacing of the ribbons, so that it is at all times proportional to the envelope of the sound currents. Now, if we regard the amount of light which passes through the average spacing of the ribbons to the film, we find that this light is considerably reduced dur ing moments of silence or of low sounds, which results in a dark sound print. A> the ribbons open up for increased sound currents, the amount of light correspond ingly increases and a lighter sound print results. Since the actual vibration of the ribbons under the action of the sound cur rents has been undisturbed in this process. the amount of change of light which reaches the film and in turn the reproduc ing photoelectric cell has been unaltered even though the total amount of light ha.1been decreased. Since the amount of change of light is unaffected, there is no volume distortion on reproduced sound a> a result of this method of recording. The extent to which the light-valve rib bons may be closed during quiet intervals is necessarily limited. They must not be completely closed, because it is not possible to construct a device which can instantane ously sample the amplitude of the sound