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236 PRINCIPLES OF CINEMATOGRAPHY
produces a negative exposure range greater than that which may be accommodated on the straight or proportional path of the exposure-transmission curve.
It is now possible to appreciate fully the need to maintain a very long straight section to the curve, which will cover a wide exposure range since, the greater the undistorted exposure range the greater will be the sound wave amplitude which can be faithfully reproduced. However, it would appear that we must not lengthen this straight section of the exposure-transmission curve by altering its angle or gamma value, since it is only possible to reproduce changes in amplitude which are in true proportion when the overall gamma value of the entire system is at unity.
The Sensitometry of Variable Density Sound Recording
It would now seem that we must first establish three factors: (1) the position on the negative characteristic curve representing a light valve exposure when no signal of any kind is passed, that is, the unmodulated unbiased exposure, (2) the degree by which the normal light valve ribbon spacing shall be biased to take care of noise reduction and, (3) the excursion along the characteristic curve caused by the valve opening and closing by amounts corresponding to the maximum signal amplitude we wish to record. Since the first factor can more easily be determined after establishing the second, we will now consider the general problems affecting noise reduction.
As has been explained previously, the noise created by film base and low emulsion densities becomes quite noticeable during passages of low sound volume. Because of this, variable density light valve recordings are rendered less noisy by applying a bias current to change the mean spacing between the valve ribbons in accordance with the 'envelope' of the frequencies applied to them and, during passages of no volume, this bias closes the ribbon separation considerably below its normal width.
Special microscopes, designed to permit the easy examination of light valve ribbons may usually be so connected to most recording machines that, for example, the valve ribbons may be observed in the microscope whilst a biasing current is applied to them. The normal unmodulated separation between light valve ribbons is usually 0-01-inch.
The amount of noise reduction usually applied is of the order of 10-db (decibels, see Chapter 7) -that is, the valve is biased to close the average unmodulated ribbon separation from 1 'O-mils