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Gamma

From Chace Audio

Gamma is a term familiar to photographers for over a century; it represents the slope of the straight-line portion of the “H&D (Hurter and Driffield)” curve. The optimization of this curve through the amount of light and time of exposure to film yields the sharpest contrast and clearest image to the photographer.

The same principles of gamma are applied when an optical sound track negative is exposed, then are reversed in the printing process. Gamma applies to both V/A (variable area) and V/D (variable density) sound tracks, but V/D is more difficult to optimize because there are many more subtleties in contrast between the light and dark areas of this type of sound track. In the early days of sound track recording, before a V/D negative would be shot, sample tracks were exposed a number of times at different lamp voltages (usually in 1/2 volt increments) in an attempt to achieve optimum exposure - these were called wedge tests. Wedge tests (similar to cross-mod tests used today) were time consuming and costly, but if the negative wasn't exposed correctly to begin with, it would become a major problem.

At the time, studios recorded directly to optical film. As a result, they could not go “back to the master” after discovering a problem as can be done today because, after finally determining the proper exposure and shooting the film, the optical recording WAS the master. Poorly optimized gamma could not be corrected except by starting from scratch and recording again.

In the early 2000s, Chace Audio released a technology called the COSP-Xi™ (Chace Optical Sound Processor - eXtended intelligence) that has an algorithm specifically designed to adjust gamma for any V/D sound track - negative or positive. Prior to the development of the COSP-Xi, V/D sound tracks had to be printed in order to be played back correctly, and this often required the aforementioned costly wedge tests even if the negative was good. In real time, the COSP-Xi's powerful DSP can correct a negative's exposure in much finer increments than wedge tests even if it suffers from poorly optimized gamma. This removes the need to print a track in order to achieve an accurate reproduction of what was originally recorded directly to the optical recorder. Currently, there is no other method to get a more original reproduction of V/D sound track negatives.

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