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526 S. S. A. WATKINS AND C. H. FETTER [J. S. M. p. E.
track is somewhat shorter than for a picture negative if the same bath is used. Dependent upon the type of system, developer, and method of control, a different negative bath formula may or may not be required for sound work.
It is of course necessary to have the proper exposure values both for the negative and positive. The exposure of the negative is controlled in the film recording machine while the exposure of the positive is controlled in the printer. It is desirable at all times to work on the linear or straight line portion of the H and D curve. If we have incorrect exposures and operate on the curved portion of this curve, deterioration in quality is likely to result evidenced again in the resultant picture by sounds which appear overloaded and full of harmonics.
However, it is a simple matter to determine the proper exposure values. In the recording machine the negative average exposure is half the maximum since the light valve has a normal opening of 2 mils and, limited by closing, has a maximum opening of 4 mils.
The density corresponding to one-half maximum exposure taken from the linear portion of the H and D curve is the proper density for the unmodulated sound track of the negative and a lamp current in the recording machine which gives this exposure is the correct one to use. Here again, however, this value is not critical as, for example, with too low a lamp current while we keep below the knee of the curve we only sacrifice slightly in volume range with the result that the full use of the emulsion properties is not made.
The proper printer point to use in printing sound track depends again on the emulsion characteristic of the positive and upon its development. In the negative the lamp current was at a point to give average exposure over the straight line portion of the H and D curve. In the positive the printer light is set to give an average density corresponding to average transmission of the positive which for all practical purposes is one-half of the maximum transmission. The maximum transmission corresponds to the density at the lower end of the straight line portion of the H and D curve. If we increase the printer light we usually may continue on the straight line part of the curve before running into the curved knee portion. Hence there is considerable latitude in choosing printer point but it appears desirable from surface noise conditions and other considerations to keep the transmission of the positive somewhere around 20 per cent.
Re-recording. — One of the most important things in connection with