Proceedings of the British Kinematograph Society (1936)

the laboratory to adhere to the specifications given to them by either the manufacturers of the particular sound system or by the studios using the equipment. There are two main types of sound recording, variable width and variable density. In the case of the former system the laboratory usually receives instructions to develop the track to a certain density, in Hre..5 most variable width systems between 1.2 and 1.5, in the positive developer. The correct exposure on the film in the recorder to achieve the required density at a norma] . developing time in the particular positive developer is obtained by exposing sound recording film to a series of lamp current changes in the recorder, developing the exposed film in the positive developer for the normal time and selecting the lamp current giving the required density. Most laboratories use the normal positive print developing machines for processing variable width sound track, but a few use a separate lay-out for this work. The Western Electric variable density recording system is much more complex in its requirements in so far as the laboratory is concerned, and it is the practice of the Western Electric Company to make their recommendations only after an exhaustive sensitometric survey of the conditions obtaining at the particular laboratory. Briefly. however, variable density track is developed in a borax type developer to a low gamma value, varying from about 0.35 to 0.45, depending upon the laboratory, and having unbiassed, unmodulated negative densities of from 0.55 to 0.65. It is necessary to maintain the sound negative gamma at a low figure, because the system requires that the overall gamma, that is, the product of the negative and positive control gammas muitiplied by the projection factor, shall be unity, and the picture requirements on the print require that the positive shall be developed to gamma values of 2.00 and over. Positive Development The development of positive film containing prints of both sound and picture is controlled in the larger laboratories entirely by sensitometric means, the practice being to maintain a pre-determined gamma value by altering the machine speed (or the replenisher flow) according to the indications obtained from sensitometric exposures on strips of positive film which are run through the machine at hourly or half-hourly intervals. The gamma values to which prints are developed vary from laboratory to laboratory. In general, however, it can be said tiat in England the lowest gamma value adhered to by any laboratory is about 2.10 and the highest 2.45, whereas in the United States these values run from about 1.9 ito 2.4, although the majority of American laboratories operate between 2.0 and 2.2. The development of master positives on lavender duplicating positive film is carried out under the same conditions as those described for positive film. The exposure requirements for this class of film vary considerably, according to the opinions held in the particular laboratory upon the vexed subject of duplicating, but Fic: 6. it is safe to say that the correct master positive after development is always denser than a normal release print on the same subject. In general, perhaps, the practice is to print the master positives about two printer points higher than would be used for the release print.