SOUND NEGATIVE AND POSITIVE PROCESSING
On optical sound film prints, image and sound are together on the same roll. Therefore, they have to undergo the same chemical treatment during the positive processing.
On B&W as well as on Colour films the positive sound track must be properly exposed in order to achieve the best result. At the beginning of the sound era, the sound head was added to printing machines. A lamp sent a light beam through the negative sound track to expose the positive stock. The first attempts showed that correct sensitometric data were needed to obtain satisfying results. The settings were different for variable-area and variable density sound tracks.
Colour films
As far as black and white films are concerned, the printing of the optical track on the positive films poses no problems and can be carried out to ideal parameters.
The coming of colour films brought enormous problems. Only the Technicolor process that used black & white stock kept the same printing routine as before.
Subtractive processes have three layers coloured Cyan, Yellow and Magenta. The colour dyes had a low density to infrared, which is one of the sensitive areas of the photoelectric reading cells.
A sound track printed on such a film could only give bad results: low level and noise modulation. It was therefore necessary to make the dark areas of the sound track opaque to infrared rays. Means were found of adding silver to the dark areas. However, half tones could not be reproduced and for that reason variable-density process was abandoned.
Colour development reveals the three layers of the emulsion:
the silver halides and the colour dyes appear at the same time.
During the following operation, (bleaching) silver is taken out of the layers
to leave in place the colour images as silver must not be left in the picture.
The sound track is then a coloured image in two of the layers (the yellow dye
layer is not exposed).
The sound track is then redeveloped to reconvert the silver halide back to
silver so that fixing will not remove it. The application of this highly
viscous developer is made by means of a wheel that runs on the sound track. It
is cleaned after action by vigorous washing.
