SOUNDTRACK APPLICATION
Colour Positive Films
Colour positive films require a single stage applicator.
After developing the film, it is bleached normally to convert the silver image of both picture and sound track to silver bromide. Before fixing the film is coated with a "stripe" of viscous, highly active developer in sound track area, by means of a steel wheel running on the sound track area.
The sound track developer converts the silver bromide back to silver. The result is a dye picture image and a combined metallic silver and dye sound track image. Usually the exposure of the sound track is limited to the top two layers of the emulsion to achieve the sharpest image.
Colour reversal print
Colour reversal print films require a double application stage.
After the first developer the sound track area is fixed by a viscous sound track fixer leaving only silver in the track. Later in the process after bleaching the film image back to silver halide the sound track is redeveloped by a sound track developer.
Thus, two application stages are required.
Defects of Application
If the developer runs into the picture area instead of being restricted to the sound track area, black streaks or patches will be visible on the edge of the screen. If breaks appear in the developer application "bead" then abrupt changes in sound level are heard. Breaks or variations width of the bead are usually caused by a dirty applicator wheel, dirt in the solution, incorrect viscosity of solution, insufficient "shaping" of the viscous bead, or incorrect levels of solution the wheel dips into.
Frequent and close variations will produce a low frequency rumble.
Recent Developments
While compiling this text, we are confronted with a new development, which is making Soundtrack Application obsolete. In facts, a new sound cell has been designed and it is in the process of progressively substituting the existing ones. The new cell is sensible to red light, which means that soundtrack can be produced by using only the cyan layer. This makes soundtrack reprocessing and application unnecessary. It is also possible to use Soundtrack Application on release prints so that they can be used on both projectors with the new or the old sound cell.
In order to know updated details about this recent development, we recommend you to contact film or sound equipments manufacturers.
It is still to be fully understood what implications this recent development will have on existing collections of soundtrack negatives, all of them having been made in order to be printed onto positive prints where the soundtrack had to be reprocessed.