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SOUND -ON-FILM EQUIPMENT
147
in this case it strikes a mirror, and from this is reflected to the photo-electric cell. This makes no difference whatsoever in the functioning of the apparatus, but merely provides for convenience of fixing the photo-electric cell. In the case of the R.C.A. equipment we have the light beam passing from the exciter lamp through the lens assembly, and the edge of the film carried round the sound drum. Inside the sound drum is the small [reflecting mirror carried on a bracket, and from this the modulated light beam is thrown
Fig. 72a. Optical System of R.C.A. Equipment
on to a photo-electric cell carried within the main casting, and not visible in the illustration. It should be mentioned that the sound drum is somewhat different in construction from what it appears in the illustration, and is cut away so that the light beam passing through the sound track is unobstructed.
The difficulty with 16 mm. sound-on-film is that there is little space available for the sound track. In R.C.A. system this has been overcome by utilizing a film with perforations down one edge only after the style of Fig. 72b. where it will be seen that the space usually allocated to one set of perforations has been completely utilized by the track. The size of the picture remaining is precisely the same as that in ordinary silent 16 mm. films.
The function of the pressure roller is merely to hold the film snugly against the sound drum, and of the idler roller