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SOME TELEVISION DETAILS 653
and Philadelphia, and New York and Chicago, but the branches from those cables are ordinary telephone line. Hence, most theatres within the foreseeable future will have to receive television signals by radio.
Television Film Projectors
(8) The television camera may be used to reproduce a motion picture rather than an actual scene. There are some advantages, when instantaneous reproduction is not necessary, in making a motion picture of the scene to be televised and subsequently televising from film.
(9) For this purpose a common photo-electric cell can take the place of the television camera. One type of television motion picture projector operates without an intermittent movement and without a shutter. A large disc with a ring of small holes near its outer edge rotates in the path of the light. As one hole sweeps across the aperture it scans one "line" of the picture horizontally, the light passing through that hole being focussed upon the photo-cell. By the time the next succeeding hole scans the picture the film has moved downward slightly, thus a lower "line" of the picture is scanned. There is no need for an intermittent movement and there is no need for a shutter. In order to obtain 30-frame scanning the speed of the projector motor is increased to provide a rate of film motion of 30 frames a second.
(10) Other devices for using motion picture film in association with television equipment include a projector without an intermittent movement but fitted with a rocking mirror which receives the light from the aperture and focuses it through a lens on a stationary mirror. A television camera in turn is focussed upon the stationary mirror.
(11) There is also a type of television projector using a modified old style Powers movement. The modification consists in the fact that the pull-down occurs at unequal intervals. Alternate frames remain stationary l/20th second and l/30th second, respectively. Alternately the cam rotates 144° and 216° before the next pull-down occurs. The over-all film speed is standard.