39 - Telecines

Abstract

The term photoconductive applies to those telecines that incorporate an electronic camera as the pickup device; a variety of camera tubes can be used such as plumbicon, vidicon, saticon etc. The light source is usually a tungsten halogen lamp and the projector will have an intermittent film transport so that the image on the film frame can be transferred to the camera tube target while the film is stationary, a necessary requirement as the camera tubes exhibit storage characteristics. The former method of transmission is called letterbox and the latter panscan. Obviously the letterbox mode does not require editing but the panscan mode does, and this editing is performed prior to transmission by pre-programming with the aid of a small computer and a memory. Prior to the advent of television, the standard film rate was 24 frames/second and this speed is still probably the most common. Obviously film produced at 24 frames/second could not be transmitted at 30 frames/second for that particular system because the increase in sound pitch and picture speed would have been unacceptable. In sprocket driven projectors, the sprockets rotate at a uniform velocity synchronized to the television sync pulses to maintain the required film frame rate. As film shrinks with age, up to 3 per cent for nitrate base and 0.5–1 per cent for acetate base, the rotational speed of the sprockets and mean film velocity falls in proportion to the film shrinkage.