Kinescope - Technology

Technology

NTSC television images are scanned at roughly 60 Hz, with two interlaced fields per frame, displayed at 30 frames per second.

A kinescope must be able to:

1. Convert the 30 frame/s image to 24 frame/s, the standard sound speed of film cameras,
2. Do so in a way so that the image is clear enough to then re-broadcast by means of a film chain back to 30 frame/s.

In kinescoping an NTSC signal, 525 lines are broadcast in one frame. A 35 mm or 16 mm camera exposes one frame of film for every one frame of television (525 lines), and moving a new frame of film into place during the time equivalent of one field of television (131.25 lines). In the British 405-line television system, television ran at 25 frames—or more correctly, 50 fields—per second, so the film camera would also be run at 25 frames per second rather than the cinematic film standard of 24 frames.

Therefore, in order to maintain successful kinescope photography, a camera must expose one frame of film for exactly 1/30th or 1/25th of a second, the time in which one frame of video is transmitted, and move to another frame of film within the small interval of 1/120 of a second. In some instances, this was accomplished through means of an electronic shutter which cuts off the TV image at the end of every set of visible lines.

Most U.S. kinescope situations, however, utilized a mechanical shutter, revolving at 24 revolutions per second. This shutter had a closed angle of 72° and an open angle of 288°, yielding the necessary closed time of 1/120 of a second and open time 1/30 of a second. Using this shutter, in 1 second of video (60 fields equaling 30 frames), 48 television fields (totaling to 24 frames of video) would be captured on 24 frames of film, and 12 additional fields would be omitted as the shutter closed and the film advanced.

Because television is a field rather than frame-based system, however, not all the information in the picture can be retained on film in the same way as it can on videotape. The time taken physically to move the film on by one frame and stop it so that the gate can be opened to expose a new frame of film to the two fields of television picture is much longer than the vertical blanking interval between these fields—so the film is still moving when the start of the next field is being displayed on the television screen. It is not possible to accelerate the film fast enough to get it there in time without destroying the perforations in the film stock—and the larger the film gauge used, the worse the problem becomes.

The problem of adapting the way the image is either displayed or captured on film, to get around the above, was solved in various different ways as time went on—improving the quality of the image.