AP mast (2K)

DIRECT TELEVISION from ALEXANDRA PALACE

by Arthur Dungate

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The Suppressed Frame System
Some Fundamental Aspects of Telerecording
by C.B.B. Wood, Designs Dept., Engineering Division, BBC



The Portrayal of Motion

It has already been stated that picture recording systems introduce some degradation of the original programme and only waveform recordings can give an accurate reproduction. In addition to degradation by loss of definition and distortion of tone gradation in the photographic process, all picture recording systems fail to portray moving objects with complete accuracy and the reason for this is to be found in the fact that such television recordings are made to motion picture standards.

The Motion Picture Film and Flicker

The standard 24-per-second frame repetition rate of the cinema is sufficient to give an excellent representation of moving objects, but unfortunately the eye is extremely sensitive to flicker at this frequency, even at very low screen brightnesses, and in order to avoid this unpleasant effect, two separate presentations of each film frame have to be made by interruption of the light from the projector while the film is stationary in the gate. This of course raises the presentation frequency from 24 c/s to 48 c/s at which the eye will tolerate about one hundred times the screen brightness for the same consciousness of flicker. In this way there is effected a considerable economy of film; it would otherwise be necessary to make all films at 48 frames per second although 24 or even 16 frames per second are adequate for the portrayal of motion, but the economy is nevertheless obtained at the expense of clearly-defined moving objects.

If the film were made at twice the standard frame frequency in order to avoid difficulty with flicker, every moving object would be portrayed in 48 different positions per second. Inability to detect individual rapid changes would cause the eye to receive the impression of smooth motion, and it would therefore follow the moving object at the average speed of the motion across the screen. In the present method, however, every image is presented in each single position twice, and, being unable to follow this discontinuous progress, the eye still moves on at the average speed of the image.

This means that after the first showing of any given film frame, the eye has moved on before it receives the second presentation. This second showing therefore cannot become superimposed in the viewer’s eye upon the persisting impression of the first presentation.

Showing how a viewer receives the subjective impression of a double image of moving objects when each film frame is presented twice, as in normal cinema projection. (a) is the actual image on the screen (b) is the subjective impression received by the viewer (20K)
Showing how a viewer receives the subjective impression of a double image of moving objects when each film frame is presented twice, as in normal cinema projection.
(a) is the actual image on the screen
(b) is the subjective impression received by the viewer

The viewer is left with the subjective impression of a double image of moving objects, the leading image being renewed by every first presentation, and the second image, trailing behind by half the frame to frame movement of the object, being renewed by every second presentation of each film frame. Depending on the nature of the moving object, and the rapidity with which it moves, this defect may appear as a clearly defined double image or it may be ohserved as an unpleasant "juddering" motion.



Still frame taken from a normal cinema film. Exposure angle of the camera shutter = 160° When projected, this film portrays motion in the manner to which cinema-goers are accustomed (19K)
Still frame taken from a normal cinema film. Exposure angle of the camera shutter = 160° When projected, this film portrays motion in the manner to which cinema-goers are accustomed

This defect is largely overcome in the cinema industry by the use of a sufficiently long exposure in the camera to cause the image of all moving objects to be smudged rather than clearly defined. This tends to merge the two parts of the double image formed subjectively in the viewer’s eye, so that although moving objects are blurred, the "juddering" motion is not apparent.

Still frame taken from cine film with exposure angie of the camera shutter only 20° When projected, this flim is unpleasant and tiring to watch; the motion is jerky and flickering (20K)
Still frame taken from cine film with exposure angie of the camera shutter only 20° When projected, this flim is unpleasant and tiring to watch; the motion is jerky and flickering

Films Reproduced by Television

The same difficulty exists when films are broadcast by television; the 50-frames-per-second interlaced television display is equivalent in this respect to the two-bladed shutter of the cinema projector. Each frame of film is displayed twice on the television screen and, being unable to follow the discontinuous progress of moving objects, the eye again forms the subjective impression of a double image. Although the long exposure, blurred image, remedy adopted when shooting the film is equally effective when films are reproduced by television, an additional effect arises in that one of the two images formed in the eye has been derived from an odd television frame while the other is from an even television frame. Where the two images do not overlap, only half the line structure will be visible and in this country the 200-lines edge to moving ohjects is one of the principal defects of televised film.

Moving Objects Portrayed by Telerecordings

It may be argued that the reproduction of motion picture film, in spite of its defects, is acceptable and that if television recordings are reproduced equally well, the result will be satisfactory. Unfortunately, however, in recording television it is not possible to adopt the long-exposure remedy for the double image of moving objects since the effective exposure of most types of television camera is very short. Individual frames of a telerecording usually contain sharp, discrete images even in the case of quite rapidly moving objects, with the result that the characteristic juddering motion is often observed.

In telerecording there is also an effect caused by exposing the film at only half the frequency of the original television display. In most recording systems two interlaced television frames are recorded in each film frame, and since each television frame portrays the moving object in a different position, a double image will be recorded wherever there is movement in the scene.

This applies to all systems where the full information of the television picture is recorded at 25 frames per second and it can easily be shown that when reproduced, such films will give the viewer an impression of no less than three images of each moving object. The suppressed frame system produces subjectively only a double image, since each frame of film is exposed to a single television frame. This is, however, not necessarily an advantage, since the triple subjective image produced by other systems appears to have some effect in reducing judder.

For repeat programmes in this country there is a case for recording at 50 film frames per second to avoid all multiple-image difficulties, and to give smooth motion equal to that of the original. While such films would be exchangeable with some countries, they would not be suitable for Canada or the U.S.A., and most telerecordings therefore seem likely to remain inferior to the original programme from the point of view of movement.

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Resolution of the Recording Process

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