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The question of frame-frequency and flicker.
Now that the British public has had the opportunity of judging for itself the progress already made in television, a quite understandable curiosity has arisen regarding the optical problems involved. For example, many viewers have been puzzled to find that although one system has 240 lines and the other 405, and one 25 frames per second against the other's 50, so far as definition is concerned there seems nothing to choose between the two systems. Indeed, many have expressed the opinion that the smaller number of lines seems to give the better definition. Whether this is so or not, the mere existence of this divergence of opinion shows that any difference can only be slight.
Another question asked by those who are home cinema enthusiasts, is why there should be any flicker on the system using 25 frames per second when the modern home cinema, showing only 16 frames per second, is virtually flickerless. The 50 frame system certainly has the advantage over the 25 in the matter of flicker, but why should there be flicker with either?
It maybe be of some assistance to the student of television if we consider a few of the optical problems surrounding the projection of a television picture, and remove certain misconceptions which exist regarding persistence of vision and flicker.
First of all, persistence of vision and flicker must be separately considered, for they are not the same problem. The 'time-lag' of the cells in the eye is such that if we project a cinema or television picture at any speed above twelve frames per second, persistence of vision will be secured, and (always providing that the picture was taken at the same speed as that at which it is being projected) movement will seem to be natural and not jerky. If now with the same apparatus we project instead of the picture a series of blank 'frames' at this rate of twelve frames per second or lower, we shall perceive upon the screen a most irritating flicker. If we vary the intensity of the light while maintaining the same frame speed, we shall soon find that the brighter the light the more irritating and pronounced is the flicker.
I should say, at this juncture, that the experiment cannot be performed with ordinary home cinema apparatus because this is designed to remove flicker. I am supposing the experiments to be performed with an apparatus which will project each single frame on to a screen, will simply move the film downwards so as to bring the next frame into position and will cover the aperture during the time the film is being so moved.
Now still projecting blank frames (with clear film) let us speed up the apparatus to find at what point flicker ceases. Many people will be surprised to find that the flicker - at least with a reasonably bright light - will persist until we reach well over 40 frames per second! It will thus be realised that flickerless projection is not obtained as soon as we reach a sufficient speed to give persistence of vision for film images, and that the eye is acutely susceptible to changes of light intensity.
The flicker problem was a great worry to the pioneers of cinematography, as it was confused with persistence of vision. For this reason early cinematograph apparatus was run faster and faster with the idea of eliminating the flicker until a speed of some 50 frames per second was reached. Flicker then ceased, but this speed was impracticable in commercial apparatus fer several reasons, such as the excessive cost of film, and the difficulty of making apparatus to work at so high a speed and still give a steady picture.
Cutting off the Light
Two-bladed shutter as used in modern cinematograph theatre projection. 24 frames per second are shown, and the shutter causes the light to be cut off 48 times per second. In this way flicker has been abolished, in spite of the fact that the frame-frequency is actually lower than that used in television.
Then it was discovered that by using a three-bladed shutter in place of the single blade variety, one blade could be used to cover the aperture while the film was being moved downward and the other two blades to cut off the light momentarily twice during the projection of each frame. Thus, sixteen frames per second could be projected on the screen while the light beam from the projector could be cut off 48 times per second. This was found to abolish the flicker while maintaining a reasonable speed of projection.
Sixteen frames per second became the standard speed in commercial cinematography until the advent of the talkies, when the speed was increased to 24 frames a second - not because there was any flicker trouble, but for the reason that the necessary frequency range could not be satisfactorily recorded on a sound track travelling at less than 90 feet a minute (the speed of travel for 24 frames per second). Home cinema apparatus for silent pictures has, however, retained the standard speed of 16 frames per second, although 24 frames is used for sound-on-film home talkie apparatus.
With the increase of frame speed from 16 to 24 it was no longer necessary to use a three-bladed shutter, for a two-bladed shutter would afford the necessary 48 maskings per second and greater light efficiency is thereby obtained. Some home cinema apparatus is provided with both two- and three-bladed shutters, the two-bladed shutter being available either for 24 frames a second projection or else for those occasions where a very big picture is being projected and the maximum light is required. As with a big picture the light intensity is proportionally reduced, the flicker is less noticeable than it would be with a normal size of picture, and the additional gain in light is worth having.
We now see why it is that in the two television systems now being projected the 25-frame system still shows flicker while the 50-frame system is practically free from it. The reader might think that the simple way would be to introduce a rotating shutter to break up the picture twice per frame in the 25 frame system, or alternatively the cathode ray itself might be cut off twice per frame, but a moment's consideration of the method of producing television pictures shows that any cutting off by either method would produce black bands in the picture.
If an attempt is made to apply cinema technique to reduce flicker by cutting off the light momentarily during projection, a black band will appear in the televised picture, the width depending on the duration of cut-off.
We are not projecting a whole frame at a time but building up the picture point by point, and those parts projected during the cut-off period would appear as black lines or bands. In my opinion there are several possible solutions of the flicker problem with the 25 frame picture, and increasing from 25 to 50 frames per second merely to reduce flicker is, I think, a wasteful use of the etheric spectrum.
When we come to the question of definition itself many other interesting problems present themselves. At the present time with both systems, and in fact with any practical television system so far suggested, definition depends not merely upon the number of lines per frame or picture, but on the sharpness or definition of the moving spot itself (whether the scanning is mechanical or electrical) on certain physiological attributes to the human eye, on the 'time-lag' of the fluorescent material on the end of the cathode-ray tube when this is used, and of course on the perfection of the transmitting apparatus. There has been a tendency in some quarters to suggest that the greater the number of lines used the better will necessarily be the definition, but it is easy to see that line frequency is only one of many factors, and the experimental transmissions at present proceeding do not show any marked difference in definition between the two systems. It is, indeed, highly probable that neither system is getting the definition possible even with the smaller number of lines used.
I have mentioned in a previous paragraph certain physiological characteristics of the human eye. In this connection it is interesting to note that the definition of a moving object in television nearly always seems better than the number of lines would suggest. Those of us who worked with 30 line television must have been very surprised at times at the detail visible, which certainly seemed in excess of that theoretically predicted. Some light on this problem can be obtained by examining a home cinematograph film of an object in motion in the foreground of the picture, such as a person walking across the field of view some 10 or 15 feet away from the camera, The picture will appear on the screen to be perfectly sharp, but if the projector is stopped on a single frame or, better still, the strip of film itself examined under a magnifying glass, it will be seen that each individual frame is considerably blurred, so much so, in fact, that the inexperienced might imagine the film would be useless for projection purposes.
A final point - for space does not permit us to discuss all the many interesting problems of definition - is the degree of detail perceptible by the average human eye, and the great difference in apparent detail between the examination of a single picture and receiving in the eye the additive result of a number of pictures projected one after the other. When the existing elements of a television picture have been-further refined it will quite probably be found that all the sharpness required for the finest detailed picture will be obtained with a number of lines not greater than 500.
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