The fundamental problems of television
The word “television” by common acceptance has come to mean the essentially instantaneous transmission, either by wire or radio, of moving pictures or images.
Essentially three steps are involved in television, namely: 1) the analysis of the light image into electrical signals; 2) the transmission of the electrical signal to the points of reception; and 3) the synthesis of a visible reproduction of the original image from the electrical signal.
The ordinary concept of a picture or image is that of a surface over which there is more or less continuous distribution of varying light and dark, the distribution changing smoothly with time to conform with motion in the picture. Here the brightness is a function of the three independent variables, X, Y and T, where X and Y are the horizontal and vertical position of any point on the picture and T is the time. Obviously, such a distribution cannot be transmitted over a single electrical communication channel where the current or voltage transmitted is the function of time only.
In order to overcome this fundamental difficulty it is necessary to take advantage of certain physiological limitations of sight to reduce the amount of information being transmitted. These limitations are the finite resolving power of the eye and the persistence of vision. If a picture is subdivided into a large number of small elements, with each element being uniformly shaded, the picture will still appear continuous, provided the elements are so small that they are not resolved by the eye. Thus a picture composed of a finite number of discrete elements is entirely satisfactory for viewing.
The illusion of continuous motion can be obtained as is done in the case of the cinema, if we form a series of static pictures in rapid succession, with one picture differing slightly from the preceding to correspond to the motion which has taken place in the scene (opera, play or sporting event) during the interval between pictures. From this it will be evident that the information known to be necessary to reconstruct a completely satisfactory visual representation of a moving picture, can be conveyed by transmission of the brightness values of a finite number of picture elements at finite rate. Therefore, the conditions found to be necessary for the transmission of moving pictures over an electrical communication channel are satisfied.
The picture to be transmitted is analyzed by the process known as scanning.
The scanning element at the transmitter end – the exploring element – moves in a continuous or discontinuous line covering the entire surface of the picture. In general the size of the exploring element is equal to a picture element or smaller than a picture element. It generates, either directly or indirectly, an electrical signal, which corresponds to the brightness of the area of the image on which it is located. As the exploring element moves along the scanning pattern over the surface of the picture the electrical signal varies forming a characteristic complex wave known as the video signal.
At the receiving end of the link there is another element – the reproducing element – which moves over the screen in a scanning pattern, which is geometrically similar to that at the transmitter. To obtain a picture at the receiver the scanning beam at the receiver must keep accurately in step with that at the transmitter: in other words, at any instant, both scanning beams must be moving over the same line of the image and must be at the same point in that time.