Hence, flicker of the individual lines is not evident.If the eye did not have this fortunate property, a television channel would have to occupy about twice as much spectrum space as it now does.The particular compromises chosen for a given television service—e.g., broadcast or cable service—are embodied in the television standards adopted and enforced by the responsible government agencies in each country.Television technology must deal with the fact that human vision employs hundreds of thousands of separate electrical circuits, located in the to the brain, in order to convey simultaneously in two dimensions the whole content of a scene on which the eye is focused.At the receiver the waves are translated back into a corresponding sequence of lights and shadows, and these are reassembled in their correct positions on the viewing screen.; that is, the brain retains the impression of illumination for about one-tenth of a second after the source of light is removed from the eye.If, therefore, the process of image synthesis takes less than one-tenth of a second, the eye will be unaware that the picture is being reassembled piecemeal, and it will appear as if the whole surface of the viewing screen is continuously illuminated.
If flicker is to be unobjectionable at brightness suitable for home viewing during daylight as well as evening hours, the successive illuminations of the picture screen should occur no fewer than 50 times per second.The higher picture-transmission rate of North America allows the pictures there to be about five times as bright as those in Europe for the same susceptibility to flicker, but this advantage is offset by a 20 percent reduction in picture detail for equal utilization of the channel.The second aspect of performance to be met in a television system is the detailed structure of the image.Aspects of appropriate compromises between the quality of the desired image and the costs of reproducing it.They must also be designed to override, within reasonable limits, the effects of interference and to minimize visual and audial distortions in the transmission and reproduction processes.
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A printed engraving may possess several million halftone dots per square foot of area.