BANDWIDTH FOR COLOR SIGNAL TRANSMISSION
The Y signal is transmitted with full frequency bandwidth of 5 MHz for maximum horizontal details in monochrome. However, such a large frequency spectrum is not necessary for color video signals.
The reason being, that for very small details, the eye can perceive onl y the brightness but not the color. Detailed studies have shown that perception of colors by the human eye, which are produced by combinations of the three primary colors is limited to objects which have relatively large colored areas (≈ 1/25th of the screen width or more). On scanning they generate video frequencies which do not exceed 0.5 MHz. Further, for medium size objects or areas which produce a video frequency spectrum between 0.5 and 1.5 MHz, only two primary colors are needed.
This is so, because for finer details the eye fails to distinguish purple (magenta) and green-yellow hues from greys. As the colored areas become very small in size (width), the red and cyan hues also become indistinguishable from greys.
Thus for very fine color details produced by frequencies from 1.5 MHz to 5 MHz, all persons with normal vision are color bling and see only changes in brightness even for colored areas. Therefore, maximum bandwidth necessary for color signal transmission is around 3 MHz (± 1.5 MHz).