Journal of the Society of Motion Picture and Television Engineers (1950-1954)

tion which describes hue and saturation.4-5 Panel 11 of the NTSG conducted a careful set of experiments,6 the results of which are summarized in Fig. 3. In making these tests all pictures were presented with a bandwidth for the luminance component extending out to 4 me. The additional information required to insert color into the picture may be thought of as "color-difference" signals; two color-difference signals taken together can be used to tell how far from gray and in what direction from gray a particular color is, relative to a gray of exactly the same luminance. If the color-difference signals have a narrower bandwidth than the luminance signal, then color gradations are not as sharp on the picture although the full sharpness of luminance gradations is preserved. The horizontal coordinate (on the chart, called "crossover frequency," in reference to the specific apparatus of the test) is the amount of bandwidth devoted to each of the two color-difference signals in a particular observation. The vertical coordinate refers to the percentage of the number of observations in which the various classes of observers rated the picture quality as "satisfactory." The curves indicate that even with a luminance bandwidth of 4 me, and the observers permitted to sit where they wished, there was no significant improvement in pictorial quality obtained by increasing the bandwidth devoted to color-difference signals above 1 me. The curves also show that this statement is true both for skilled and for lay observers. In the tests made by Panel 11 both color-difference signals had the same magnitude in any one observation. Subsequent tests, the results of which have not yet been published, have indicated that along a particular direction of color difference the bandwidth may be decreased to approximately ^ me without noticeable impairment of the picture, especially if the bandwidth in another direction is retained at the value of 1 me or perhaps slightly greater. We also note that the eye responds only slowly to changes in color; thus small rapid fluctuations about a correct color value go unperceived, whereas corresponding fluctuations about the correct brightness value would be perceived immediately as flicker. This latter point indicates that greater exposure to electrical disturbance may be permitted the color component of the signal without impairment of performance than may safely be permitted to the monochrome component of the signal. Sharing of the Channel The luminance component of a colortelevision signal has the task of controling, element by element, the brightness of the received image. It is most important for successful color-television transmission that this task be well discharged. The standards of the Federal Communications Commission for monochrome television provide the means for discharging this task well, and the use of these standards should certainly be examined as a basis for a successful colortelevision system. It has been known for a long time that a normal monochrome television signal by no means completely fills its channel;7-8 the spectrum consists in general of a component at each harmonic of the line-scanning frequency, with each such component being accompanied by a cluster of smaller components spaced from the main component by the fieldscanning frequency. Figure 4 illustrates a small section of the spectrum, showing the region in the vicinity of the eighty-second and eighty-third harmonics of the line-scanning frequency. The dotted line half-way between the groups illustrates the absence of any signal information at this region. The signal spectrum consists therefore of groups of components at the successive harmonics of the scanning-line frequency, or bet A. V. Loughren: Color Television 325