Service manual

STP 11-25R13-SM-TG
S - 21
(4) The terms monochrome and luminance are synonymous. They are also often referred to as the
brightness signal. Every monochrome signal (video) contains nothing but the variations in amplitude of
the picture signal; these amplitude variations at the picture tube produce changes in light intensity at the
screen.
b. The Color Signal
. The second component of the television signal is the color signal itself. This,
as we have just seen, is interleaved with the black and white signal. To determine what information this
portion of the total signal must carry, let us first see how the eye reacts to color, since it is the eye, after
all, for which the color image is formed.
(1) A number of studies have investigated the color discerning characteristics of the human eye.
Briefly, here is what they found: the typical human eye sees a full color range only when the area or
object is relatively large. When the size of the area or object decreases, it becomes more difficult for the
eye to distinguish between colors. Thus, where the eye required three primary colors, now it finds that it
can get along very well with only two. That is, these two colors will, in different combinations, provide the
limited range of colors that the eye needs or can see in these medium-sized areas.
(2) Finally, when the detail becomes very small, all that the eye needs or can discern are changes
in brightness. Colors cannot be distinguished from gray; in effect, the eye is color-blind.
(3) These properties of the eyes are put to use in the NTSC color system. First, only the large and
medium-sized areas are colored; the fine detail is found in black and white. Second, as we shall see
later, even the color information is regulated according to bandwidth; that is, the larger objects receive
more of the red, green, and blue than the medium-sized objects.
(4) The color signal takes the form of subcarrier and an associated set of sidebands. The
subcarrier frequency is 3.579545 MHz or rounded off, 3.58 MHz. This represents a figure, which is the
product (approximately), of 7,875 Hz multiplied by 455. The value 7,875 is one-half the line frequency of
15,750 Hz, and if we use an odd multiple of 7,875 Hz as a carrier, the frequency falls midway between
the harmonics of 15,750 Hz. If we use even multiples of 7,875, we would end up with 15,750 Hz or one
of its harmonics; this would place the color signal at the same points (throughout the band) as those
occupied by the black-and-white signal. Refer to Figure S-20. By taking an odd multiple of 7,875 Hz, we
cause the signal to fall in between the bundles of energy produced by the first signal, and the two do not
interfere.
(5) It is highly desirable to have the color subcarrier frequency as high above the picture carrier as
possible in order to minimize the interference with the black and white video information (see Figure S-
20). Research has shown that the average energy level of the black and white video falls very rapidly
with increasing video frequencies. By placing the color subcarrier as high as possible in Figure S-21, the
different energy levels minimize possible interference. Practical limits set the upper limit to 3.6 MHz.