(3) The terms monochrome and luminance are synonymous.
They are also often
referred to as the brightness signal. Every monochrome signal contains nothing but
the variations in amplitude of the picture signal, and these amplitude variations
at the picture tube produce changes in light intensity at the screen.
b. The second component of the television signal is the color signal itself,
which is interleaved with the black and white signal.
To determine what
information this portion of the total signal must carry, we will examine how the
eye reacts to color, since it is in the eye where the color image is formed. The
color-discerning characteristics of the human eye have been thoroughly studied and,
briefly, here is what is known:
(1) 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 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) When the detail become very small, all that the eye needs to (or can)
discern are changes in brightness. Colors cannot be distinguished from gray, and
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
fendered in black and white.
Second, even the color information is regulated
according to bandwidth; that is, the larger objects receive more of the green, red,
and blue than the medium sized objects.
(4) The color signal takes the form of a subcarrier and an associated set of
sidebands.
The subcarrier frequency is 3.579545 MHz or rounded off to 3.58 MHz.
This represents a figure which is the product of 7875 Hz multiplied by 455. The
value 7875 Hz is one-half the line frequency of 15750 Hz, and if we use an odd
of 15750 Hz. If we use even multiples of 7875, we would end up with 15750 Hz or
one of its harmonics, and this would place the color signal at the same points
throughout the band as those occupied by the black and white signal (fig 1-14). By
taking an odd multiple of 7875 Hz, we cause the signal to fall in between the
bundles of energy produced by the first signal, and the two do not interfere.
(a) 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 (fig 1-15).
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