Service manual

STP 11-25R13-SM-TG
S - 24
a. Transmitting symmetrical or double sidebands of chrominance would mean that color definition
would be limited to 0.5 MHz and color would be absent in all fine detail in the picture. However, it is
possible to transmit the chrominance sideband vestigially; that is, with one side band suppressed the
same as the black and white signal. Although this would extend the color definition to possibly 1.5 MHz,
it is practical to transmit only one of the chrominance signals vestigially. One of the color difference
signals would then extend from 0 to 1.5 MHz, while the other color difference signal would be limited from
0 to 0.5 MHz.
b. Remember that the color difference signals are the signals that can be produced by subtracting
the Y signal from each color camera signal. For example, the Y signal is inverted so that all values are
negative (-30R, -59G, -11B). This is algebraically added to each camera tube output in turn, thus
producing three chrominance signals: R-Y, G-Y, and B-Y.
c. Only two color difference signals plus the Y signal need be transmitted to reproduce the color
elements of a televised scene. One of the color difference signals may be developed in the receiver,
since any two of the color difference signals contain all the necessary color information. Thus, only the
R-Y and the B-Y signals need to be transmitted.
d. Notice in Figure S-22 that if R-Y were the extended sideband, fine detail would appear as either
a bluish-red (magenta) or a bluish-green (cyan). On the other hand, if B-Y were the extended sideband,
only reddish-green or yellowish-green colors would be produced. Fine color detail can be represented
accurately to the eye by orange or cyan for any color.
e. If this principle were to be used by R-Y and B-Y transmission standards, it would still be
necessary to transmit extended sidebands of both color difference signals, since both are necessary to
reproduce either orange or cyan.
f. It appears, then, that color definitions using the color difference signals must be limited to 0.5
MHz of sideband extension (upper), and therefore cannot, by their inherent colors, represent full fidelity of
the televised scene. The eye cannot detect all colors accurately for all sizes of picture detail. Color areas
beyond a certain fineness of detail can be accurately represented to the eye by either orange or cyan,
since the eye cannot distinguish any color other than one of these. For extreme fineness of detail, the
eye cannot detect any color sensation. Only a brightness variable is necessary to represent the object as
having color.
g. Logically then, if the axis of the color subcarrier signals were shifted in-phase from the
reference subcarrier so that one axis would represent orange and cyan, with the other axis at right angles
to it, then many transmission problems would be solved. The orange-cyan axis could then be extended
to 1.5 MHz; since the fine detail conveyed by this extended bandwidth need only be either orange or cyan
to represent accurate color reproduction to the eye, full color fidelity would be possible in picture detail up
to 1.5 MHz of picture definition.
h. By readjusting the phase of the chrominance signals and transmitting one signal vestigially with
extended bandwidth, all colors are faithfully reproduced in picture detail from 0 to 0.5 MHz. Beyond this
point, one of the chrominance signals drops out. With the orange-cyan signal still present to 1.5 MHz
effectively, the eye still sees all color though only orange and cyan are transmitted. The eye cannot
distinguish color in picture detail represented by video frequencies beyond 1.5 MHz. With no
chrominance signals transmitted beyond this point, the brightness signal will accurately convey the
illusion of color in video detail from 1.5 to 4.1 MHz.