Service manual

STP 11-25R13-SM-TG
S - 16
(7) When primary colors were selected for color television work, it was found that those primaries
must, of necessity, be limited by the color phosphors that were available for the picture tube. Figure S-14
shows the actual location of the primaries (red, green, and blue) that are used in color television.
(8) These points represent the primaries selected by the NTSC. They define a triangle within the
boundaries of the Chromaticity Diagram. The area within the triangle represents the range of colors that
are obtainable when these primaries are used. In the NTSC triangle, red has a wavelength of
approximately 610 nanometers, green is approximately 540 nanometers, and blue is approximately 470
nanometers.
(9) At first glance, this triangle appears much smaller than the gamut of colors obtainable when
ideal primaries are used. If we give Figure S-14 a closer inspection, we see that the NTSC primaries fall
very close to the saturated colors on the Chromaticity curve. The red primary, for example, is actually on
the curve.
S-15. Summary.
We have just covered the colors and wavelengths of light along with the qualities of color: hue,
saturation, and brightness. Their importance becomes increasingly evident as we proceed into the study
of the TV transmission system.
a. The Chromaticity Diagram was also covered. There were two major reasons for this:
(1) The diagram is based upon actual tests of human vision.
(2) The NTSC triangle, which can be put to practical use in servicing work, is based to a great
extent on the Chromaticity Diagram.
b. Since, in the color television system, the desired colors are produced by an additive color
mixing action, basic rules of color mixing have been presented. A basic knowledge of these rules will
provide helpful in analyzing and adjusting color television systems.
Section V. COMPATIBILITY AND REVERSE COMPATIBILITY
IN THE NTSC COLOR TV TRANSMISSION SYSTEM
S-16. Compatibility.
a. One of the primary requirements of a color television system is to provide a high definition black
and white picture for present standard black and white receivers without any modifications to the
receivers. This means that:
(1) A color telecast must provide a full 6 MHz black and white signal with the same amplitude
modulation, sync, and blanking characteristics as does any ordinary standard monochrome telecast.
(2) The chrominance information, which includes the hue and saturation variable of color, must be
transmitted within the standard 6 MHz television channel.
(3) The transmitted chrominance information must not in any way cause objectionable interference
with the black and white signal (the brightness variable of color).
b. At first glance, this seems to be a difficult task since the 6 MHz channel is apparently already
well filled. However, it is possible to transmit the chrominance information along with the monochrome
signal within the standard 6 MHz television channel.