(1) It appears then, that color definitions using the color difference signals
must be limited to 0.5 MHz of sidebands 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.
(2) 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 be extended to 1.5 MHz, and 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.
(3) By readjusting the phase of the chrominance signals and transmitting one
signal vestigially with extended bandwidth, all colors are 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.
9. High definition color transmission.
The readjustment of chrominance signal
phases is called high definition color transmission and is the system of color
transmission used today.
Since the chrominance axes in this system are not the
same as R-Y and B-Y, they have been given new titles. One is called the "I" signal
since it is nearest in phase to the burst or the reference subcarrier, and the
other is called the "Q" signal as it is in quadrature with the I signal.
a. Figure 1-18 is a vector diagram of the I and Q color coordinates and their
relationship to the old R-Y/B-Y system.
The I and Q signals are produced with
their phase coordinates 90 degrees from each other.
The +I signal is 57 degrees
from the reference subcarrier, with -I at 237 degrees (180 degrees from +I). The
+Q signal is displaced 57 degrees + 90 degrees, or 147 degrees from reference
subcarrier, with the -Q signal 180 degrees from it, or 327 degrees from the
reference subcarrier.
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