818 Chapter 28
television system launched in the United States in 1953, although, for practical reasons,
the engineers eventually resorted to an electronic dot-sequential signal rather than
achieving this in the action of the tube. This technique is considered next.
28.6.1 NTSC and PAL Color Systems
If you’ve ever had to match the color of a cotton thread or wool, you’ll know you have
to wind a length of it around a piece of card before you are in a position to judge the
color. That’s because the eye is relatively insensitive to colored detail. This is obviously
a phenomenon of great relevance to any application of color picture reproduction and
coding; that color information may be relatively coarse in comparison with luminance
information. Artists have known this for thousands of years. From cave paintings to
modern animation studios it is possible to see examples of skilled, detailed monochrome
drawings being colored in later by a less skilled hand.
The fi rst step in the electronic coding of an NTSC color picture is color-space conversion
into a form where brightness information (luminance) is separate from color information
(chrominance) so that the latter can be used to control the high-frequency color
subcarrier. This axis transformation is usually referred to as RGB to YUV conversion and
it is achieved by mathematical manipulation of the form:
Y 0.3R 0.59G 0.11BU m (B Y )V n (R Y )The Y (traditional symbol for luminance) signal is generated in this way so that it as
nearly as possible matches the monochrome signal from a black and white camera
scanning the same scene. (The color green is a more luminous color than either red or
blue and red is more luminous than blue.) Of the other two signals, U is generated by
subtracting Y from B: for a black and white signal this evidently remains zero for any
shade of gray. The same is true of R Y. These signals therefore denote the amount
the color signal differs from its black and white counterpart. They are therefore dubbed
color difference signals. (Each color difference signal is scaled by a constant.) These
signals may be a much lower bandwidth than the luminance signal because they carry
color information only, to which the eye is relatively insensitive. Once derived, they are