Color difference signals

In analog television, the video signals obtained by subtraction of the luminance value from the primary color signals.

A color signal conveys picture information for each of the red, green, and blue components of an image (see the article on color space for more information). However, these are not simply transmitted as three separate signals, because: such a signal would not be compatible with monochrome receivers (an important consideration when color broadcasting was first introduced). It would also occupy three times the bandwidth of existing television, requiring a decrease in the number of television channels available. Furthermore, typical problems with the signal transmission (such as differing received signal levels between different colors) would produce unpleasant side effects.

Instead, the RGB signals are converted into YUV form, where the Y signal represents the lightness and darkness (luminance) of the colors in the image. Because the rendering of colors in this way is the goal of both black and white (monochrome) film and black and white (monochrome) television systems, the Y signal is ideal for transmission as the luminance signal. This ensures a monochrome receiver will display a correct picture in black and white, where a given color is reproduced by a shade of gray that correctly reflects how light or dark the original color is.

The U and V signals are "color difference" signals. The U signal is the difference between the B signal and the Y signal, also known as B minus Y (B-Y), and the V signal is the difference between the R signal and the Y signal, also known as R minus Y (R-Y). The U signal then represents how "purplish-blue" or its complementary color "yellowish-green" the color is, and the V signal how "purplish-red" or its complementary "greenish-cyan" it is. The advantage of this scheme is that the U and V signals are zero when the picture has no color content. Since the human eye is more sensitive to detail in luminance than in color, the U and V signals can be transmitted in a relatively lossy (specifically: bandwidth-limited) way with acceptable results.

In the receiver, a single demodulator can extract an additive combination of U plus V. An example is the X demodulator used in the X/Z demodulation system. In that same system, a second demodulator, the Z demodulator, also extracts an additive combination of U plus V, but in a different ratio. The X and Z color difference signals are further matrixed into three color difference signals, (R-Y), (B-Y), and (G-Y). The combinations of usually two, but sometimes three demodulators were:

  • (I) / (Q), (as used in the 1954 rca CTC-2 and the 1985 rca "Colortrak" series, and the 1954 Arvin, and some professional color monitors in the 1990s),
  • (R-Y) / (Q), as used in the 1955 rca 21-inch color receiver,
  • (R-Y) / (B-Y), used in the first color receiver on the market (Westinghouse, not rca),
  • (R-Y) / (G-Y), (as used in the rca Victor CTC-4 chassis),
  • (R-Y) / (B-Y) / (G-Y),
  • (X) / (Z), as used in many receivers of the late '50s and throughout the '60s.

In the end, further matrixing of the above color-difference signals c through f yielded the three color-difference signals, (R-Y), (B-Y), and (G-Y).

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Color difference signals
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Adapted from content published on wikipedia.org
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Last modified on October 18, 2020, 1:42 pm
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