page_title: Analog television
section_title: Color video signal extraction
hierarchical_section_title: Analog television / Receiving signals / Color video signal extraction
context_page_description: Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog signal.
  Analog signals vary over a continuous range of possible values which means that electronic noise and interference may be introduced. Thus with analog, a moderately weak signal becomes snowy and subject to interference. In contrast, picture quality from a digital television signal remains good until the signal level drops below a threshold where reception is no longer possible or becomes intermittent.
  Analog television may be wireless or can be distributed over a cable network as cable television.
  All broadcast television systems used analog signals before the arrival of DTV. Motivated by the lower bandwidth requirements of compressed digital signals, since the 2000s a digital television transition is proceeding in most countries of the world, with different deadlines for cessation of analog broadcasts.
context_section_description: 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 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).
  The R, G, B signals in the receiver needed for the display device (CRT, Plasma display or LCD display) are electronically derived by matrixing as follows: R is the additive combination of (R-Y) with Y, G is the additive combination of (G-Y) with Y, and B is the additive combination of (B-Y) with Y. All of this is accomplished electronically. It can be seen that in the combining process, the low resolution portion of the Y signals cancel out, leaving R, G, and B signals able to render a low-resolution image in full color. However, the higher resolution portions of the Y signals do not cancel out, and so are equally present in R, G, and B, producing the higher definition (higher resolution) image detail in monochrome, although it appears to the human eye as a full-color and full resolution picture.
  In the NTSC and PAL color systems, U and V are transmitted by using quadrature amplitude modulation of a subcarrier. This kind of modulation applies two independent signals to one subcarrier, wit
caption: Color signals mixed with video signal (two horizontal lines in sequence)
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