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Color killer

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Color killer

The color killer is an electronic stage in color TV receiver sets which acts as a cutting circuit to cut off the color amplifiers when the TV receives a monochrome signal.[1]

Monochromatic transmission

When a receiver is tuned to a monochrome transmission, the displayed scene should have no color components. However, there are two factors which result in color display even during monochrome transmission:

  • a high frequency component of the luminance signal
  • a high frequency of external noise

When the frequencies of these signals equals the frequency of the color subcarrier wave, they may create an irritating color dot pattern like confetti, which interferes with the monochrome picture.

Color transmission

In a color TV waveform, a reference pulse, called the burst[1] is transmitted along the back porch portion of the video signal. If the transmitted signal is monochromatic, then the burst is not transmitted. The color killer is actually a muting circuit in the chroma section which supervises the burst and turns off the color amplifiers if no burst is received (i.e. when the signal is received signal is monochromatic.)[2]

Equation

In NTSC and PAL transmissions, the color TV signal can be represented as:[2]

E(t) = E_y(t)+ a_1\cdot(E_b(t)-E_y(t))\cdot \sin(\omega t)+ a_2\cdot(E_r(t)-E_y(t))\cdot \cos(\omega t)

In this equation a_1 and a_2 are attenuation factors, E_y is the luminance signal, (E_b-E_y) and (E_r-E_y) are the so-called color difference signals and \omega is the angular frequency of the color carrier. \omega is within the luminance bandwidth.[3]

Color eraser (mekhikon)

In the 70's, the Israeli government considered the import of color televisions as frivolous and a luxury that would increase social gaps. Therefore, the government ordered the Israel Broadcasting Authority to erase the color from color-taped telecasts by erasing the "burst phase" signal. The "damaged" signal triggered the "color killer" mechanism, installed in color TV sets to prevent the appearance of color. This method was named mekhikon (Hebrew: מחיקון‎ "eraser").

Color anti-eraser (anti-mekhikon)

Soon after its introduction of the "Color eraser", special TV sets with an anti-mekhikon (Hebrew: אנטי-מחיקון‎ "anti-eraser") device were offered. This device re-constructed the burst phase signal according to several known standards. The client had to turn a switch until the pictures on the screen appeared in natural colors. According to a report in Yediot Aharonoth from January 1979[4] clients had to manipulate the switch every 15 minutes on average in normal conditions, or up to 10 times an hour when special problems occurred, in order to restore natural colors or if the picture suddenly turned black and white.

Based on information from owners of electricity appliance stores, the report estimated that 90% of those who bought color TV sets also bought the anti-mekhikon device, whose price ranged between 2,500 and 4,000 Israeli lirot (the TV set itself cost 40-50 thousand lirot).[5]

Eventually, the mekhikon idea was proven to be futile, and the Israeli television stopped using it in 1980, allowing color transmissions to be received freely.

Notes

  1. ^ Burst is actually a 10 period color carrier signal.
  2. ^ In SECAM, a similar equation can be used except that the modulation system is FM instead of QAM.

References

  1. ^ thefreedictionary.com
  2. ^ Bernard Grob-Charles E.Herndon:Basic television and Video systems, Glanceo McGraw Hill, 1998, ISBN 978-0-02-800437-2, p. 456–458.
  3. ^ For more complete equation see Reference Data for Radio Engineers, Howard W. Sams Co, ISBN 978-0-672-21218-5, pp. 30–14.
  4. ^ Report from 19 January 1979 by Leah Etgar on Yediot Aharonot's economic supplement ("HaLirot Shelkha"), partially cited in "The Future that was: Anti-Mekhikon", a historical review by Gal Mor, Ynet, 7 June 2004 (in Hebrew)
  5. ^ Ten thousand Israeli lirot in January 1979 equal about 7,000 Israeli new shekels in May 2011 prices, according to the calculator available at the website of the Israel Central Bureau of Statistics.
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