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.
In a color TV waveform, a reference pulse, called the burst,[a] 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 received signal is monochromatic.)
In this equation and are attenuation factors, is the luminance signal, and are the so-called color difference signals and is the angular frequency of the color carrier. is within the luminance bandwidth.
Color eraser (mekhikon)
In the '70s, 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 cease broadcasting in color. As it was impractical to strip out the chrominance signal from material recorded in color, this was accomplished by simply omitting the burst phase signal from the broadcast. 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").
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 knob until the pictures on the screen appeared in natural colors. According to a report in Yediot Aharonoth from January 1979 clients had to adjust the knob 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, which added about 5–10% to the price of the television.
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.
- Bernard Grob-Charles E.Herndon:Basic television and Video systems, Glanceo McGraw Hill, 1998, ISBN 978-0-02-800437-2, p. 456–458.
- For more complete equation see Reference Data for Radio Engineers, Howard W. Sams Co, ISBN 978-0-672-21218-5, pp. 30–14.
- 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)