Color killer


Color killer

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

Contents

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[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 signal is received signal is monochromatic.)[2]

Equation

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

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 a1 and a2 are attenuation factors, Ey is the luminance signal, (EbEy) and (ErEy) are the so called color difference signals and ω is the angular frequency of the color carrier. ω is within the luminance bandwidth.[3]

References

  1. ^ [1]
  2. ^ Bernard Grob-Charles E.Herndon:Basic television and Video systems,Glanceo McGraw Hill,1998,ISBN 0-02-800437-X p 456-458
  3. ^ For more complete equation see Reference Data for Radio Engineers, Howard W.Sams Co inc,ISBN 0-672-21218-8, pp 30-14

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.

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