Subject :- Television Engineering

Topic :- Color TV

Difficulty :- Low

• 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.

• 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.

• Colour Killer Circuit As the name suggests this circuit becomes ‘on’ and disables the chroma bandpass amplifier during monochrome reception. Thus it prevents any spurious signals which happen to fall within the bandpass of the chroma amplifier from getting through the demodulators and causing coloured interference on the screen. This colour noise is called ‘confetti’ and looks like snow but with large spots in colour.

• The receiver thus automatically recognizes a colour or monochrome signal by the presence or absence of the colour sync burst. This voltage is processed in the AFPC circuit to provide a dc bias that cuts off the colour killer circuit.

• Thus when the colour killer circuit is off the chroma bandpass amplifier is ‘on’ for colour information. In some receiver designs the colour demodulators are disabled instead of the chroma bandpass amplifier during monochrome reception.