Color difference concept
The color difference concept describes a method of color signal processing in a color television . It was used in the first US American color televisions based on the NTSC system from 1954 and was also used in devices based on the PAL and SECAM color television standards until the 1970s.
The three color picture tubes - cathodes for the primary colors red , green and blue are controlled together with the brightness signal Y. This is identical to the black and white image signal, is transmitted in full bandwidth and fed to the color picture tube from the Y amplifier (also called video amplifier ).
In the color part, the three color signals are obtained ( color type and color strength ), but reduced by the brightness part Y. It is only about the pure color information RY, GY and BY, also called color difference signals. They are amplified in the color difference output stages and fed to the Wehnelt cylinders of the three jet systems.
With the color difference concept, the black and white image is colored afterwards .
advantages and disadvantages
The color difference concept has the advantage that it only requires a broadband video output stage with approx. 5 MHz for the brightness signal, while the color difference output stages can be designed with a narrow band with approx. 1.5 MHz. The circuitry effort is less than with the RGB concept that is common today . In addition, a color television with the color difference concept can often still reproduce a clean black and white picture even if there is a fault in the color difference output stages.
However, one problem is the correct representation of saturated colors in relation to pastel colors in darker parts of the picture. Due to the not always completely linear increasing characteristics of the beam currents in the color picture tube depending on the Wehnelt voltages, you have to set a little more color saturation for the pastel colors (the color image appears more colorful than it actually is). On the other hand, if the pastel colors are correctly reproduced, the saturated image areas appear paler.
literature
- H. Achterberg et al. (1967): VALVO color television. Receiver circuits. Published by VALVO GmbH.
- Dieter Nührmann (1967): color television book. Color television technology based on the NTSC-PAL system. Stuttgart.
- Gunnar Zaydowicz (undated): Color television. Part 2: the recipient. Minden.
- WA Holm (1964): Color television technology without mathematics. Philips technical library. Eindhoven.
- Otto Ackermann (1972): Television in Colors. Leipzig.