Color bar signal
The color bar signal is an electronically generated test signal of color television technology , which is used as a test line , as an independent test image and as a component of test images for setting the encoder and decoder.
Significance for color television standards
A color bar signal plays a special role in which the white bar is at 100% of the white level (height of the black-and-white jump), while the level of the color value signals for the colored bars is only 75% of the white level. In the basic version, the individual bars are sorted from left to right according to descending brightness ( luminance ). The level reduced to 75% takes into account that average image content rarely has a color saturation of 100%, which is understood here to mean that the color in question is located on the edge of the color triangle spanned by the basic colors . The saturation, which is limited in practice, is therefore mapped to a saturation of 100% with a level reduced to 75% for the color television standards SECAM and PAL . The parameters of these color television processes relate to this signal. The term "colorful" was used by color television technicians only in a pejorative sense. Therefore, despite the percentage, no color was spoken of. The positive sense was expressed with "colored".
With SECAM , the nominal frequency swings are reached with this signal or set on the coder and measured with a Secamscope (French: Sécamscope). The color difference signals (R − Y) and (B − Y) are mapped to signals in the description of the model using two factors, which are designated as D R and D B , respectively. Although these factors are specified in the color television standard, they are actually fictitious because they refer to the "75% color bar" as the input signal, in which the frequencies of the modulated color signal are measured directly.
With PAL , the luminance signal, together with the quadrature-modulated color signal , reaches the white level for the colors yellow and cyan, i.e. 100%. Because of the negative modulation used in Germany , the white level should not be significantly exceeded, as otherwise the temporarily blanking of the video carrier would lead to sound interference (required residual carrier). The image carrier is used in almost all television sets to generate the audio intermediate frequency of 5.5 MHz. With negative modulation, large overdrive can lead to blanking of the picture carrier, undershooting a minimum level has an impact on the level of the IF and becomes audible via the demodulator as 50 Hz interference (half frame rate ). The designation YUV refers to the color difference signals evaluated with the necessary factors. One cannot therefore speak of YUV in connection with SECAM.
Practical use
The color bar signal used to be sent regularly (alternating with the convergence grid ) to give workshops and interested viewers the opportunity to adjust the color saturation. It was also part of the test lines as well as more complex test images.
If the color saturation is set correctly, all relevant colored bars in the decoded signal have the same level and the same brightness on the screen with only one activated color separation (exception: 100% white bar). The differences in the areas close to the blanking level or cut-off point of the picture tube can be seen more easily .
To improve the saturation setting, there was a variant with a split white bar with 100% and 75% or only with 75% (EBU test signal).
In black and white reproduction, the color bar signal is used as a gray step to check the gradation .
Different variants
The colors of the color bar were also combined in different orders and in different geometric arrangements.
For NTSC there were bars with linear combinations of the three basic colors, which allowed a matrixed setting of the gain in the two color difference channels.
swell
- ↑ RFZ standard for SECAM (Central Radio and Television Office of the Deutsche Post)