The Monochrome Image
As discussed in the previous segment, the three components of a video image are luminance, chrominance, and hue. In order to set the color aspects of the monitor properly, it is essential that the black and white (luminance) aspect of the monitor be set first as a base or reference. The principle is that video is an additive color system and white is created when all three primary colors are in proper balance. Therefore, the initial step in setting up a color video monitor is to ensure the purity of white. It must not have a color tint, but be pure white. Once this is set, the color video monitor will be in correct color balance.
Since the color information is encoded on a separate carrier, it is possible on some monitors to turn off the color burst, removing color information from the image and giving a pure monochrome image. Once the image is monochrome, the contrast control must be adjusted for a bright image. However, if the contrast is too high, the whites will “bloom” or bleed into the darker parts of the picture. Once contrast is set, the brightness should be adjusted to account for the ambient light in the room. The brightness of the monitor should be reduced when viewing in a darker environment and increased when the ambient light is bright.
On the monitor, the color bar image will appear as four grey bars separated by three black bars, the I and Q chips if using EIA split field bars, and black. As the hue control is adjusted, the inner two bars will change, becoming lighter or darker. On some monitors, the hue control is also labeled phase. The hue or phase must be adjusted until the inner two bars appear the same. Adjusting the saturation or chroma control will affect the two outer bars. Phase and saturation must be adjusted until all four bars appear the same.
To aid in setting the color balance, some monitors have a switch marked “Blue Only.” This switch is designed to turn off the red and green displays, and simultaneously change the remaining image to monochrome. Therefore, the image, while using the blue only display, will appear monochrome. |