חיפשתי ומצאתי הסבר של JOE KANE
In video we allow signals to go slightly above white and a bit below black so that there won’t be hard clipping at these boarders. Hard clipping is easy to spot in pictures. It took the creation of Video Essentials, where its producer was interested in picture quality, to force the use of the full video specification, allowing information to exist above white and below black. Unfortunately, there are still DVD player manufacturers who do not conform to all of the rules of good video performance
Otherwise, the reality of missing information below black from a DVD player has a lot to do with how well the display device is calibrated. As picture information approaches black it runs into a hard clip with players that won’t go below black. The soft clip of a properly adjusted TV set will produce a better picture than the hard clip of a DVD player. Of course, without being able to see the reference pattern below black, a properly calibrated display device is difficult to obtain. This effect is more easily seen, and far more disturbing, when there is also a clip in white. Clipping in black often goes hand-in-hand with clipping in white. On the white side of the scale, details in light areas of the picture will be destroyed. Clouds in the sky which have been relegated to patches of solid white are certainly a good example of where white clipping happens.
http://www.videoessentials.com/ve_d_faqvideo.php