OTHER TECH

Grayscale Matters

Adjusting the grayscale is probably the best-known part of calibration. But reviewing a bit of TV history will help you understand why it's so important.

We're deep into the transition from analog to digital TV. (All analog over-the-air TV transmissions are supposed to end on February 17, 2009.) When you consider that a top priority for the committee that established the NTSC standards in 1953 was to make sure that black-and-white sets would still be able to receive signals once color was introduced, the system has served us well. The solution was to lay the color information on top of the black-and-white picture information (a format called "composite video") - leaving the TV to separate the signals in order to make sense of them.

Even in component-video format (the way HD signals are broadcast), black-and-white signals are separate from the color ones. The green-colored connector on a component-video cable doesn't really carry the color green to your TV, but instead sends the luminance, or grayscale, information. The blue connector carries the blue-minus-luminance (B-Y) color-difference component, the red the red-minus-luminance (R-Y) component. The TV's color decoder adds and subtracts these three signal components to extract the three color primaries, red, green, and blue (RGB), in the correct proportions.

In video, every color and shade is produced by combinations of red, green, and blue light at different intensity levels. Full black is the complete absence of light, and an equal mix of colors at maximum intensity produces peak white. The grayscale is the range of steps between full black and peak white. Ideally, the only thing that changes from one shade of gray to the next is the intensity of the light, not the color mix. This is important because you can't have an accurately reproduced color picture if your gray isn't set correctly.

The signal's intensity is measured in IRE (named after the Institute of Radio Engineers). Full black is 0 IRE for HDTV (the NTSC standard was 7.5 IRE), and full white is 100. The grayscale makes up the transitions between 0 and 100 IRE.

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