OTHER TECH

Lately it seems as though every component in a fully tricked-out home theater system wants to dink with the video - the DVD player, the receiver, the TV. Usually whatever is being done is described as some sort of upconversion. What does that mean, though? And perhaps more important, is it always a good thing?

The types of video processing commonly described as upconversion fall into three basic categories: • Transcoding. Changing one signal format to another, such as composite-video to S- or component-video. • Deinterlacing. Converting an interlaced video signal to progressive-scan, such as from 480i to 480p or 1080i to 1080p. • Scaling. Changing the signal from one display resolution to another, such as from 480p to 720p. Understanding how each of these works will help you decide when upconversion is necessary or desirable (and when it's not) and where in the signal chain the processing is best applied. At the end, I'll give you some basic rules for figuring all that out.

Transcoding: The Background Although video transcoding might seem to be a recent innovation - a feature built into some A/V receivers - it actually has a long history; every TV set ever sold has had to do it just in order to work. That's because video has never been transmitted in the same format in which it is acquired and displayed. TV cameras and displays are fundamentally RGB devices, meaning that they use three primary colors - red, green, and blue - in various proportions to represent all the others, including black, white, and all the shades of gray in between. We see fine detail mainly in grayscale, or black-and-white, however, so using full-bandwidth color signals to represent it isn't very efficient.

The solution is component-video, which consists of a full-bandwidth black-and-white, or luminance, signal, and two narrower-bandwidth "color-difference' signals, created by subtracting the luminance signal from the red and blue signals. TV sets have reciprocal color-decoder "matrices" that add and subtract these three signals in various proportions to regenerate an RGB signal to drive their displays. The letter used to designate the luminance signal is Y, so the color-difference signals are Y-R and Y-B, which are abbreviated to Cr and Cb for digital video and Pr and Pb for analog video. Hence, digital component-video is called YCrCb and analog component video is called YPrPb.