CompuServe Messages

John, I would imagine that you already know this but I thought it was worth posting. This is a side-bar to an article in AV Video, March 1994, written by Rick Lehtinen. "Digital Video is Video Whether in the ROM chips of a test generator or in the three chips in a camera head, video starts out as three separate signals: red, green and blue. RGB is somewhat clumsy to use, so various algebraic reductions of it are greatly preferred. This is done first by synthesizing a black-and-white image, called luminance, or Y. As all colors contribute to Y, one of the primary colors can now be expressed by subtracting Y from each of the other two primaries. Knowing the values of Y, R minus Y and B minus Y lets us figure out the value of green whenever we need to. Y, R-Y and B-Y are recognizable as the channels in an analog component video signal, such as Betacam, Betacam SP or M-II. Were you to digitize the Y, R-Y and B-Y signals, take one sample of Y for each sample of R-Y and another sample of Y for each sample of B-Y, you would end up with four samples of Y to each two samples of the color signals (4:2:2). This is CCIR 601 video, which is the basis for D-1 and D-5. Adding compression before recording yields digital Betacam and Ampex DCT. Continuing the analog path, one can combine the R-Y and B-Y signals to produce the chrominance signal, C. Video can be expressed by working with the Y and C signals alone. This is S-video or Hi8. At this time, there is no standard for digital Hi8. The last step on the ladder is to develop composite video signals (NTSC and PAL) by combining Y and C into one signal and adding sync. If this is digitized, it becomes D-2 and D-3. In all cases, the digital video signal is nothing more – and nothing less – than a pixel-by-pixel representation of an analog video signal. This is the big difference between real digital video and CIF (340 X 240) alternatives offered by the computer industry. R.L. " As I said, I figured you already knew this, but I thought it was well written and worth sharing on the forum. 🙂 Fred