YCbCr is the digital counterpart to analog YPbPr component video. (YCbCr converted to analog is YPbPr). Digital RGB converted to analog is usually referred to as VGA.
SD and HD DVDs are encoded in 8bit YCbCr 4:2:0. After/during decoding it's upsampled to YCbCr 4:2:2. If RGB output is required the YCbCr is upsampled again to 4:4:4 (sometimes done only once 4:2:0 -> 4:4:4) and a standard and simple transform converts to RGB 4:4:4. If done properly, you'll never notice the difference between the two. The advantage of YCbCr 4:2:2 is that it can be sent as 10bit (or 12bit) video via HDMI (all versions). RGB 4:4:4 is restricted to 8bit (except for the new deep color formats). However, if your display takes 8bit video and then upsamples to 10bit or higher for display, you may only need 8bit video. RGB is also the only format used with DVI (with a few exceptions).
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