Originally Posted by Kodaz:
“...I was trying to figure out why they hadn't just used RGB for the component signal, and considered that sync affecting the quality probably *wasn't* an issue, nor the reason.”
RGB is a live studio transmission format. A studio TV camera uses red, green and blue image sensors. RGB maintains that separation right through the production floor and gallery chain to the display monitors.
Thinking back to the early days of TV prior to home recorders or even studio VTRs then everything not on film was broadcast live. The problem came when trying to record. Early studios were just monochrome of course. Colour TV was still in its infancy, so the problem of recording RGB which would take up three times the recording space could be worked on at a later date.
The first studio VTRs from the mid 50's recorded in monochrome. A couple of years later it became possible to record in colour. Composite video was used as a way to solve the data storage issue. Th Luma channel retains full bandwidth and the colour information is compressed and multiplexed in to the Luma channel to form a "colour under" format. Add sound and, to all intents and purposes, you have a single frequency version of the same signal that is picked up by an analogue aerial. Composite video is what we have all been using in VHS recorders.
The caveat of a colour under system is that the Luma and Chroma data are effectively mixed together. There's no way to retrieve the original RGB signal. That's not so much of an issue for home recording, but it is an issue for studio editors. Each successive dub produces more significant signal losses.
The solution was what we now refer to as Component video. Luma and Chroma were kept separate. The colour information was matrixed and compressed to a two channel colour difference signal and then recorded on a parallel stripe on the tape. Luma and Chroma never mixed. So despite the loss in colour resolution it is possible to de-matrix the signal back to a slightly lower resolution version of the original RGB. Analogue Component video VTR still only recorded at SD resolutions, but since there wasn't a HD format in wide use at the time then that was never an issue.
Component now forms the heart of the recording format used in both the analogue and digital worlds. DVDs and Blu-rays store recorded data in digital Component video. So despite the possibility to use RGB in analogue SCART and digital HDMI, the originating signal is in Component format.
Originally Posted by Kodaz:
“(*) AFAIK- though don't quote me on this- YPbPr to RGB should (in theory) be a mathematically lossless conversion. But, of course, any analogue processing at all is going to result in *some* quality loss.”
Be it analogue or digital, Colour Difference Component still uses subsampling for Chroma so it never quite gets back to the original RGB signal format.