Video compression is commonly thought of as being unique to digital, but it’s also been around since the early days of analog. Compression has just become more sophisticated since then, in particular: 4:2:2, 4:1:1 and 4:2:0 chroma subsampling.
Vision specialists have long known that our eyes are much less sensitive to color resolution than we are to brightness resolution.
Early monitors showed images by progressively scanning across each horizontal line of pixels, usually from top to bottom in rapid succession. As each line was scanned, values would be sent less frequently for chroma than for luma.
Since chroma subsampling effectively decreases color resolution, it will be most visible near the edges of sharp color transitions.
Consumers have become more discerning, and have grown to expect more when it comes to image quality. Moving beyond the traditional 4:2:0 compression, used with DVD and Blu-ray, has therefore been a recent trend. Ultimately, feeding full 4:4:4 data into a modern digital codec makes the most of that imagery—and has the potential to noticeably improve image quality.
See full details and examples of Chroma Subsampling usage on the RED 101 Article: CHROMA SUBSAMPLING TECHNIQUES