RGB to YCbCr conversion and back is well defined, and RGB->YCbCR->RGB should give you back the original RGB values, no matter what the colorspace was.

Lossless roundtrip conversion is only true for schemes like YCoCg-R [1]. It is not true for Rec.601, Rec.709, etc. because these standards require quantizing to 8bits (actually even less than 8bits due to the 16-235 thing).

[1] https://www.microsoft.com/en-us/research/wp-content/uploads/...

I'm not talking about mathematically exact reconstruction, I'm talking about not being able to see the difference.

I first noticed it around 2014 on a 1080p Dell display (maybe ~24") with an i7 MacBook Pro of the day.

It was the poor non-crisp pixels that led me to even knowing there were even RGB/YPbPr modes.

was it maybe with 4:2:2 or 4:2:0 subsampling? at 4:4:4 there should be no difference