Borrowing and correcting (Mpeg1 versus Mpeg2 placement) Manao's nice ascii art :-
Code:
X X X X ----> Top luma
O O ----> Top chroma
* * * * ----> Interpolated luma
X X X X ----> Top luma
+ + ----> Interpolated chroma
* * * * ----> Interpolated luma
What is conceptually so difficult about this, practicality is another matter