This format dates back to the days of the ActionMedia II adapter and comprises an NxN plane of Y samples, 8 bits each, followed by (N/4)x(N/4) V and U planes.
Horizontal | Vertical | |
Y Sample Period | 1 | 1 |
V Sample Period | 4 | 4 |
U Sample Period | 4 | 4 |
Positive biHeight implies top-down image (top line first)
ATI has a codec supporting this format that you can download from here.