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.
|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.