Unfortunately, I have made the test of using the command to set the forced subpic 0, and changed the subpic 0 stream attribute to forced. But it doesn't work on my Sony.
I have also tested with "normal" and "normal caption", without any result.
Seems that the Sony DVD-S725D cannot handle the "forced" subpic.
Or is it something more to do? Or a flag in the VOB?
