I think the rounded edge is just a detail, as far as I can tell, it doesn't make any difference in copy protection.
And I think that "codec" is another way (software way) to read the disc files, probably another File System or somethink like that?
But I think it must contain another obfuscation (defected positions again?), Nintendo loves that. Or maybe they grow tired and now they use another feature.
Probably that unknown black layer...
**Ippen... Shinde miru?**