Okay, but to be fair I immediately recognised the "ready" signal 0xAA or 0x55 as "oh, it just toggles the bit every clock. That sounds oddly useful. You can probably sync a clock or something on that." I've also run into situations where I'm counting bits and the flip-flop doesn't actually get set until the /next/ clock pulse, so having the phase change bit arrive one cycle early seems like a kludge around that sort of thing.

Ya gotta remember that Sony is hardware people so they do inscrutable things in software that probably turn out to make silicon implementation easier.