I know the answer to this is practice, practice, practice, but I'd just like to understand what's going on from a technical point of view. When I play A in the upper register often there is a harsh "grating" kind of quality to it, which I think is the low A coming through. The B above and the G below are fine. It is not a leak etc because I have played a couple of other horns with the same result. I think it is the first sign that my embouchure is tiring because it doesn't happen at first when I start playing. But then why are the higher notes above A ok when they should be harder to play when tired? Is high A just a trouble spot due to the inherent design of the sax, in the same way that middle D can be?