Ancient Greek diphthongs

Ancient Greek Sculpture
Originally uploaded by Mr G’s Travels

I’m teaching myself a tiny bit of ancient Greek at the moment.

There’s one thing I’ve been puzzling over with regards to the pronunciation.

My problem is with the short and long diphtongs.

In an unstressed position the difference is very clear: ?? [ai] vs. ? [a?i].

However, both can receive the circumflex accent: ?? and ?. The circumflex accent is supposed to be a rising-falling pitch, i.e., [???].

But I find that this is hard to achieve without a certain amount of vowel length.

In other words, ? is easy to pronounce as [?a?a?i], but what about ??? Is that [??a?i] or [?a??i]?