[SDL] callback issue

[Ryan C. Gordon]

> The audio callback routine I have assigned fires ok but the length 
> parameter passed to it is not 512 as I would expect, instead it is 340...!?

SDL is probably converting to whatever format the audio hardware can 
handle behind the scenes, which means it needs less samples from you 
each callback.

You can't count on the callback to ask for a specific amount of data.

--ryan.