> > > > It's not unusable, but IIRC it can get to several ms of jitter. > > > > > > Why is that? The USB iso clock is every ms IIRC, so naively you > > > would expect the maximum jitter to be just under 1ms (if the bus was > > > saturated by audio transfers), and less in proportion to the degree of > > > saturation. > > USB transfers always wait for the beginning of the next frame, so the > maximum jitter is never less than 1 ms, even on an otherwise free bus. > > > Yes, one would expect that (if there are no other bulk transfers), but > > somehow this does not seem to be the case: > > > > http://www-2.cs.cmu.edu/~eli/papers/icmc01-midiwave.pdf > > These measurements include the jitter added by the drivers and by the > high-quality realtime-capable (yeah :-) Windows 98 scheduler. > > I did some similar measurements under Linux, and it seems the jitter > isn't bigger than the expected 1 or 2 ms (2 because MIDI through > involves two USB transfers).
Well, I still think they probably should have used interrupt of isochronous transfer mode, but 2 ms jitter is quite usable (although a < 1 ms jitter would have been better and possible even with USB I think). --ms
