[linux-audio-dev] a question re: the MIDI spec

[Martijn Sipkema]

> > > > 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