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

[Clemens Ladisch]

Martijn Sipkema wrote:
> > > 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).


Regards,
Clemens