13 Sep
2004
13 Sep
'04
6:22 a.m.
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