Re: core midi time stamping
Re: core midi time stamping
- Subject: Re: core midi time stamping
- From: Brian Willoughby <email@hidden>
- Date: Sat, 16 Jul 2005 16:40:35 -0700
[ >A custom driver is required to get better MIDI latency performance than
[ >this over USB.
[
[ Careful with terminology there. Depending on who you are talking to
[ (as in most Musicians who use computers), latency means the time it
[ takes to echo back an input event for monitoring purposes. No fancy
[ driver stuff can improve that over USB. Ideally for USB, it would be
[ 1 msec average (1/2 msec for input and 1/2 msec for output). In
[ practice, it seems to be more like 2-3 msec.
You're absolutely right. A custom MIDI driver cannot change the latency
introduced by USB, only improve the accuracy of the time stamp. For recording,
an accurate time stamp can effectively remove latency after the fact, but as
you point out this is not helpful during real-time monitoring.
But I would disagree that latency is as specific as your monitoring loopback
example. Latency refers to any delay in transmission of data events.
[ >I have no experience with the limitations of FireWire for MIDI time
[ >stamping.
[
[ The isochronous heartbeat it 1/8 msec. If MIDI is done
[ isochronously, then it would insert 1/16msec average latency. If
[ it's not isochronous (MIDI really shouldn't be, because we don't want
[ MIDI packets dropped), then it could be faster, but 1/16th msec is
[ probably faster than the OS can schedule; so, I suspect the hardware
[ isn't going to be the limiting factor.
I think the key question is whether FireWire delivers a time stamp with the
data. If not, then FW suffers from the same design flaw as USB, and the 125
usec heatbeat is equivalent to high-speed USB.
Brian
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