[wdmaudiodev] USBAudio MIDI Confusion
- From: Tim Roberts <timr@xxxxxxxxx>
- To: wdmaudiodev@xxxxxxxxxxxxx
- Date: Thu, 04 Oct 2007 14:22:47 -0700
I'm confused. I watched with interest the discussion about USBAudio
MIDI over the last few weeks, because I had a client who needed some
help. I now have his device in hand. This is basically a drum pad,
with no external MIDI ports; it generates MIDI, and it accepts a limited
number of MIDI control commands, all via USB.
So, he set up his descriptors to have an embedded IN jack (connected to
the bulk OUT pipe) and an embedded OUT jack (connected to the bulk IN
pipe). This enumerated fine, but it didn't show up as a MIDI device.
That is, midiInGetNumDevs was 0, and midiOutGetNumDevs was 1 (the
default MIDI device). I could bring it up in graphedt and see two pins
on the filter, but I couldn't connect them. Interestingly, if we plug
this into a Mac, it is recognized and configured, and the Mac starts
sending bulk reads.
On a whim, I tried adding an external IN and external OUT jack to the
descriptors, identical to the sample given in the USBAudio MIDI spec
appendix B. Now, it DOES appear in midiInGetNumDevs and
midiOutGetNumDevs. However, when I do midiInOpen and send some blank
buffers, I never see any bulk reads from Windows. I tried running
MaxMSP to read from the device, and I still never see any bulk IN
reads. I've tried XP and Vista.
What does it take to get convince USBAudio to start sending me bulk IN
packets?
(On a side note, I've been amused to see that almost everyone is
propagating the same mistake from the descriptors in that appendix. The
sample "Class-specific MS Interface Descriptor" in section B.4.2 has an
element wTotalLength, which is supposed to be the length of itself plus
all of the jack and element descriptors. For this example, that's
7+6+6+9+9, which would be 37, or hex 0x25. And yet almost everyone has
0x41 in that field, copied from the erroneous sample, regardless of how
long their descriptors really are. They only way to get 0x41 is to
include the endpoint descriptors, which is wrong. I assume USBAudio
ignores this field.)
--
Tim Roberts, timr@xxxxxxxxx
Providenza & Boekelheide, Inc.
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