Re: OS X USB/Midi Driver bugs?
Re: OS X USB/Midi Driver bugs?
- Subject: Re: OS X USB/Midi Driver bugs?
- From: Doug Wyatt <email@hidden>
- Date: Mon, 31 Jan 2005 16:17:15 -0800
On Jan 18, 2005, at 20:59, Kurt Revis wrote:
It's sadly not uncommon for MIDI device drivers to have problems
sending and receiving sysex. Some of my users have reported having
every 4th byte dropped, or 2 bytes dropped out of every 256, or
similar (but harder to track down) issues. Generally, updating the
driver software or switching to a different MIDI device makes the
problem go away.
I recently spoke with a developer who was experiencing this problem
with a device in development.
It turns out that the device does not support one of the messages in
the USB-MIDI spec (CIN 0xF: single byte unparsed). That would certainly
explain the symptoms.
The Apple USB-MIDI driver will send this CIN on occasions where it has
1 or 2, but not 3, bytes to send in the middle (not the end of a
sysex). These occasions end up being frequent when the client has
called MIDISendSysEx, which happens to give data to the driver in
256-byte chunks. 255 is a multiple of 3, so there ends up being one
byte sent with CIN 0xF for every 255 bytes sent in 3-byte chunks with
CIN 4.
I don't know what class-compliant devices might be out there with
similar issues. The developer had an indication that the Windows class
driver may be doing something similar.
Doug
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