Monday, June 18, 2007

XP/Uno drops input MIDI running status messages

The problem occurs when using WhorldRC to switch video clips quickly in the PlayerFF plugin. The problem first appeared when shortcut keys were added to WhorldRC for triggering recent clips. Before that it wasn't possible to switch clips fast enough to cause the bug. It seems that when MIDI messages are output close together in time, Windows uses MIDI running status, otherwise not. This was verified using the DOS mididump program. The running status messages are NOT received by the MIDI input callback under XP with the M-Audio Uno. This is true using both the driver that shipped with the Uno, and the Windows default driver. Note that with the latest M-Audio driver (MA_CMIDI_WDM_4.2.03v4.exe), the callback DOES receive events for the running status messages, but they are strangely garbled.

Attempts to reproduce the problem using the X-Session as input were not succesful. Perhaps it's the particular nature of WhorldRC's output (b0 00 xx b0 01 xx) that causes the bug? The most likely suspects are the Uno itself, its driver, or XP's USB MIDI driver. One way to test this would be by using a different MIDI device.

For the moment the workaround is a hack to WhorldRC: it sends a note off command after each set of bank/clip commands commands. This is a bit wasteful but WhorldRC is a low-bandwidth app and it seems to have no side effects.

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