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THE DIFFICULTIES OF HANDLING MIDI CORRECTLY

• MIDI:
  • non-streaming
  • synchronous wire protocol
  • bandwidth utilization varies dramatically
  • maximum 1 byte/0.33 msec.
• Audio
  • streaming
  • abstract memory access protocol
  • constant bandwidth
  • typically 100's-1000's bytes/msec.
• correct MIDI timing:
  • 0.3 msec resolution required
  • MIDI msg received at a given audio transport position should be on wire when we return to that position.
• can't be done from audio thread: mutual blocking issues
• can't be done from GUI thread: GUI operations block MIDI I/O
• scheduler can be in user-space or in kernel (eg. ALSA sequencer)
• standard kernels: can't accurately schedule queued MIDI data
  • standard kernel timer: 0.01secs or 0.001secs
  • RTC: power-of-2 Hz
  • audio interrupt stream: multiples of 1/{44100,48000,96000} secs.