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This type of MIDI port fetches all of its data from inside ::cycle_start(), and delivers it to a FIFO connected to another thread (typically a control surface). Unlike regular MidiPorts, which will be read from inside a Session::process() call, these ports will read their data once per AudioEngine::process() cycle. They therefore cannot use MidiPort::get_midi_buffer() which scales and adjusts event timestamps as if the data is being accessed from within Session::process(). It is still an open question whether or not AsyncMIDIPort::cycle_start() should still scale event timestamps by speed. In some respects it seems more appropriate to do so, and the reading thread (e.g. a control surface) doesn't care about the "nframes" limit on timestamps that exists for calls within a Session::process() tree. For now, leave the timestamps unscaled by speed.
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http://ardour.org/development.html