Re: Forcing an absolute timestamp for every midi event.

["Martijn Sipkema" <msipkema@sipkema-digital.com>]

> Hi! I wanted to ask, how about forcing
> an absolute timestamp for _every_ midi event?
> I think this would be great for softsynths,
> so they dont need to work with root/schedfifo/lowlatency
> to have a decent timing. Not allways you are willing
> to process midi at the lowest latency possible.
> I say because you dont really need all that if you
> sequence in the computer and control softsynths and maybe
> some external device. 
> This way, the softsynth gets the event with the timestamp,
> gets the current time, substracts the audio delay (latency) to that
> and just mixes internally in smaller blocks processing each 
> event in the right time.

I'm not sure what you mean, but below is what I think would be the
best approach for audio/MIDI I/O.

----

UST = unadjusted system time
MSC = media stream count

see also:

http://www.lurkertech.com/lg/time/intro.html

(synchronous) Audio I/O API
- Callback based.
- All buffers in a callback are of the same size.
- All frames with a particular MSC occur at the same time. (I don't think
 OpenML requires this, EASI does have this with its 'position')
- The audio callback provides an MSC value for every buffer corresponding
 to the first frame in the buffer.
- The (constant) latency (in frames) between an input and an output can be
 seen from the difference between their MSC values.
- The audio callback provides an UST value for every input buffer
 corresponding to the end of that buffer/start of the next buffer.
- The UST value for the start/end of an output buffer can be estimated.

MIDI I/O API
- MIDI messages are received with a UST stamp.
- Timestamps are measured at the start of the message on the wire.
- MIDI messages are either sent immediately or scheduled to UST.
- MIDI messages must have monotonically increasing timestamps if
 scheduled.

For a software synthesizer the MIDI messages received at some UST
can be mapped to a corresponding MSC value and then rendered at
a constant offset from this MSC, most likely the audio I/O latency
(the audio I/O latency may not be the same for all inputs/outputs,
also since MIDI messages always arrive late a small extra latency
should be introduced in this case by increasing the MIDI message's
UST by a constant value in order to compensate for the MIDI interface
and scheduling latency so they arrive a little early instead of late
in order to reduce jitter with MIDI messages arriving near buffer
bounderies).
MIDI -> audio output latency would then be slightly higher (say 2ms,
noting that transmitting a note-on message already takes 3*320usec)
then audio I/O latency.



--martijn





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