Subject: Re: [linux-audio-dev] midi events in jack callback (was: Reborn)
From: David Olofson (david_AT_gardena.net)
Date: Thu Aug 22 2002 - 04:21:20 EEST
On Friday 16 August 2002 00.19, Bob Ham wrote:
> On Thu, 2002-08-15 at 20:17, Paul Davis wrote:
[...timestamps...]
> ...and only make calls on those that are stamped "now".
> Except that removes the ability for the softsynth to accept events
> in the future, but I'm not sure how useful that would be.
Events stamped with a time outside the buffer you're currently
processing should (depending on how the event system is
implementated) either be left alone until it's time to process them,
or just never happen. That's all there is to it. (At least in a
callback driven, "audio oriented" real time system.)
> I envisioned a system where a sequencer object delivers midi events
> to the softsynth. Ie, all the time-specific magic would be in the
> sequencer object, rather than the synth/wave/whatever objects.
> Thinking more about it, tho, I can see this being bad. Each object
> would have at least two threads, and those with jack callbacks
> having another. Lots of room for things going wrong, time wise.
Yes indeed. I don't think you'll have much fun at all with any kind
of design that tries to run the event processing and the audio
processing in separate threads. :-)
> I
> wanted it so that an object isn't necessarily a jack client.
> Perhaps I'll have to change that. More thinking to do, I fear.
Well, unless JACK is going to provide some kind of ports that can be
used to pass timestamped events, how about dropping the extra
threads, and just pass the events through lock-free FIFOs? Read
events from inside the audio callback, and stop when you see a
timestamp that's past the end of the current buffer.
Just one "minor" issue left: The actual timestamping. If the events
originate from a thread that doesn't run as a JACK client (real time
MIDI input, perhaps), you'll have to keep track of "audio time" some
other way. Knowing the sample rate, keeping track of JACK "buffer
cycles" and checking the current time...?
//David
.- M A I A -------------------------------------------------.
| Multimedia Application Integration Architecture |
| A Free/Open Source Plugin API for Professional Multimedia |
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.- David Olofson -------------------------------------------.
| Audio Hacker - Open Source Advocate - Singer - Songwriter |
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