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Re: MIDI synchronization issues

From: Chris Wilson <cwilso@google.com>
Date: Mon, 1 Oct 2012 17:38:56 -0700
Message-ID: <CAJK2wqVfd+DMMr-2cYaFEA7JeTaXtFk8SG+=KmHm9=eFbhmcDA@mail.gmail.com>
To: Srikumar Karaikudi Subramanian <srikumarks@gmail.com>
Cc: James Ingram <j.ingram@netcologne.de>, public-audio@w3.org, Jussi Kalliokoski <jussi.kalliokoski@gmail.com>, Joseph Berkovitz <joe@noteflight.com>
Srikumar,

yes, that's what I had in mind with this approach initially.  You need a
callback that's going to occur some amount of time BEFORE the next prequeue
"chunk" is due.

On your [1] - yes, they are unreliable; that's the major reason we even
have timestamps in sending MIDI messages.

On Mon, Oct 1, 2012 at 5:20 PM, Srikumar Karaikudi Subramanian <
srikumarks@gmail.com> wrote:

> Hi James,
>
> The tick() function in the gist is not the right approach, I think. If we
> consider a MIDI sequence that has drum hits spaced 1 sec apart and use
> PREQUEUE = 500ms (anything less than 1 sec actually) , then the tick() will
> try to land itself just in time to send out the drum hit, by which time it
> may already be too late [1] ... unless I'm reading the gist code wrongly.
>
> A better approach would be to ask the system for *regular* tick()
> callbacks with a period that is *independent* of the message time stamps.
> Then if we have a PREQUEUE that is some multiple (ex: 3x) of the tick()
> interval, we can catch up if a large jitter happens and the time stamps
> become useful to the backend high precision scheduler.
>
> The best way to get reliable periodic callbacks in a browser is to use
> requestAnimationFrame(). If my reading of your gist code is right, all that
> is needed is to change the last line from -
>         window.setTimeout(tick, delay);
> to -
>         webkitRequestAnimationFrame(tick); // Gets called at 60Hz usually.
>
> Then, when time stamps begin to be supported in the MIDI implementation,
> PREQUE = 50ms can give the rock solid timing promised by the underlying
> implementation. This will also allow a JS player to respond to global tempo
> changes quickly if there is a tempo slider in the UI.
>
> Furthermore, in current browsers, it is not possible to guarantee a small
> enough upper limit on the time difference between a generated event and a
> JS function called in response. This means that even if you have a "MIDI
> message sent" callback installed, you may receive that late anyway. It is
> easier (imho) to assume that the message goes out as expected and plan to
> do the necessary UI. If the UI is wrong, it would be wrong only up to
> PREQUEUE.
>
> Best,
> -Kumar
>
> [1] setInterval and setTimeout are just plain unreliable for music timing
> in the browser as far as I've seen. I've measured standard deviations of
> 400ms for such "target times" in Chrome some time ago. In Node.js, though,
> setInterval is rock solid (deviations ~< 0.4ms) while setTimeout is as
> goofy as in Chrome. (Node.js has a MIDI npm module btw.)
>
> On 2 Oct, 2012, at 2:29 AM, James Ingram <j.ingram@netcologne.de> wrote:
>
> > Hi Chris,
> >
> > I'll put it another way: There is going to be a synchronization problem
> when the timestamp property on sendMiDIMessage is implemented and PREQUEUE
> is increased.
> >
> > Lets say the sendMIDIMessage timestamps are working as advertised, and
> we increase PREQUEUE to 500. That would mean in [1] that the
> reportTimestamp function would only be called ca. every 500ms, and would
> not be able to report the timestamp as often as it needs to. There is no
> way, as things stand, for the world outside the while{} loop to report the
> sending of events inside the loop at the time they are actually sent. I
> think this problem needs thinking about now. We don't need to wait for
> sendMIDIMessage timestamps to be implemented.
> >
> > Whatever the answer is going to be, I think it also needs to take empty
> messages into account. Empty messages have lots of uses when synchronizing
> with other things going on on the same computer. Maybe one of MIDI's more
> obscure messages could be hijacked for the purpose, but the state of the
> output device should not change. Maybe someone has an idea in that
> direction?
> >
> > Hope that's a bit clearer.
> >
> > best,
> > James
> >
> > [1] https://gist.github.com/3806262
>
Received on Tuesday, 2 October 2012 00:39:25 UTC

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