Re: Exposing a playbackPosition property on AudioBufferSourceNode

Hey Chinmay,

Although I've specifically asked for this feature before - notably, in
playing around with DJ software on Web
Audio<http://webaudiodemos.appspot.com/wubwubwub/index.html>,
I found this would have been really useful - because exactly as you said,
it gets challenging to keep track of current time when playbackrate keeps
changing (I *do* keep track in that app, even through linear ramps of
playback rate, but the math starts getting more challenging when you use
log ramps).

However, I would point out that you're not likely to have "sample-accurate
sample indices" - just because while you're off twiddling with something,
the current time may change.  Also, you can't really hack at looping like
this, for the same reason we don't can't really have an efficient
"loopEnded" event - because it may happen at far too high a frequency for
us to fire DOM events and expect results.  In keeping with the rest of the
API, of course, this should be a time not a sample index.

We could expose something that would be useful as you suggested - to know
where the playback was when you called "stop", or to even to schedule ahead
- but I'd caution against it being a panacea.  I'd also suggest that this
should NOT be a writeable parameter, and certainly should not be an
AudioParam - it would conflict with playbackRate then.

Would you like to file an issue on the spec?
https://github.com/WebAudio/web-audio-api/issues

-Chris


On Fri, Feb 21, 2014 at 12:51 AM, Chinmay Pendharkar <
chinmay.pendharkar@gmail.com> wrote:

> Hello,
>
> Let me first thank everyone in this group for all the work they've put
> into the WebAudioAPI. I'm must say I'm enjoying using the WebAudioAPI both
> as a developer and consumer.
>
> I'm trying to implement a interactive effect/synthesis library using the
> WebAudioAPI, and I found myself looking for a `playbackPosition` property
> on the AudioBufferSourceNode which exposes the sample index which is being
> played currently.
>
> A simple use case would be to implement a pause like functionality on
> AudioBufferSourceNode, which would allow the playback to resume at the
> sample accurate position where it had been stopped (the `offset` property
> in the start() method would be handy here) albeit using another instance of
> AudioBufferSourceNode. There are many other situations where
> AudioBufferSourceNode's playbackPosition would be useful, especially when
> used with looping.
>
> Using the currentTime to keep time to calculate the position doesn't work
> if the playbackRate of the AudioBufferSourceNode is being changed.
>
> I did find an old email on this list from 2011 talking about this. But
> there didn't seem any other discussion about such a property.
> http://lists.w3.org/Archives/Public/public-audio/2011OctDec/0143.html
>
> I haven't found any other method to get sample accurate playbackPosition.
> Am I missing something? Or is this something that can be added to the WD?
>
> -Chinmay Pendharkar
>