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Re: [web-audio-api] OfflineAudioContext and ScriptProcessorNodes (#69)

From: Olivier Thereaux <notifications@github.com>
Date: Wed, 11 Sep 2013 07:29:36 -0700
To: WebAudio/web-audio-api <web-audio-api@noreply.github.com>
Message-ID: <WebAudio/web-audio-api/issues/69/24244243@github.com>
> [Original comment](https://www.w3.org/Bugs/Public/show_bug.cgi?id=22723#16) by Srikumar Subramanian (Kumar) on W3C Bugzilla. Mon, 22 Jul 2013 17:35:15 GMT

(In reply to [comment #16](#issuecomment-24244239))
> (In reply to [comment #15](#issuecomment-24244234))
> > (In reply to [comment #7](#issuecomment-24244176))
> > > If every node used this "wait for all inputs before running" logic, then
> > > script nodes with buffer sizes greater than 128 need not impose a delay in
> > > their signal paths. 
> > 
> > I just realized a subtlety in this. If a script processor node's
> > onaudioprocess reads computed values from AudioParams, then the perceived
> > k-rate for those AudioParams will be determined by the block size set for
> > the script node and not the fixed 128-sample-block in the spec. Not only
> > that, it will look like a filter-type script node (with input and output) is
> > prescient and anticipates animated AudioParams, because the the
> > onaudioprocess will only get to run once enough input chunks have
> > accumulated, meaning the values of some of these k-rate AudioParams could
> > already have advanced to a time corresponding to the end of the script
> > node's buffer duration.
> No, according to the spec the implementation must do 128-frame block
> processing all the time, which means that for example if we have 1024 frames
> to fill up for a ScriptProcessorNode, we need to call the block processing
> code 8 times, and each k-rate AudioParam will be sampled at the beginning of
> each block.

That holds only for the native nodes, doesn't it? With the real-time context, script processor nodes with buffer sizes > 128 (which is all the time) already have a lower k-rate than the native nodes if they read computed values of AudioParams within their onaudioprocess callbacks.

Anyway, to ensure that the k-rate is uniform at least during offline processing, it looks like the only way is to raise onaudioprocess events for each 128-sample-frame block. The event dispatcher better put up some performance :)

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