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Re: Concerning the gap-less output of real-time generated audio in JavaScript

From: Chris Rogers <crogers@google.com>
Date: Mon, 11 Jul 2011 12:00:15 -0700
Message-ID: <CA+EzO0=4oZC+5J7h5K029bpTgP=z3qPoMwZgjd=Wwi-jyi4rCA@mail.gmail.com>
To: Grant Galitz <grantgalitz@gmail.com>
Cc: public-audio@w3.org
On Sun, Jul 10, 2011 at 11:11 PM, Grant Galitz <grantgalitz@gmail.com>wrote:

> I'll briefly compare the mozilla audio data api and the web audio api and
> run through this list of what can be improved upon in web audio.
> - Web Audio does not allow resampling, this is a major thorn in probably a
> couple people's butts, because I have to do this in JavaScript manually. If
> there is a security concern for bottlenecking, then I'd assume we could
> throw in some implementation-side limitations on the number of concurrent
> supposed resampling nodes that could be run at the same time.

I agree that it would be useful to allow the creation of AudioContexts with
user-settable sample-rates.  It could be as simple as:

var context = new AudioContext(sampleRate);

where sampleRate must have some kind of reasonable upper and lower bound.

> - Web Audio forces the JavaScript developer to maintain an audio buffer in
> JavaScript. This applies for audio that cannot be timed to the web audio
> callback, such as an app timed by setInterval that has to produce x samples
> every x milliseconds. The Mozilla Audio Data API allows the JS developer to
> push samples to the browser and let the browser manage the buffer on its
> own. The callback grabbing x number of samples every call is not a buffer on
> its own, that's the callback sampling the whole buffer of what I'm talking
> about. Buffer ring management in JavaScript takes up some CPU load and it
> would always be better in my opinion to let the browser manage such a task.

If sample-rate conversion is taken care of as proposed above, then the CPU
overhead of managing a simple ring-buffer in JavaScript should be extremely
small and can be implemented in just a few lines of code.  I understand that
in your current implementation, you're also dealing with sample-rate
conversion which is slower and complicates your code.  But a simple
ring-buffer is not very complex.

> - "The callback method knows how often to fire," this is a fallacy, even
> flash falls for this issue and can produce clicks and pops on real-time
> generated audio (Even their docs hint at this). This is because by the time
> the callback API figures out a delay, its buffering may be premature due to
> previous calculations and may as a result gap the audio. It is imperative
> you let the developer control the buffering process, since only the
> developer would truly know how much buffering is needed. Web Audio in chrome
> gaps out for instance when we're drawing to a canvas stretched to fullscreen
> and a canvas op takes a few milliseconds to perform, to a reasonable person
> this would seem inappropriate. This ties in basically with the previous
> point of letting the browser manage the buffer passed to it, and allowing
> the JS developer to buffer ahead of time rather than having a real-time
> thread try to play catch-up with an inherently bad plan.
> - Building up on the last point, in order to achieve ahead-of-time
> buffering, I believe it would be wise to either introduce a stub function
> that allows samples to be added at any time without waiting for a callback,
> just like mozWriteAudio, OR to allow the callback method to be called when
> buffering reaches a specified low point *specified* by the developer. This
> low point is not how many samples are to be sent to the browser each
> callback, but lets the API know WHEN to fire the callback, with the firing
> being at a certain number of samples before buffer empty.

I like your second idea of having an internal buffer (in the implementation)
whose size can be specified by the developer.  This buffer size is
independent of the callback size.  There could also be a mode where this
internal buffer can automatically adjust its size depending on runtime
characteristics, but this mode could either be enabled or disabled.

> I hope we can use some or all of these points listed in providing a proper
> API for real-time generated audio output in JavaScript in a 21st century
> browser. :D

I think we can.  My apologies for not yet implementing the ability to choose
sample-rates for an AudioContext.  It'll come...

Received on Monday, 11 July 2011 19:00:42 UTC

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