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

From: Grant Galitz <grantgalitz@gmail.com>
Date: Mon, 11 Jul 2011 21:37:10 -0400
Message-ID: <CAD8zUBbum855NC3qbR7umB27BfFcO=GNUZAzLuOttnm2Uvcb3w@mail.gmail.com>
To: Chris Rogers <crogers@google.com>, public-audio@w3.org
yes, minimum buffering to be set.

On Mon, Jul 11, 2011 at 9:36 PM, Chris Rogers <crogers@google.com> wrote:

>
>
> On Mon, Jul 11, 2011 at 6:29 PM, Grant Galitz <grantgalitz@gmail.com>wrote:
>
>>
>>
>> ---------- Forwarded message ----------
>> From: Grant Galitz <grantgalitz@gmail.com>
>> Date: Mon, Jul 11, 2011 at 9:23 PM
>> Subject: Re: Concerning the gap-less output of real-time generated audio
>> in JavaScript
>> To: Chris Rogers <crogers@google.com>
>>
>>
>> So we are thinking about a callback based system that allows buffering
>> ahead of time, that allows resampling, and uses some form of a buffer ring
>> for sample count safety and management optimization? Buffering real time
>> without allowing the developer to specify a minimum amount felt like a bad
>> plan to be implemented, due to many blocking issues for a callback to be
>> launched without major delays (single threaded woes that we need to
>> implement APIs around indeed...).
>>
>
> I'm not quite sure I follow what you're saying.  But just to clarify what I
> meant, I'm proposing *allowing* the developer to specify the "minimum
> amount" in the buffering.  So when the hardware drains the buffer to below a
> certain threshold (settable by the developer) then the callback will be
> fired.  Isn't that what you wanted?
>
> Chris
>
>
>
>
>
>
>>
>>
>> On Mon, Jul 11, 2011 at 3:00 PM, Chris Rogers <crogers@google.com> wrote:
>>
>>>
>>>
>>> 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...
>>>
>>> Chris
>>>
>>>
>>>
>>
>>
>
Received on Tuesday, 12 July 2011 01:37:38 UTC

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