W3C home > Mailing lists > Public > public-audio@w3.org > July to September 2013

Re: [web-audio-api] The callbacks of the decodeAudioData API are underspecified (#189)

From: Olivier Thereaux <notifications@github.com>
Date: Wed, 11 Sep 2013 07:30:17 -0700
To: WebAudio/web-audio-api <web-audio-api@noreply.github.com>
Message-ID: <WebAudio/web-audio-api/issues/189/24244748@github.com>
> [Original comment](https://www.w3.org/Bugs/Public/show_bug.cgi?id=20039#3) by Ehsan Akhgari [:ehsan] on W3C Bugzilla. Thu, 29 Nov 2012 21:00:36 GMT

(In reply to [comment #3](#issuecomment-24244744))
> (In reply to [comment #2](#issuecomment-24244739))
> > (In reply to [comment #1](#issuecomment-24244733))
> > > (In reply to comment #0)
> > > > Here is a list of problems that come to my mind right now.
> > > > 
> > > > 1. It's not clear whether the callbacks have to be asynchronous or not.  For
> > > > example, if for some reason the implementation decides that it's unable to
> > > > decode the audio right away, should it just call the failure callback
> > > > directly, or call it the next time that the execution hits the event loop.
> > > 
> > > I'm not sure I completely understand the question.  The decoding is meant to
> > > be asynchronous, so in some sense will not happen "right away".  So neither
> > > the success nor the error callback will be called directly, but at some
> > > later time (a later time in the event loop).  The failure callback should be
> > > called if there is an actual error in the decoding, meaning that the audio
> > > data is not one of the known and supported formats (depending on what the
> > > user agent supports), or that the audio data is corrupt.
> > 
> > Let me clarify the question.  Consider this test case for example:
> > 
> >   var cx = new AudioContext();
> >   var buf = new Uint8Array(100);
> >   // buf is filled with zeros, and therefore, not a valid audio stream
> >   var decodeFailed = false;
> >   cx.decodeAudioData(buf.buffer, function(x) {
> >     }, function(x) {
> >       decodeFailed = true;
> >   });
> >   alert(decodeFailed);
> > 
> > The question is, should decodeFailed be true or false after the call to
> > decodeAudioData.  The reason that I started to think about this is that in
> > the Gecko implementation, we sniff the buffer synchronously as it's a
> > relatively cheap operation, so in this case we will know immediately that
> > the decoding operation will fail.  Now, we have the option of calling the
> > failure callback immediately which means that decodeFailed will be true when
> > decodeAudioData returns, or calling it the next time that we hit the event
> > loop, which means that decodeFailed will be false when decodeAudioData
> > returns.
> 
> I see.  I would say that decodeFailed will be false, since the callback will
> only happen the next time we hit the event loop.

Sounds good.  Do you want me to edit the spec to reflect this?  (And also my point 3 in comment 0 perhaps?)

> > > > 2. It's not clear what the AudioBuffer callback passed to the failure
> > > > callback should contain.  For example, if the audio's type cannot be
> > > > sniffed, what should be passed to the failure callback?  What if the source
> > > > buffer doesn't contain valid samples and the decoding operation fails
> > > > half-way?
> > > 
> > > I think it's best that there is *no* AudioBuffer in the failure case.
> > 
> > In that case, we need to change the Web IDL to offer two distinct callback
> > types, like this:
> > 
> >   callback DecodeSuccessCallback = void (AudioBuffer decodedData);
> >   callback DecodeErrorCallback = void ();
> > 
> >   interface AudioContext {
> >     // ...
> >     void decodeAudioData(ArrayBuffer audioData,
> >                          DecodeSuccessCallback successCallback,
> >                          optional DecodeErrorCallback errorCallback);
> >     // ...
> >   };
> > 
> > I personally think that this makes way more sense.  Please let me know if
> > you want me to change the Web IDL accordingly.  Thanks!
> 
> Yes, agreed this makes more sense, and feel free to edit the IDL.

Done: https://dvcs.w3.org/hg/audio/rev/cb492d3bd589.

---
Reply to this email directly or view it on GitHub:
https://github.com/WebAudio/web-audio-api/issues/189#issuecomment-24244748
Received on Wednesday, 11 September 2013 14:39:02 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 21:50:11 UTC