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Clarification on the latency bug in Moz Audio

From: Grant Galitz <grantgalitz@gmail.com>
Date: Wed, 20 Jun 2012 12:49:53 -0400
Message-ID: <CAD8zUBYCBNy2_z++pP2xpMECCUx5u15ALa2Xttd91aT8treTSw@mail.gmail.com>
To: public-audio@w3.org
Say you want to keep a latency of 100 ms in your audio app. If an API like
the Web Audio API or the custom Adobe Flash bridge decide they need more
than 100 ms of latency, you can fill in with silence when you decide not to
push out more samples. With the Mozilla Audio API, Firefox on multiple OSes
will decide not to play your audio stream if it's not "filled up" to the
buffering amount it likes. Instead, you won't be able to correctly discern
that latency Firefox wants and as a result this will corrupt your own app's
internal buffering, as a function like mozCurrentSampleOffset will not
function correctly as you'd expect, due to this hidden buffering.
Therefore, in order to fix this problem, firefox needs to use a callback
based API instead, as the audio library interface will still be able to
determine its buffering position (Because it will keep its own buffer
directly instead of passing to the browser for mgmt).

Take this as an example for a callback API:

1) Browser requests 500 ms of latency via multiple callbacks one after the
2) Web App only wants to have 100 ms of latency.
3) 400 ms of silence inserted and 100 ms of audio data inserted.
4) Now that the latency has been "filled," the callback API will request in
proper intervals. The buffer management for a callback api has to be done
inside JS, so the web app will always know how many samples it has left.
The problem with moz audio here is that moz audio was responsible for
buffer management and gave back incomplete information to the web app so it
had issues.
Received on Wednesday, 20 June 2012 16:50:23 UTC

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