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

From: Ian Ni-Lewis <ilewis@google.com>
Date: Tue, 12 Jul 2011 09:44:45 -0700
Message-ID: <CAHzqrxoHYYozA50njiHP4D4SVxmXTDckvvOGHKm4Uw151rwbiw@mail.gmail.com>
To: Philip Jägenstedt <philipj@opera.com>
Cc: public-audio@w3.org
On Tue, Jul 12, 2011 at 1:25 AM, Philip Jägenstedt <philipj@opera.com>wrote:

>
> The callback method is basically a pull model. AFAICT, the only way to
> achieve low latency is by having the callback fire very late, increasing the
> risk of missing the deadline.
>
> Mozilla's (old) API is a push model. AFAICT, the only way to achieve low
> latency is by filling up very little data at a time, again increasing the
> risk of gaps.
>
> Perhaps an overwriteable ring buffer model is more complicated, but it is
> very flexible in allowing the application to pick the (necessary) trade-off
> between latency and risk of gaps.
>
>
And an overwritable ring buffer is the worst of both worlds; the only way to
achieve low latency is to correctly guess when the callback is going to
fire. :-)

I'm kidding, of course. It doesn't really matter. All of the methods under
discussion are more or less isomorphic. You can make a push system look like
a pull system if you only ever push one or two buffers; you can make a pull
system look like a push system by managing your own list or ring buffer and
handing off little pieces of it in the callback.

Let's be frank: the only way to achieve low audio latency is to service the
audio at a high, predictable rate.  Whether you service that via pull or
push is not particularly germane to a discussion of latency. In the past
I've tended to favor the push model (whether via linked list or ring
buffer), but in the end it doesn't matter. However you design the system,
latency is pretty much just a function of service rate and jitter. A system
that can fire its service routine every 10ms plus or minus 1ms is going to
have at least 11ms of latency. If jitter goes up to 10ms, then you have at
least 20ms of latency. You have to size your buffer to account for the
maximum jitter because jitter by its nature unpredictable; you don't know
whether or by how much you're going to miss the deadline until you've
already missed it.

Going to an overwritable ring buffer doesn't reduce the latency; it just
gives the app more rope to hang itself. The important question shouldn't be
"how do we increase the application's flexibility." It should be "how do we
make it possible for implementations to run reliably at high speed with low
jitter?"
Ian





> --
> Philip Jägenstedt
> Core Developer
> Opera Software
>
>


-- 
Ian Ni-Lewis
Game Developer Advocate
Android Developer Relations
Received on Tuesday, 12 July 2011 16:45:31 UTC

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