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DynamicsCompressorNode and latency

From: Robert O'Callahan <robert@ocallahan.org>
Date: Fri, 6 Jul 2012 13:54:10 +1200
Message-ID: <CAOp6jLbvCAFMU6sR-uttbVj1OGry-PpxWGbjUoG7mwiiXUQe3Q@mail.gmail.com>
To: public-audio@w3.org
In http://www.w3.org/2012/05/30-audio-minutes.html Chris R said: "The
dynamics processer has a small look-ahead latency."
This small remark has large implications. No other node does any kind of
lookahead or latency compensation right now. Supporting lookahead causes
problems when you have cycles (depending on the total delay around the
cycle(s) compared to the total lookahead in the cycles). There are also
issues where a node doing lookahead has looked ahead in some source stream,
and then something changes in the graph which means the data it looked it
isn't actually valid anymore. (Given that chaining nodes with lookahead can
require arbitrary lookahead on a source node, a whole host of issues would
arise.)

It had seemed to me that Web Audio was designed around the assumption that
the output of a node for sample T could only depend on the values of its
input nodes at samples T' <= T. I'm disturbed to find that's not the case.
I hope I've completely misunderstood the situation.

In any case, the details of this "look-ahead latency" need to be in the
spec to ensure that everyone does it the same way. Currently it isn't
mentioned in the spec at all.

Rob
-- 
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Received on Friday, 6 July 2012 01:54:38 GMT

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