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Re: Extending MediaRecorder to record from Web Audio node faster than real time?

From: Harald Alvestrand <harald@alvestrand.no>
Date: Wed, 20 Aug 2014 12:57:56 +0200
Message-ID: <53F47F34.40209@alvestrand.no>
To: public-media-capture@w3.org
On 08/19/2014 10:23 AM, John Lin wrote:
> Hi all,
>   Currently MediaRecorder only records data from media stream, AIUI [1], is a real time source. That means recording would take as long as content duration.
>   Some use cases, such as cropping and cropping 1st half of an one hour speech audio clip, would not be very useful if saving the result needs such a long time to complete.
>   Web Audio API already defines OfflineAudioContext [2] to support processing faster than real time use cases.
>   By adding a new Constructor to MediaRecoder API:
>     Constructor(AudioNode node, optional unsigned long output = 0, optional MediaRecorderOptions options)
>   web applications can implement use cases that need to save processed audio with OfflineAudioContext and MediaRecorder.
>   What do you think?

Hmm.... this has more hair than most things, but mostly behind the
scenes; it's already possible to specify using

destiationNode = new MediaStreamAudioDestinationNode()
recorder = new MediaRecorder(destinationNode.stream)

But having MediaStreams that are not linked to a clock at all - we might
have to think about that; what happens if we link a stream from an
OfflineAudioContext to a PeerConnection?

Since it's an apparently legal thing to do, it SHOULD have a well
defined behaviour - but I'm not sure whether the result should be "no,
you can't do that" or whether it should do something useful in all cases.

> [1] final paragraph of http://www.w3.org/TR/mediacapture-streams/#introduction
> [2] http://webaudio.github.io/web-audio-api/#the-offlineaudiocontext-interface
> X
> John
Received on Wednesday, 20 August 2014 10:58:31 UTC

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