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Re: Proposal: Media streams, media stream tracks and channels

From: Harald Alvestrand <harald@alvestrand.no>
Date: Tue, 08 Nov 2011 15:05:09 +0100
Message-ID: <4EB93715.7000506@alvestrand.no>
To: public-webrtc@w3.org
On 11/08/2011 11:57 AM, Stefan Håkansson LK wrote:
> On 11/08/2011 01:15 AM, Harald Alvestrand wrote:
>> Discharging a task taken on at the TPAC meeting, some possible words on
>> what a media stream, a media stream track or a channel is....
>> This is based on the introduction section in section 3.1 of the current
>> API editors' draft.
>> The|MediaStream
>> <http://dev.w3.org/2011/webrtc/editor/webrtc.html#mediastream>|interface
>> is used to represent streams of media data, typically (but not
>> necessarily) of audio and/or video content, e.g. from a local camera or
>> a remote site. The data from a|MediaStream
>> <http://dev.w3.org/2011/webrtc/editor/webrtc.html#mediastream>|object
>> does not necessarily have a canonical binary form; for example, it could
>> just be "the video currently coming from the user's video camera". This
>> allows user agents to manipulate media streams in whatever fashion is
>> most suitable on the user's platform.
>> Each|MediaStream
>> <http://dev.w3.org/2011/webrtc/editor/webrtc.html#mediastream>|object
>> can represent zero or more tracks, in particular audio and video tracks.
>> Tracks can contain multiple channels of parallel data; for example a
>> single audio track could have nine channels of audio data to represent a
>> 7.2 surround sound audio track.
>> <new text below>
>> All tracks in a MediaStream are presumed to be synchronized at some
>> level. Different MediaStreams may or may not be synchronized.
>> Each track represented by a|MediaStream
>> <http://dev.w3.org/2011/webrtc/editor/webrtc.html#mediastream>|object
>> has a corresponding|MediaStreamTrack
>> <http://dev.w3.org/2011/webrtc/editor/webrtc.html#mediastreamtrack>|object. 
> Nit: why do you say "represented by a MediaStream" in this section but 
> "in a MediaStream" in the previous one? "In a" sounds better to me.

I copied this from "Each MediaStream object can represent zero or more 
tracks" 2 paragraphs further up. I would prefer to use language that 
says that a MediaStream contains zero or more tracks - as long as this 
doesn't confuse anyone when one track can be contained in multiple 

>> A MediaStreamTrack represents content comprising one or more channels,
>> where the channels have a defined well known relationship to each other
>> (such as a stereo or 5.1 audio signal), and may be encoded together for
>> transmission as, for instance, an RTP payload type.
>> A channel is the smallest unit considered in this API specification.
>> <end new text>
>> Would including this text help add any clarity after our discussions at
>> TPAC?
> I think it helps. I am not convinced that the channels must be exposed 
> in any of the webrtc API's. The Audio API that Chris presented enables 
> access to the channels (in audio tracks) if there is a need to.
Once we're sure we have consensus to reference the Audio group's Audio 
API, and that API is clear on how to deal with MediaStream tracks, I 
think we should add a paragraph saying that audio tracks can be 
manipulated by using the relevant Audio API functions.

Received on Tuesday, 8 November 2011 14:05:50 UTC

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