- From: Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>
- Date: Thu, 10 Nov 2011 16:08:55 +0100
- To: public-webrtc@w3.org
On 11/10/2011 03:30 PM, Francois Daoust 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. >> >> >> >> 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. > > To ensure I get things right... > > With this definition left and right channels of a stereo audio > signal can be equally represented as: 1) two channels within a > MediaStreamTrack. 2) two MediaStreamTrack objects. In principle it could even be two different MediaStreams... > > The first case is the most likely representation given the > well-known relationship between both channels in a stereo audio > signal. The two channels may or may not be encoded together for > transmission. Agree. > > The second case is not the most likely representation but is still > possible. In this case, can the two MediaStreamTrack objects still > be encoded together for transmission? I would assume that it is not > possible. Would it make sense to clarify that a MediaStreamTrack > object is always encoded on its own for transmission (or is it > self-evident)? I think we still have some details to work out. There is e.g. the question on how much of this control that is actually available to the web developer. Can she/he influence if one/many tracks/streams are used? And if a MediaStreamTrack (with e.g. two audio channels) will have a joint encoding or not on the transport layer will probably depend on the available codecs. Some support stereo/multichannel, others require separate encodings. > > Francois. > > >> >> 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? >> >> Query: What other examples of embedded channels would be useful to >> add? Are there good examples of audio or non-audio data embedded >> in video tracks? >> >> Harald >> >> >> >
Received on Thursday, 10 November 2011 15:09:27 UTC