Re: TV Control API and getUserMedia

(speaking as an individual)

To me it seems that using getUserMedia, as specced now, with constraints 
to select a tvtuner as the source is a bit clumsy, since the constraints 
you can supply are of type "MediaStreamConstraints" which in turn has 
(up to) one entry for a video track and one for an audio track.

If you were to extend getUserMedia to also be able to accept a (e.g.) 
"TVTuner" constraint it could perhaps look a little better.

Also, changing channel by applying a track constraint seems (as you 
point out) clumsy and non intuitive. Here, you could perhaps define 
constraints that apply on TVMediaStreams instead?

To me, your 'Alternative proposal' actually seems cleaner.

Now I am far away from my area of knowledge, but anyway: is there any 
specific reason for not going along the path of Media Source Extensions 
and define a TV Tuner in a similar way? To me it seems that the only 
uses of TVMediaStream is a) recording and b) text tracks. But a) will 
(?) be solvable by combining mediacapture-fromelement with 
mediacapture-recodring, and for b) I had the feeling that there was 
already support for handing text tracks in the video element itself.

Br,
Stefan

On 16/12/16 15:03, Francois Daoust wrote:
> Dear Media Capture task force,
>
> The TV Control Working Group is re-modeling the TV Control API
> specification around sources. That new model is well aligned with the
> model used in Media Capture and Streams. In fact, it would seem
> possible to re-use Media Capture and Streams interfaces as-is.
> However, some details do not seem entirely right when we do that, and
> we would like to get your feedback on the following questions.
>
>
> Context ----- The TV Control API specification defines an API for
> sourcing audio and video media, such as TV and radio from broadcast,
> IPTV, or other sources, and allows presentation of the media using
> the <video> and <audio> HTML elements:
>
> http://w3c.github.io/tvcontrol-api/
>
> The API typically produces MediaStreams, with methods to switch from
> one TV/radio channel to the other, and various classes to retrieve
> associated channel and program metadata.
>
> The current API is designed around the notion of tuners but the group
> agreed to a re-design centered on the notion of sources. To keep
> things simple, a TV source is something attached to a broadcast
> signal (which could come through a cable, terrestrial antennas,
> satellites, etc.) that can be tuned to a specific channel to produce
> a MediaStream, composed of a set of MediaStreamTracks for that
> channel. Users may switch the source to another channel at any time.
>
> Looking at it from a getUserMedia perspective, it seems possible to
> consider TV sources as input devices, and channels as a constraint
> that could be applied to the main video track. This would lead to the
> following code:
>
> var source = null;
>
> navigator.mediaDevices.enumerateDevices() .then(function
> (devicesInfo) { source = devicesInfo.find(function (deviceInfo) { //
> Or use "getCapabilities" to detect support for the "tvchannel" //
> constraint if kind cannot be extended? return deviceInfo.kind ===
> 'tvsource'; }); return source.getChannels(); }) .then(function
> (channels) {}) var channel = channels[0]; secondChannel =
> channels[1]; navigator.mediaDevices.getUserMedia({ video: { deviceId:
> source.deviceId, tvchannel: channel } }) }) .then(function (stream)
> { tvStream = stream;
>
> // Render the stream document.getElementById('video').srcObject =
> tvStream;
>
> // Switch to another channel return
> tvStream.getVideoTracks()[0].applyConstraints({ tvchannel:
> secondChannel }); }) .then(function () { // Stop stream
> tvStream.getVideoTracks()[0].stop(); });
>
>
> This triggers a few questions though.
>
> Questions -----
>
> 1. Attaching to "enumerateDevices"? -- We need some way to enumerate
> the TV/radio sources but it feels a bit strange to mix TV/radio
> sources with camera/microphone sources. Use cases that want to get
> media from a camera are roughly disjoint from use cases that want to
> tune to a particular TV channel. Shouldn't APIs rather be separated?
>
> If not, what would be the proper way to distinguish TV sources?
> Should we introduce a new "tvsource" kind of source for instance?
>
>
> 2. Channel constraint at the track level? -- Similarly, it does not
> seem natural to apply the constraint to change the channel at the
> track level. The channel "constraint" rather seems to apply at the
> MediaStream level, as it is going to affect all tracks. Are we trying
> to push the constraint model too far?
>
> The same comment applies to the "stop" method, which again is going
> to affect all tracks of the MediaStream in our case.
>
>
> 3. TV specific semantics? -- While it works, is also seems
> counter-intuitive to "apply a constraint" to switch from one channel
> to another. Developers would rather expect something like a
> "tuneToChannel" method instead.
>
>
> 4. Extending or wrapping MediaStream? -- The TV Control API extends
> MediaStream to add buffering. This seems to preserve the spirit of
> MediaStreams and there has already been exchanges about that in the
> past.
>
> We may need to introduce other attributes at that level, such as an
> "isRecordable" property to tell whether the channel may be recorded.
> This is also where the "stop" method mentioned above would fit. Such
> extensions seem of different nature. We're wondering whether it could
> be preferable to introduce an extra layer that wraps the MediaStream
> and exposes TV specific properties.
>
>
> Alternative proposal ----- All in all, an alternative proposal that
> separates the APIs, moves operations back to the source level, and
> creates a wrapping tuner class around MediaStream could lead to the
> following code:
>
> var source = null; var secondChannel = null; var tvTuner = null;
>
> navigator.tv.getSources() .then(function (sources) { source =
> sources[0]; return source.getChannels(); }) .then(function (channels)
> { var channel = channels[0]; secondChannel = channels[1]; return
> source.tuneToChannel(channel); }) .then(function (tuner)) { tvTuner =
> tuner;
>
> // Render the stream document.getElementById('video').srcObject =
> tvTuner.stream;
>
> // Switch the source to another channel, re-using the same tuner
> return source.tuneToChannel(secondChannel, tuner); }) .then(function
> () { // Stop stream and release resource tvTuner.stop(); });
>
> That alternative proposal re-uses MediaStreams, but is less aligned
> with getUserMedia. What do you think?
>
> Feedback through email is fine. Feel free to send feedback on GitHub
> if preferred. This is being tracked by the following issue on our
> side: https://github.com/w3c/tvcontrol-api/issues/4
>
> Note that we would be happy to schedule a call as needed to clarify
> questions and thoughts.
>
> Thanks, Francois.
>
>
>

Received on Monday, 19 December 2016 10:13:08 UTC