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Re: Shared Motion - multi-device synchronization and media control for the Web.

From: Ingar Mæhlum Arntzen <ingar.arntzen@gmail.com>
Date: Wed, 17 Dec 2014 09:57:48 +0100
Message-ID: <CAOFBLLq-vzSjDfH8DSVBL-ZxNym5xh2QBvPduXYG1ZJiiS+hXg@mail.gmail.com>
To: Sangwhan Moon <smoon@opera.com>
Cc: "public-web-and-tv@w3.org IG" <public-web-and-tv@w3.org>, Njål Borch <njaal.borch@gmail.com>, Dominique Hazael-Massieux <dom@w3.org>, François Daoust <fd@w3.org>
Hi Sangwhan

You are correct, seekTo operations introduce asynchrony/errors. Even worse,
in a distributed system it is hard to ensure that seekTo operations occur
at the same time. Shared Motion could help with this though. By delaying
operations until a specific point on a shared clock (i.e. motion), you
could mask effects of differences in network latency among devices and
approximate simultaneous performance of the seekTo operation.

However, we are not doing it like that, as it would introduce un-necessary
latency in user interaction, thereby hurting end-user experience.

Instead we are performing seekTo operations as soon as they are available
on different devices. This leads to the media being visibly out of synch
for a brief moment (after seekTo).

Crucially though, the presence of and ideal media clock (shared motion)
means that our JavaScript synch wrappers know locally exactly what the
error is. So this error is quickly compensated by temporary tuning the
playbackRate or doing adjustments to the playback offset (if variable
playback rate is not available, or if the errors are above some limit).

Monitoring errors and adjusting for them is something one has to do
continuously anyway, as media elements playback drift at different rates.
So, "recovering" after seekTo doesn't even require logic that wasn't needed
in any case.



2014-12-17 9:32 GMT+01:00 Sangwhan Moon <smoon@opera.com>:
> On Wed, Dec 17, 2014 at 5:31 PM, Sangwhan Moon <smoon@opera.com> wrote:
>> On Fri, Dec 12, 2014 at 9:50 PM, Ingar Mæhlum Arntzen <
>> ingar.arntzen@gmail.com> wrote:
>>> Dear IG Members
>>> We would like to present ourselves to this forum, as we share your
>>> interest in improving the Web as a platform for broadcast and multi-device
>>> media, and because we have some contributions which you might find relevant.
>>> My collegue (Njål Borch) and myself (Ingar Arntzen) are researchers
>>> working for NORUT (Northern Research Institute), Tromsø, Norway. Over the
>>> last couple of years we have focused on timing, synchronization and media
>>> control in multi-device media. Currently NORUT is in charge of the
>>> workpackage that deals with this topic in MediaScape, a FP7 EU project
>>> aiming to provide a fundament for multi-device Web applications. The
>>> consortium includes BBC R&D, Vicomtech, IRT, NEC, NORUT, BR and W3C.
>>> To the point: We have invented and developed the concept of  "Shared
>>> Motion", a generic mechanism for synchronization and media control in
>>> time-sensitive, multi-device Web applications. This mechanism has already
>>> been included as fundamental component in the multi-device architecture
>>> explored within the MediaScape project.
>>> To give you a rough idea what this is about:
>>> - Shared Motion synchronizes *globally*, thus multi-device
>>> synchronization is not limited to Intranet or specific network carrier.
>>> - Shared Motion synchronizes across Internet with errors < 10ms, and
>>> works fine even under poor network conditions (e.g. edge - albeit a modest
>>> reduction in precision may be expected)
>> This is a "out of plain curiosity" question, but it wasn't clear from the
>> paper how the < 10ms synchronization
>> deals across devices using different codecs - would it be possible to
>> share how that bit works?
>> While I am not a expert in this area, from my experience if a
>> synchronized seek request happens between
>> two devices that are decoding using different codecs there is a high
>> chance of the different streams having
>> different i-frames, which will require the decoder to advance to a
>> i-frame, which could end up making the two
>> devices go out of sync.
>>               ^
>>          Seek request
>>                  ^
>>        Stream 2 Decode Starts
>>                           ^
>>                 Stream 1 Decode Starts
> That should have looked like this: (Silly rich text mail clients.)
>               ^
>          Seek request
>                  ^
>        Stream 2 Decode Starts
>                           ^
>                 Stream 1 Decode Starts
> --
> Sangwhan Moon [Opera Software ASA]
> Software Engineer | Tokyo, Japan
Received on Wednesday, 17 December 2014 08:58:16 UTC

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