[Widex] suggestions re timestamps
Dave Raggett <dsr@w3.org> Fri, 24 March 2006 17:14 UTC
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Date: Fri, 24 Mar 2006 17:14:40 +0000
From: Dave Raggett <dsr@w3.org>
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Subject: [Widex] suggestions re timestamps
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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 In the face to face meeting on Thursday, we discussed the potential problems with how REX specifies timestamps for updates that are intended to be applied at a future time. REX currently specifies these in terms of the renderer's clock as milliseconds since the start of 1970 GMT. The problem is the likelihood of a significant clock skew between the renderer and the server, and the latency incurred by the protocol. This makes it difficult to the server to set the appropriate timestamps when the update is to be applied in a relatively short time interval into the future. In principle, the server may be able to estimate the clock skew and the network latency from a sequence of interactions with the renderer, but this is tricky and subject to uncertainties. W3C's SMIL language deals with this by allowing you to specify event timestamps in terms off timed offsets from other events (the beginning or end of a give presentation). SMIL isn't itself a solution to Widex as it doesn't deal with the means to transport DOM events and updates. Widex/REX could use the SMIL event-value syntax, which includes a reference to a named event and a signed time offset. This would be very general, but comes with an implementation overhead that may not be acceptable in all environments. A simpler approach would be use non-negative timed offsets from an implicit reference event. You would be able to designate that a given event is the new reference point, replacing the previous reference point. The initial reference point is the start of the session. This technique avoids the need to record the times of events as they are applied. You only need to record the time of the current reference point. The syntax for identifying an event as a reference point would involve a new attribute. The existing timestamp would be refined as a non-negative integer denoting the number of milliseconds from the current reference point. For multimedia presentations, the audio track is often used as the reference timebase since human perception finds disruption to the audio more disturbing than to the visual presentation. A question for the W3C REX task force is whether they are interested in synchronizing SVG updates to streaming audio, and if so whether the presentation can be paused/resumed. If yes, then this should be taken into account in the semantics for timestamps, e.g. the clock should be paused along with the presentation. Comments? Dave Raggett <dsr@w3.org> W3C lead for multimodal interaction http://www.w3.org/People/Raggett +44 1225 866240 (or 867351) -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.1 (GNU/Linux) iD8DBQFEJCkFb3AdEmxAsUsRAmG1AKCiQWEkIYxLYmAOhQIYlxmhnByyEgCgn3bl +M+Bs9+lJX/0L5mtiu2xYvM= =UZF3 -----END PGP SIGNATURE----- _______________________________________________ Widex mailing list Widex@ietf.org https://www1.ietf.org/mailman/listinfo/widex
- [Widex] suggestions re timestamps Dave Raggett
- RE: [Widex] suggestions re timestamps Vlad.Stirbu