[Matroska-devel] Opus in Matroksa Cont.

Ralph Giles giles at thaumas.net
Wed Apr 10 01:42:20 CEST 2013


On 13-04-08 7:58 PM, Steve Lhomme wrote:

> Just an addition to the conversation for now as I didn't see it
> mentioned. One of the major use case of Opus (+VP8) in Matroska/Webm
> would be to stream a WebRTC stream from a website as a read only video.

Yes. Or just making a local recording of a WebRTC session.

> If it's live it would be important to minimize the playback delay to a
> minimum. That plays against having a timecode shift applied by default.

The pre-skip value is typically used at the start of a stream to account
for algorithmic delay, so this timecode shift is in fact necessary got
get correct a/v synchronization.

The delay is typically only a few hundred samples, though, so the error
if this is incorrect is a fraction of a video frame duration.

Correct pre-skip handling is more important when it's been used for
editing, or to signal the need for full pre-roll at the start of the
stream, as happens when recording begins in the middle of an established
session. At 80 ms the shift accounts for noticeable sync error if not
applied.

 -r


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