Re: [Cellar] AV1 mapping Matroska

Moritz Bunkus <moritz@bunkus.org> Mon, 02 July 2018 07:48 UTC

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From: Moritz Bunkus <moritz@bunkus.org>
To: Codec Encoding for LossLess Archiving and Realtime transmission <cellar@ietf.org>
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Date: Mon, 02 Jul 2018 09:47:45 +0200
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Subject: Re: [Cellar] AV1 mapping Matroska
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Hey,

> Looking at the term "coded video sequence" (CVS) in other codecs (H.264
> and H.265) it seems it's a common term. And for those codec one Segment
> correspond to one CVS, with the parameters of that CVS stored in the
> CodecPrivate (SPS + PPS for H.264 for example). So we should probably go
> in that simple way.

I have quite a lot of h.264 samples here, mainly M2TS from DVB, where
SPS/PPS change mid-stream, often multiple times. For such files the first
occurrences of SPS/PPS make up the AvcC in CodecPrivate, but all key frames
are still prefixed with the currently active SPS/PPS — because that's the
only way to signal that stuff has actually changed.

Requiring a new segment each time those parameters change would make for
quite a bad experience, mainly due to two reasons:

1. There's no index listing where each segment starts making seeking inside
   multi-segment files all but impossible.
2. Software support for multi-segment files is pretty much non-existent.

I'm very pessimistic 2 would ever change, even if we mandated it in the AV1
mapping.

m.