[opus] preskip and seeking suing Opus
jmvalin at jmvalin.ca
Thu Aug 15 11:21:11 PDT 2013
If you're controlling the encoding and disabling all prediction, you can
reduce convergence time down to about 3 ms.
On 08/15/2013 12:09 PM, Bob Estes wrote:
> Yes, that's a start.
> Ultimately, though, I'm hoping to reduce the 80ms requirement, and am
> trying to get a handle on what state in the decoder must converge and
> what complications I might be up against. I'm also only considering
> CELT-based encodings if that simplifies things.
> I know that Opus can do inter-frame energy envelope prediction, but that
> dependency can be eliminated by using intra-frames.
> There's also a dependency introduced by the MDCT overlap of CELT.
> Without the previous frame, this could be a major term in the
> convergence requirement.
> Other than that, I'm not sure what other dependencies I would need to
> Is their a paper or some studies or discussions available about where
> the 80 ms comes from?
> I'm a fairly new member to this group, but did scan the mailing list
> archives for similar topics ...
> Thanks again,
> -Bob Estes
> On Thu, Aug 15, 2013 at 5:30 AM, Ralph Giles <giles at thaumas.net
> <mailto:giles at thaumas.net>> wrote:
> On 13-08-14 10:09 PM, Bob Estes wrote:
> > I've been studying the Opus code and documentation for a while and
> > seen it mentioned several times that Opus uses pre-skip to allow the
> > codec to converge. What convergence are they referring to? Rate
> > control? Energy envelope prediction after seeking?
> Not rate control, but there are a number of predictors running in the
> decoder which need some amount of data to converge with their expected
> values when the encoder and decoder would be out of sync for whatever
> Pre-skip can be used by a muxer to describe how much data must be run
> through the decoder before valid output is obtained at the start of a
> file. This could be e.g. 80 ms if the muxer is creating a file from the
> middle of another one (lossless editing, webrtc stream recording).
> More commonly it's used to account for resampler and algorithmic delay
> so the output can be phase-aligned with the input.
> > Also, I'm also interested in the seeking behaviour of the decoder.
> > If, say, I want to skip ahead in an Opus bitstream, can I start
> > at any intra frame without artifacts?
> You must start feeding data to the decoder about 80 ms before the
> intended seek point, and discard the output. This is for convergence as
> described above, but for seeking you can use the general 80 ms rule as
> described in the Ogg Opus draft. This is referred to as pre-roll, and
> doesn't need to be specially specified by the muxer like pre-skip since
> it's constant and applies to any seek point, not just the start of
> Does that help?
> opus mailing list
> opus at xiph.org
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