[opus] Channel Mapping Family for Ambisonics

Christian Hoene christian.hoene at symonics.com
Tue Apr 19 17:34:13 UTC 2016


Ambisonics are good for the recording but what is about the playback?

In addition to an Ambisonic channel mapping, isn't it worthwhile to think about
some object based audio coding, too? 

But then again, that would be much more work than just adding a new channel


> Michael Graczyk <mgraczyk at google.com> hat am 18. April 2016 um 04:29
> geschrieben:
> Hello,
> We (Google) have been experimenting with configuration and adjustments to
> CELT-only Opus that give good results for compressing ambisonic audio
> signals [1]. Based on our results so far, we would like to use Opus to
> encode spatial audio. We hope to make it easy/possible to use libopus with
> other common tools and software modules (ffmpeg/libav in particular).
> Based on my reading of the libopus code and the IETF spec, it seems one
> reasonable option would be to add a new "Channel Mapping Family" for
> ambisonic audio [2]. The mapping family would indicate to the decoder that
> the audio is ambisonics and the channel mapping array would indicate which
> ambisonic channel (W, X, Y, etc) corresponds to which coded stream. This
> representation is analogous to Opus headers for surround sound.
> There are a few caveats though. Although we believe we can achieve good
> compression at first without changing the bitstream or the decoder, we
> would like the flexibility to potentially modify both if potential
> improvements are compelling enough to impress you (specifically, we have a
> pre/post transform that would require sending compressible side
> information). Would changing either the bitstream or encoder require adding
> yet another channel mapping? Would it require a new Opus version number?
> To summarize, should we add a new channel mapping for ambisonics? If not,
> what should we do?
> [1] https://en.wikipedia.org/wiki/Ambisonics
> [2] https://tools.ietf.org/html/draft-ietf-codec-oggopus-14#section-5.1.1
> Thanks for your input,
> Michael Graczyk
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