[Vorbis-dev] Mapping = 1 Ambisonic Vorbis flag

[Oliver Oli]

On Thu, Sep 11, 2008 at 5:23 AM, Richard Lee <ricardo at justnet.com.au> wrote:
>>> When we start distributing 4th order stuff (which requires >16 speakers) we can get around this by having missing channels simply silent.  Vorbis compression is very efficient at coding silence.  Thanks to Gregory Maxwell for this excellent suggestion on the sursound forum.
>
>>Fons killed that proposal already on the sursound list. The Ambisonics decoder have to be setup before the playback starts.
>
> For higher orders, Gregory's scheme, has silent channels.  The player would need to look at the first sec. or so to determine the order for horizontal and vertical.

This would be a clever hack, if we had already defined and implemented
Ambi mapping in Vorbis limited to 3rd order. But I don't understand
why we just don't define a proper channel mapping, that don't waste
channels with silence for horizontal Ambisonics. With 255 channels we
could go up to 127th order horizontal, this would require 14 Bit for
the channel mapping per channel. Would this possible to implement?

>> I don't see any reason why a flexible universal channel mapping approach is a VERY bad idea. Just don't call it Ambisonics channel mapping. There are hybrid formats, how could you describe these with a simplistic Ambisonics-only channel mapping? I'm thinking of B+ Format (Ambisonics + Stereo) or maybe WXY(Z) plus a center channel and optional LFE as a 5.1 alternative.
>
> As I understand it, Ogg is a container and may have several streams eg Video, vorbis sound, FLAC sound, subtitles, karaoke etc.
>
> The sensible way to do mixed modes like Ambi with dedicated CF speaker channel is to have the extra channel and its metadata in a separate Ogg stream.  This way, Vorbis Ambi & AmbiDecLib have a clean interface and only deal with Ambi type signals.

I have to agree. Sometimes it's hard to see the obvious and simple solution.