[Flac-dev] (Universal) Ambisonic implementation

[Brian Willoughby]

On Oct 4, 2009, at 16:48, James Cloos wrote:
> I always thought that the best way to put ambisonic media in flac  
> was to
> use an ogg container, with the 0-order w channel in one flac  
> stream, the
> three first-order channels in a second flac stream, the five second- 
> order
> channels in a third flac stream and the seven third-order channels  
> in a
> forth flac stream.

Excellent idea!

> An alternative would be to group the 0-order and first order into the
> first stream and continue as above for the second and third order  
> channels.

This was my first thought, but I did not know how well received the  
idea would be.

> Keeping the same-level channels together should help the compression,
> but different-order channels are likely to have less inter-dependence.

Very true.  I'm not sure how the three (?) first order channels would  
allow FLAC to take advantage of inter-dependence, because I cannot  
remember the limits of the current algorithm.  Certainly, stereo is  
analyzed, as are mid-side and a few other variations.  I'm just not  
sure whether any advantage is gained beyond two channels.  Archives  
of this list should have comments from Josh about the current  
algorithm's capabilities.

> Limiting each flac stream to just same-order channels requires four
> flac streams for a sixteen-channel third-order ambisonic, rather
> than just two streams, but makes it easier to drop the higher-order
> channels when desired.

Agreed.

> Allocation of forth and higher order ambison channels into the flac
> streams is left open, but higher orders are increasingly difficult
> to record and therfore are increasingly rare.  Forth and higher level
> may be a purely theoretical concern.

This also makes sense.

Perhaps you should write up an official recommendation!

Brian Willoughby
Sound Consulting