[Vorbis-dev] Re: Decoding for ambisonic Ogg audiob

[Richard Lee]

>The following sane options exist:

A) Simply output the B-format audio
B) Produce a downmix
  1) Mono.
  2) Stereo  blumlein crossed pairs
  3) Stereo UHJ
  4) binaural
C) Produce speaker feeds
  1) Fully generalizable speaker feed decoder
      (such as http://www.kokkinizita.net/linuxaudio/adec-pict.html)
  2) G-format (fixed decode for the 5.1 layout)

C) Produce speaker feeds

As we are talking surround sound, this is the most important support that is missing.

There are various levels of complexity in deriving speaker feeds from Ambisonic B-format.

I've just updated the Wiki Ambisonic Decoding page to show the simplest options which just need a panner.

The more sophisticated decodes are detailed in my "Ambisonic Surround Decoder" and "SHELF FILTERS for Ambisonic Decoders"

I'm sorry these are so obfuscating, but as I say very plainly, these are aimed at DSP gurus wanting to write software Ambisonic Decoders.

Just be thankful there are no glaring mistakes which will result in sub-optimal decoders.

Generating a UHJ signal these days is only sensible to get a stereo output.  But there are other simpler good stereo decodes.

My recommendation is to implement the simple 4.0, 5.0 & 7.0 decoders I've described on Wiki in the 'core' library now.  I recommend the 4.0 with a very low level CF over a Pentagon for general release.  But these are all good for playing Ambisonic material on most surround systems.

When we have Ambisonic gurus who are Ogg gurus, they can add the enhancements in "SHELF FILTERS ..." and other more sophisticated speaker decoders.

Fon's decoder is the only software full Classic Ambisonic Decoder apart from the secret Meridian stuff.  It is an excellent starting point for a fully programmable Ambisonic Speaker Decoder.

>Because these mapping 1 multichannel files will not playback on things
like hardware players, is there any aspect of the Vorbis subset
specification which you believe we should explore breaking, in effect
defing a new vorbis subset for ambisonic audio?

This is similar to what a stereo only player should do if faced with a multi-channel file.  If this was designed before multi-channel files were invented, it probably wouldn't have ANY strategy to deal with it.

Present players like Windoz Media Player tend to mix anything they don't understand into stereo with good and bad results.

Are you saying that existing Vorbis players don't have a similar strategy if faced with "Mapping <>0"?

If they do have a sensible strategy, we can adopt

A) Simply output the B-format audio

and leave the Ambisonic Speaker Decode to the new "core" Ambisonic library.

After all, if we intend to use Ambi technology to help compress surround sound in films, it is not unreasonable to expect a player to install a new decoder to take advantage of this.

The new decoder could output whatever Dolby speaker assignments apply to the punters surround system including Zillion.1



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