On 19/04/16 12:17 PM, Timothy B. Terriberry wrote:> We should probably start keeping track of the space of invalid TOC > sequences somewhere global so that people don't define conflicting > extensions.I started such a page at https://wiki.xiph.org/OpusExtensions -r -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 455 bytes Desc: OpenPGP digital signature URL: <http://lists.xiph.org/pipermail/opus/attachments/20160419/54f56bd2/attachment.sig>
Alright, here is a somewhat formal definition of the channel mapping I had in mind: ------ Channel Mapping Family 2 Allowed numbers of channels: (1 + l)^2 for l = 1...15. Explicitly 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225. Ambisonics from first to fifteenth order. Each channel is assigned to an ambisonic component in Ambisonic Channel Number (ACN) order. The ambisonic component with degree n and ambisonic index m corresponds to channel (n * (n + 1) + m). Channels are normalized with Schmidt Semi-Normalization (SN3D). In SN3D, the ambisonic component of degree n and index m is normalized according to sqrt((2 - delta(m)) * ((l - m)! / (l + m)!)), where delta(0) = 1 and delta(m) = 0 otherwise. ------ I chose ACN order and SN3D normalization because these seem to be the most common, and are used by the Ambix format and by Google's spatial audio RFC: https://github.com/google/spatial-media/blob/master/docs/spatial-audio-rfc.md I have a couple of questions about the definition and language. 1. Should I be more explicit about what "ambisonics" is, what normalization is, and what each channel really means? 2. Do I need to list the specific meaning of each possible channel count? For example, should I write "4 channels: First order ambisonics" similar to what is done for surround sound? 3. Should we define whether downmixing should occur when Ogg Opus players do not support the channel mapping? Since channel 1 is a mono channel, it is always possible to play something reasonable. Please let me know what you think. Thanks, Michael Graczyk
Hi Michael, On 04/25/2016 05:32 AM, Michael Graczyk wrote:> Channel Mapping Family 2 > > Allowed numbers of channels: (1 + l)^2 for l = 1...15. > > Explicitly 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, > 225. Ambisonics from first to fifteenth order.Would it make sense to allow an arbitrary number of channels and just "truncate" the list of channels. For example, two-channel ambisonics would be W plus X and three-channel would be W, X and Y. The idea is that you would get these mappings for free -- if there's any use for them anyway.> 1. Should I be more explicit about what "ambisonics" is, what > normalization is, and what each channel really means?For these kinds of things, you can just use references.> 2. Do I need to list the specific meaning of each possible channel > count? For example, should I write "4 channels: First order > ambisonics" similar to what is done for surround sound?I'm not sure I understand what your asking here.> 3. Should we define whether downmixing should occur when Ogg Opus > players do not support the channel mapping? Since channel 1 is a mono > channel, it is always possible to play something reasonable.It's not absolutely required, but it would indeed be nice if you could define a simple way that players can downmix. I guess mono is just "take the W channel", but maybe a reasonable stereo would be nice too. Cheers, Jean-Marc