Re: channel layouts and up/down mixing

It would be wrong to assign a specific channel ordering based only on the
number of channels.
You also need to have an identifier to disambiguate different surround
formats that have the same
numbers of channels.

For instance, quad (front left, front right, back left, back right) vs LCRS
(left, center, right, surround)
(both of these are directly supported by Pro Tools), or 11.1 vs 10.2.

Any such identifier should clearly reference the standard it is based on
(ITU-R, SMPTE, Dolby, etc.),
so that the correct speaker layout can be determined directly from the
source standard document.
Note that some standards have the same number of channels, and same speaker
placements, but
different channel orders.

If there is no standard, the format does not exist and should not be
supported.

No assumption should be made that all surround formats can be described as
subsets of some
master collection of speaker positions, as is done in the specification of
the multi-channel Wave file
format (
http://msdn.microsoft.com/en-us/library/windows/hardware/gg463006.aspx).
Doing so assumes a permanent limit on human creativity as applied to
speaker layouts, and is bound
to miss something that will be used in the future. For instance, the
multi-channel Wave file format
cannot describe the 22.2 format promoted by NHK. Nor can it describe 10.2.

This is a complex topic that should not be oversimplified. Oversimplifying
will cause endless
compatibility problems with the various professional formats, both current
and yet to come.

All IMHO of course.

Sincerely,
   Frederick


On Tue, Jan 15, 2013 at 12:44 PM, Ralph Giles <giles@mozilla.com> wrote:

> On 13-01-15 10:36 AM, Chris Rogers wrote:
>
> > What Tim says sounds reasonable.  I think we should be able to come up
> > with good channel orderings for the currently undefined ones he
> > mentions.
>
> To be clear, what we settled on for FLAC is:
>
> 1 channel: mono.
> 2 channels: left, right.
> 3 channels: left, right, center.
> 4 channels: front left, front right, back left, back right.
> 5 channels: front left, front right, center,
>             back/surround left, back/surround right.
> 6 channels: front left, front right, front center, LFE,
>             back/surround left, back/surround right.
> 7 channels: front left, front right, front center,
>             LFE, back center, side left, side right.
> 8 channels: front left, front right, front center, LFE,
>             back left, back right, side left, side right.
>
> This is based on identifying the standard Dolby surround speaker sets
> for home theatre with the the channel designations and order in
> http://msdn.microsoft.com/en-us/windows/hardware/gg463006.aspx
>
> The 'back/surround' hedge is to clarify compatibility with media formats
> and frameworks like Apple's CoreAudio which use the 'surround'
> terminology. For 4, 5 and 6 channel speaker arrangements the distinction
> between side, back, rear, and surround isn't meaningful.
>
> As Tim says, there is confusion with 7 and 8 channel setups between
> different standards as to whether 'surround' maps to 'side' or 'back',
> and whether the rear centre channel in 6.1 is in line with the other two
> surrounds or not. I think selecting 'side' and 'back' makes clear enough
> how to map these channels to any physical speaker arrangement.
>
> > Another thing to note is that the user agent may
> > have to do some amount of "channel swizzling" at the very final stage
> > when talking to the audio hardware to switch the Web Audio ordering of
> > channels into the actual physical ordering on that specific machine
> > (which can vary depending on configuration).
>
> Yes, of course.
>
>  -r
>
>