[flac-dev] WAVEFORMATEXTENSIBLE_CHANNEL_MASK is not described

[Martin Leese]

lvqcl wrote:
...
>  From FLAC 1.2.1 documentation:
>
> 0000-0111 : (number of independent channels)-1. Where defined, the channel
> order follows SMPTE/ITU-R recommendations. The assignments are as follows:
> 1 channel: mono
> 2 channels: left, right
> 3 channels: left, right, center
> 4 channels: left, right, back left, back right
> 5 channels: left, right, center, back/surround left, back/surround right
> 6 channels: left, right, center, LFE, back/surround left, back/surround
> right
> 7 channels: not defined
> 8 channels: not defined
>
> It was more obvious that "where defined" means 1-6 channels, and 7 and
> 8-channel
> files have undefined channel assignment. Now all possible numbers of
> channels have
> defined channel map, so the text should be (IMHO):
>
> 0000-0111 : (number of independent channels)-1. The channel order follows
> SMPTE/ITU-R recommendations. The assignments are as follows:

Note that the channel order may not be defined.  An
example of this is in the Microsoft documentation
defining WAVEFORMATEXTENSIBLE.  The
final example is titled "Multiple mono streams (no
speaker location)", and has a (valid) channel
mask of zero (dwChannelMask = 0).

Also note that in the changelog at:
    https://xiph.org/flac/changelog.html
it states:
    Now properly supports ... WAVEFORMATEXTENSIBLE multichannel input,
performing necessary channel reordering both for encoding and
decoding. WAVEFORMATEXTENSIBLE channel mask is also saved to a tag on
encoding and restored on decoding for situations when there is no
natural mapping to FLAC channel assignments.

So the WAVEFORMATEXTENSIBLE channel
mask is saved ONLY when the channel order
does not follow SMPTE/ITU-R recommendations.

Having channel assignments in two different
places is bad practice.  In the FLAC spec there
is "1011-1111 : reserved".  Why not specify one
of the reserved codes to mean "channel
assignment is specified in the
WAVEFORMATEXTENSIBLE channel mask; go
look there".  The problem is how to now specify
the number of channels.

Regards,
Martin
-- 
Martin J Leese
E-mail: martin.leese  stanfordalumni.org
Web: http://members.tripod.com/martin_leese/