[flac-dev] Define 6.1 and 7.1 channel mappings

[Ralph Giles]

On 13-01-01 4:36 PM, Tim W. wrote:

> - 4 channels: left, right, back left, back right (FL FR BL BR)
> - 5 channels: left, right, center, back/surround left, back/surround right
>               (FL FR FC BL BR or FL FR FC SL SR, same order so doesn't matter)
> - 6 channels: left, right, center, LFE, back/surround left, back/surround right
>               (FL FR FC LFE BL BR or FL FR FC LFE SL SR, same order so doesn't matter)

I looked at the 'flac' command-line front-end code today, and it uses
Back Left and Back Right for 4 channel but Side Left and Side Right for
5 and 6 channel files when writing out WAVE from FLAC files without a
WAVFORMATEXTENSIBLE_CHANNEL_MASK metadata tag.

As you say, the order is the same, and for less that 7 channels the
physical speaker configurations are equivalent.

The encoder accepts both Side and Back in the input channel mask, but
passes them on in the tag, of course. If the input is AIFF the support
is more limited, rejecting any multichannel file unless the undocumented
--channel-map=none is passed, followed by some dead code saying only 1,
2, 3, and 5 channels can be unambiguously mapped.

> - 6.1: FL FR FC LFE BC SL SR (L R C LFE Cs Ls Rs)
> - 7.1: FL FR FC LFE BL BR SL SR (L R C LFE Rls Rrs Ls Rs)

Again, I'm suggesting we update the spec to define these as the default
channel maps for 7 and 8 channel FLAC files in the absence of the
overriding WAVFORMATEXTENSIBLE_CHANNEL_MASK metadata tag, and modify the
included 'flac' front-end to write out those masks when decoding such
files. Likewise, we would modify 'flac' to accept files with those
channel masks as input when encoding.

Untested patch attached.

 -r
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