Martin Leese wrote:
Stefan Schreiber wrote:
...
To offer a backward-compatible extension of a < UHJ extended > AAC
stereo file, you would have to include the T and Q audio channels as 3rd
or 4th audio stream, somewhere. (Probably you could "label" such a file
as stereo, the first 2 channels being L and R. Include some tags/flags
in the header that there are one or two further < extension > audio
channels, which would have to be decoded by a UHJ decoder. The decoder
could be an app running on a smartphone, and the output could be a
binaural version of the surround or actually LRTQ 3D audio recording.)
If this "audio channels" approach doesn't work, use the "data"
extensions of .mp4. (T and Q are not direct audio channels, so this
might actually be the formally correct approach... Because T and Q go
into some decoder, as extension < data >.)
The sections quoted above are the key, to my
mind. A problem with 3- or 4-channel UHJ is,
what do decoders that are unaware of
Ambisonics do with the extra channels? With
other file formats, they would treat the file as
multi-channel and mix all the channels down to
stereo. With T and Q included in the mix, this
would produce a mishmash.
This problem of inadvertent mix down is why I
have been pushing for so long (without any
success) for a way to specify in multi-channel
files the preferred mix down to stereo. See,
for example:
http://members.tripod.com/martin_leese/Audio/StereoMix_chunk.html
This is meta data and channel denomination hell, but actually these
problems don't matter so much in our case. Because the "preferred
downmix" is already in LR, you have only one or two more channels which
you have to embed without breaking existing decoders (hardware and
software).
Somebody would need to produce AAC test
files containing T and T+Q, and see what
existing stereo decoders actually do.
If existing
decoders cannot be made to ignore T and Q
(by fiddling with the file format) then the idea of
including T and Q is dead.
http://en.wikipedia.org/wiki/Advanced_Audio_Coding
"AAC supports inclusion of 48 full-bandwidth (up to 96 kHz) audio
channels in one stream plus 16 low frequency effects (LFE, limited to
120 Hz) channels, up to 16 "coupling" or dialog channels, and up to 16
data streams."
Probably you would use the data channels, to be on the safe side of
backward-compatinilty. This is if you stay in AAC format, which can be
used as audio part in some M4P. (M4P is the most general term. MP4 would
be some AVC video file with some audio, and maybe additional data like
subdata etc. You could have timecodes and so on.)
I would propose two solutions anyway, because .aac and .m4a/.m4p offer
different possibilities and actually applications. (You could have more
than 4 channels in .m4p, if needed.)
Best,
Stefan
P.S.: .M4P is a better format than Wave-EX. It is a container format
which allows you to extend both much easier and more flexible, forget
all this "chunk" stuff which might work or not.
The ISO doesn some good standardization, once more...
_______________________________________________
Sursound mailing list
[email protected]
https://mail.music.vt.edu/mailman/listinfo/sursound
