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 >.)
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.
Certainly, but I see many ways to achieve this.
Note that .aac is one thing, and .m4a and .m4p as container formats are
something different. (Because Apple seems to mix these things a bit up,
a decoder will play a "aac" stereo file in any of these variants, and it
will be the same thing anyway. Speaking of extensions, it is not always
the same thing. )
...
- The UHJ article already mentions that the T channel could be
bandwidth-limited.
Geoffrey Barton said some time ago that a
bandwidth-limited T-channel resulted in some
unwelcome compromises in the design of the
3-channel UHJ decoder. This may not be
such a problem with software decoders as you
could just include two separate decoders, one
for 2.5 channels and another for 3. However,
this would mean a lot more work.
I question whether the gain from band-limiting
T is worth the pain.
No, I already wrote it is not worth it. (Better to use a lower
AAC/HE-AAC bitrate for the full T/Q channel/channels, IMO.)
Best,
Stefan
P.S.: Of course you would have to prove such a concept. If you have at
least three ways to "fiddle" and two ways don't use hidden "audio"
channels at all, things should really work.
http://en.wikipedia.org/wiki/MPEG-4_Part_14
"The existence of two different filename extensions, .MP4 and .M4A, for
naming audio-only MP4 files has been a source of confusion among users
and multimedia playback software. Some file managers, such as Windows
Explorer, look up the media type and associated applications of a file
based on its filename extension. But since MPEG-4 Part 14 is a container
format, MPEG-4 files may contain any number of audio, video, and even
subtitle streams, making it impossible to determine the type of streams
in an MPEG-4 file based on its filename extension alone. In response,
Apple Inc. started using and popularizing the .m4a filename extension,
which is used for MP4 containers with audio data in the lossy Advanced
Audio Coding (AAC) or its own lossless Apple Lossless (ALAC) formats.
Software capable of audio/video playback should recognize files with
either .m4a or .mp4 filename extensions, as would be expected, since
there are no file format differences between the two."
Almost any kind of data can be embedded in MPEG-4 Part 14 files
through private streams. A separate hint track is used to include
streaming information in the file.
Which is the option which leads to >= 320kbps mode, as well. (I could
figure this out. Not necessary as response to your posting.)
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