Hi Stephan, Please note:
AAC/HE-AAC profile 1 uses Spectral Band Replication, which means that top octave information is generated from lower frequency content using "hints." I'm unsure of the impact this would have on ambisonic decoding. I guess one could filter out the replicated contents and treat it as a band-limited channel. AAC/HE-AAC profile 2 uses parametric stereo, which is similar to Ogg Vorbis Square Polar Mapping (described here http://xiph.org/vorbis/doc/stereo.html). This destroys phase information and I think would be unstable for ambisonic content. Can it be turned off in the encoder? Aaron Heller (hel...@ai.sri.com) Menlo Park, CA US On Fri, Aug 2, 2013 at 9:39 AM, Stefan Schreiber <st...@mail.telepac.pt>wrote: > 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<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.) > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: <https://mail.music.vt.edu/**mailman/private/sursound/** > attachments/20130802/49823d7a/**attachment.html<https://mail.music.vt.edu/mailman/private/sursound/attachments/20130802/49823d7a/attachment.html> > > > > ______________________________**_________________ > Sursound mailing list > Sursound@music.vt.edu > https://mail.music.vt.edu/**mailman/listinfo/sursound<https://mail.music.vt.edu/mailman/listinfo/sursound> > -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20130802/ee40d393/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
