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 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. ... > - 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. Regards, Martin -- Martin J Leese E-mail: martin.leese stanfordalumni.org Web: http://members.tripod.com/martin_leese/ _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound
