Sampo Syreeni wrote:
On 2017-01-09, Stefan Schreiber wrote:
- Backward-compatible (to stereo) forms of Ambisonics are very
probably possible.
They are not "probably possible". That's what the BHJ version of UHJ
*is*.
Yes. But you still have to put L/R + 1 or 2 ext. channels into some <
stereo
MP3 or AAC file.
...
That is then not much of a problem either, if you think about it.
Pretty much the only formats we have to think about are RIFF WAVE
(.wav) and MPEG2 layer 3 (.mp3). Just maybe MPEG2/4 AAC (ISO/IEC
13818-7:2006).
Wav, especially in its modern versions, supports hiding T. It also
supports a media tag to discern between just UHJ encoded stereo with a
hidden channel, from ordinary stereo with one hidden channel. We
already used it to encode B-format, in Richard Dobson's .AMB format,
derived from WAVE_FORMAT_EXTENSIBLE
(https://en.wikipedia.org/wiki/Ambisonic_data_exchange_formats).
AAC is pliable as well. Of MP3 I'm not too sure, but I think it might
be. (I'm too drunken and tired to delve into that now. If you want
certainty, remind me later. I do think I have the relevant ISO
standards laying around somewhere on my disks.)
I came basically to the same conclusions. (AAC implementation possible,
some MP3 "hack" maybe...)
But to implement this, you or I or anybody would need the (detailled)
format specifications.
(Hidden) Extension channels are known and applied in other cases. (DTS,
True HD's 5.1 system over stereo "core", etc.)
Best,
Stefan
UHJ was always a hierarchy scheme. Even so you have to find some
space for T and Q channels. It is all just about some "practical
space problems", not Makita theory itself... Just IT and standard
related stuff, not math! ;-)
Funnily, Gerzon also built up another hierachy of sorts, incompatible
with UHJ. That's the frontal stereo one for early HDTV work. I never
understood why he didn't bring it under the ambisonic compatibility fold.
DirAC has the problem that it currebtly can be used "just as
research" tool. (I won't discuss this here. But you know this as well
as I do. )
Yes. It might be patent encumbered as well. And I don't much like the
idea that the decoder's input is effectively a directed cardioid --
that should be dealt with pretty much as the dual of an optimal decoder.
But yeah...
Maybe. But before we "just" would need some good binaural decoder for
some standard surround formats; 5.1 and FOA surely included.
If you "just want to do it", I can easily contribute the research,
math and inner loops. On the framework and integration side I'm not
too good.
I might come back to this later and accept your kind offer... :-)
Until 2morrow, and many thanks for your thoughtful and constructive
posting! (I have to think about this quite a bit more....)
What the list is for, but also a bit sorry for my usual rant-length. :)
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