Aaron Heller wrote:
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 ([email protected])
Menlo Park, CA US
(HE-AAC, Vorbis, Opus, FLAC)
To step things a bit up:
http://people.xiph.org/~greg/opus/ha2011/
The comparison of HE-AAC and Opus happens here at 32kbps/channel. (I was
talking about transparent or near-transparent bitrates, but if we just
talk about streaming or mobile streaming, say at 128kbps, you still have
some options... )
Opus is an official Internet (IETF) format, by now. I am not ignoring
Opus, FLAC etc., but wrote my first posting (mainly) from an AAC
perspective, because we talked about < established ways of audio
delivery >.
(The proposed format is basically codec and format agnostic. It is the
stereo backward-compatible version of B format at 1st order, which could
"easily" be extended to 3rd order, at least from a theory perspective.
Important is that there is always some direct relationship between the B
format and the stereo-extension" UHJ version. XYWZ < ---- > LRTQ is
therefore the same, the latter a different presentation of XYWZ. You
could extend this scheme to Fu-Malham B format up to 3rd order,
introducing the corresponding "UHJ" versions. If you use 3rd order
horiz. or 3h1p variants which are backward-compatible to stereo, you
don't have more channels than for Dolby/DTS 7.1 variants, and the
bitrates won't have to be higher. You could easily have all this at
640kbps, if we talk about 7-8 channels. Note that this is "just" some
framework, not necessarily the 1st step. Don't kill the messenger - i.e.
me! Philosophically and mathematically speaking these things have always
existed somewhere... Ok, this was maybe just the Platonic view. :-) )
Gregory Maxwell and colleagues:
1. Does the Opus format (sic!) allow 2 audio channels and (in some
form) "data" extensions? (Audio data, I might add.)
2. What will be the container format for Opus? (I heard: Ogg)
3. What is the situation for FLAC? File format? Container format?
(It is always best to ask typical questions like these to the format
developpers themselves. I didn't ask for the "2.5 channel case", to
simplify the following discussions, if they should happen.)
Thanks
Stefan
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