On Fri, Mar 2, 2012 at 4:40 PM, Jörn Nettingsmeier
<netti...@stackingdwarves.net> wrote:
> On 03/02/2012 08:02 PM, j.l.ander...@phonecoop.coop wrote:
>> A few naive HOA Qs I'm hoping to gain some insight on...
>> 1) Order (M) is related to minimum number of loudspeakers by:
>>
>> (M + 1)^2 for 3D
>> 2M + 1 for 2D
These are not correct. To get the HF localization (rE) aligned with
the LF localization (rV), you need at least four speakers for 2-D and
six for 3-D, for first-order playback. For an illustration of the
1st-order, 3-D case, see
http://www.ai.sri.com/ajh/ambisonics/tetra-v-cube.html
http://www.ai.sri.com/ajh/ambisonics/tetra-rVrE/tetra.pdf
[...]
> if you have to play back lower order material as well (such as a native
> soundfield recording), i found it is advisable to have a separate low-order
> decoder which uses fewer speakers, for better clarity and less phasing.
> aaron heller disputes this, he claims to have observed no detrimental
> effects in vastly over-specified systems, and if you look at simulations
> where N->oo, he should be right, but in practice i have found systems with
> many more speakers than strictly necessary to be significantly worse in
> terms of phasiness... maybe others can comment and clarify.
I would not say that I've heard no detrimental effects on large arrays
-- I've heard all sorts of problems, but in every case I've been able
to attribute it to other reasons: bad decoder, errors in speaker
positions, feeding 1st-order signals into a HOA decoder, and so forth.
For example, decoders where the HF/LF balance is set using the
"conservation of total energy" approach from Daniels' thesis,
emphasize HF more and more as the number speakers goes up. I find
this causes the tonal balance on large arrays to be wrong, as well as
creating near-head artifacts (indicative that the sound at left and
right ears is very different). Reducing the HF/LF balance by 2 to 4
dB fixes this. I use the Stravinsky and Beethoven recordings I posted
to Ambisonia to judge this, as I am quite familiar how they sound when
reproduced correctly.
In reports in the literature, I find that either an incorrect decoder
has been used or there is no succinct statement about the decoding
criteria employed, e.g. Solvang's 2008 JAES paper. (v 58 #4, p. 267).
Same with anecdotal reports.
Aaron (hel...@ai.sri.com)
Menlo Park, CA US
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