To clarify. There is no "upmixing" in Ambisonics (at least none that
does not involve more sophisticated beamforming and therefore changes
the spatial properties of the captured sound field).
In general, the number of sensors of the array determines the maximum
Spherical Harmonic order ("B-format") that can be extracted from the raw
microphone signals ("A-format"). That is (order+1)^2=mics for the case
that the sensors are arranged according to a Fliege-Meier grid, which is
the most efficient arrangement.
Higher Ambisonics orders could be extracted from the captured signals.
However, these "B-Format" signals will contain an increasing amount of
spatial aliasing (boost towards higher frequencies). Such a higher-order
decomposition will have seemingly higher spatial resolution, however the
additional information is composed purely out of artifacts (the spatial
aliasing) that have nothing to do with the original sound field.
Therefore, extracting or rendering at a higher order than the microphone
array allows is mathematically possible but conceptually not beneficial
and possibly perceptually unfavorable.
In that way, the OctaMic seems to provide a reasonable solution for
second order nevertheless. To be pedantic, the frequency range in which
some apparent microphone signals can be __accurately__ transformed into
the desired Ambisoncs (SH) representation depends on many aspects. There
not only the physical arrangement, but also the employed processing
plays an important role. The decomposition is no hidden secret but still
can be performed with different care or aspects in mind (especially in
terms of the employed modal radial filters which compensate for the
scattering of the microphone array). Therefore it could be interesting
to also experiment with different software solutions there in order to
yield different results.
Nevertheless, there is one aspect to distinguish when looking solely at
the reproduction part of an Ambisonics pipeline. A first order signal
for example can be reproduced without drawbacks (and actually without
any additional processing) at second order. However, one should not term
this "upmixing", since the higher order components ("B-Format") of the
Ambisoncs signals will simply be left at zero (since this information is
not available).
Such decoded reproduction signals could nevertheless utilize the higher
dentistry of for example a second order loudspeaker grid. If this
actually yields any technical or perceptual benefits I don't know, since
I am not so familiar with loudspeaker reproduction.
Regarding binaural rendering "directly in the SH domain", there is no
reason or benefit to render at a higher order than the source material.
For the case that the binaural renderer follows the "virtual
loudspeaker" approach, the only limiting factor of the reproduction
resolution (following the resolution of the source material) would be
the utilized HRTF data set.
In any case, "upmixing" or decomposition of the microphone signals at a
higher order than the array architecture supports is not reasonable.
I also noticed that people are arguing over different ways on how to
utilize the microphone signals that you have available. To be frank,
lower order signals, especially first and even second order, are not
great in preserving the spatial properties of the captured sound field.
That is true in terms of instrumental accuracy due to physical and
mathematical limitations, but also perceptual fidelity.
The limitations have different implications for either reproduction
technique, e.g. spatial resolution, preservation of timbre, source
externalization, etc..
However, low order signals is what we mostly (or only begin with to)
have available at the moment. But since there is no universally "right"
way to treat them, the only option is to make things sound as "nice" as
possible at the end of the day. :)
I hope this was useful to some to understand the decomposition of the
microphone signals and the connected implications a bit better.
Hannes Helmholz
(PhD Student, somewhere)
Can make mistakes. Also just human. Please forgive.
On 2020-10-23 15:10, Stefan Schreiber wrote:
I think COMPASS should upsample from 2nd order to 3rd order (and higher).
However, the Octomic is not giving full 2nd order. (2h1v)
You still could set the missing element (FuMa R element, ACN 8) to 0 and
do this.
Has anybody tried this?
Best,
Stefan
----- Mensagem de Steven Boardman <boardroomout...@gmail.com> ---------
Data: Fri, 23 Oct 2020 14:04:21 +0100
De: Steven Boardman <boardroomout...@gmail.com>
Assunto: Re: [Sursound] ORTF-3D With Higher-order Ambisonics
Para: Surround Sound discussion group <sursound@music.vt.edu>
Hi Len
I know Harpex is only first order, and a calibrated 2nd order mic will
produce better first order patterns. I was referring to just Tetramic
when I mentioned 1st order...
My point about Harpex was; you tried to do ORTF with only one Octomic,
and it wasn’t good.
Did you try to do 3D ORTF with 2 Octomic, experimenting with
positions, and which position each Harpexed Octomic synthesises?
Maybe Octomics on an opposing corner with Harpex from each decoding
alternately into the other corners?
The two Octomics could each have the 2 straightforward decodes (to
obtain 4 vertexes). But each of the other corners have 1 spaced decode
from each Octomic.
So the other vertexes are more deco-related from using alternate
Octomic spaced Harpex decodes.
Hope that makes sense, or shoot me down with bag of crap…:)
PS don’t suppose you have a super-cardioid response of the Octamic?
The CCM41 capsules used in the 3D ORTF are excellent, with pretty dam
good polar response upto 16k. (Obviously they wouldn’t be anyway near
the same in an array being so close to each other:)
https://schoeps.de/en/products/ccm/ccm-microphones/supercardioids/ccm-41.html
On 23 Oct 2020, at 03:16, moskowitz <lenmoskow...@optonline.net> wrote:
Steven Boardman wrote:
Have you tried it with first order?
I would of thought using one Harpexed octomic to synthesise all the
spaced
mics, would be a bit blurry, did you try with two?
Harpex only operates at first-order.
We used OctoMic's first-order B-format as input to Harpex.
We've used two OctoMics for ORTF. Since OctoMic's cardioid patterns
are so stable and consistent, and because pointing angles can be
fine-tuned in post, two OctoMics make probably the world's finest
ORTF array.
Len Moskowitz (mosko...@panix.com)
Core Sound LLC
www.core-sound.com
Home of OctoMic and TetraMic
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