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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