What I've always done is to keep the channel sets of different ambisonic order separate, then design a decoder for each channel set, and mix the speaker feeds. Perhaps that's overkill.
On Sun, Jun 11, 2023 at 4:25 PM Sampo Syreeni <de...@iki.fi> wrote: > On 2023-06-01, Jan Jacob Hofmann wrote: > > > is it possible/ reasonable to mix ambisonic encoded information of > > different order? > > It's possible and it's reasonable, and as Fons Adriansen said above, at > the rather high orders you're talking about, it's not much below > optimality either. This has also been talked about in the past, with the > — granted, a bit of a shocking — revelation to me and some others, that > actually orders mixed this way do *not* automatically decode optimally > in either decoder. > > But theoretically, this ought to be purely a decoding side issue. When > you're mixing into or in B-format, you're essentially dealing with an > isotropic approximation of a soundfield, around a central point. That > approximation is always a physical one, and in ambisonic work, it's > going to be orthogonal by the basic math. If you want to add extra > directional accuracy, you'll add orders to your directional > decomposition. If you can't or won't, then you don't. But in the end, > the fact that the (3D) Fourier-Bessel series rightly normalized (too) > preserves the power of point sources, and is an isotropic decomposition > of an inbound far field, guarantees that the *only* thing you lose in > lower order is directional accuracy. When going to B-format, the one > meant to capture the physics, mixing two orders cannot lose anything. > > So the real trouble comes when decoding B-format into D-format. If you > have a set of first order, POA signals, you have one particular, optimal > equation set for how you'd lay the sound out over your speakers. If you > had a second order HOA signal, running into something like 5.1, the > optimal set differs quite a lot, especially in the higher frequencies, > since the theory doesn't work by easy interference principles there, but > by second order psychoacoustical ones, coming from the stereo work of > Makita. Solving the problem optimally becomes rather finicky. > > Then, solving it for mixed orders (not usually a term used for this > situation, but for leaving out certain spherical harmonics, e.g. for > horizontal, pantophonic work), is even messier. How could we know in > decoding only, blindly, that we have a superimposition of say first and > second (arbitrary?) order signals, so that we could apply the optimum > decoding rule to them all, at the same time? > > I've been toying around with this problem for a decade or so, and > haven't found a satisfactory solution to it all. My intuition says > this has something to do with non-negative matrix factorization and > convex optimization, but even if that's it, I'm not quite there yet. > > From Dolby Surround and HARPEX -like things I've been toying around with > doing them in the pure spherical harmonical domain to arbitrary order; a > generizable infinite order decoder; in DirAC kind of stuff I've been > toying around with just tensoring the STFT/MDCT-domain with the > directional Fourier domain, complexly; and then some classical LTI DSP > statistical learning and information/compression/rate-distortion theory > on top. In an effort to solve the problem of how to make full spatial > audio pack well. > > And then there was NFC-HOA. I was already making some progress, but that > totally stopped me. In that one, you an mix several orders of signals, > but suddenly you can't mix ones of separate radii. Fuck, back to the > drawing board for me as well. :/ > > > The sound-information (synthesized) is encoded in Ambisonic 7th order > > while the spatial reverberation of that very sound is encoded „only“ > > to third order. > > In fact Fons asked you already: why go to such a high order? You'd need > an extraordinary number of speakers to utilize such a signal. Also, an > extraordinary computing power and a lot of real life meaasurement of > your speaaker rig to even align your decoding solution optimally. > Whereas in low, matched order, you can do it right with a day's > computation time. > > > Reason for doing so: My reverberant information comes from several > > directions in space. If these would not have to be encoded all up to > > 7th order, it would save some calculation time and computation effort. > > They really don't have to. Take a look at Ville Pulkki's DirAC work, > here in Finland. The gist of it is that it reconstructs both specular > sources and reverberation, separately. The first part is identified via > time coherence, averaging, much like Dolby Surround does it in its four > constrainted channels, and like HARPEX does it better in the ambisonic > work. > > Ville's work however is fully general and frequency dependent in its > source recognition. And it goes beyond: it actually tries to identify > reverberant modes from a SoundField, by using the imaginary axis of the > Fourier transformation in time to recognize reverberant modes. Which has > also been discussed years before on-list, when Angelo (I think) talked > about his car interiors. > > > Also the reverberant information may well be more „blurry“ in respect > > to the actual sound, as it may stay in the background of perception > > anyway. > > So in reverberation, why not try out a SoundField, for a measurement? > The original Ambisonic mic? Because it's actually calibrated to measure > not only the pointwise pressure, as its W, but also velocity in XYZ. > The latter are where you get the reverberant, echoing, reactive field > measurements from. > > > But my emphasis is on the question, if a decode of 3rd *and* 7th order > > information - yielding in one encoded file - would be mathematically > > correct if it comes to the decoding of the higher order content. Would > > there be missing something (maybe an overall lower amplitude of the > > third order content)? > > As said, it will not be. As your order goes higher, in the higher order > decoder, you'll get better and better decodings at the higher > frequencies. Just as Fons Adriansen said, above. Doing it right, you > will necessarily start to approach the far field diffraction limit of > your array, both low and high. > > However, at the same time, your decode for the lower order will not be > psychoacoustically optimal, and won't approach it by these principles. > If you mix in lower order content, it won't decode optimally without > severe extra work. At something like 3-7 differentiation, you probably > won't hear the difference, but if you mix together even first and third > order, you definitely will; an optimal third order decoder does not work > well with a first order superimposed signal to the degree a specialised > first order (esp. four (panto) or six (peri)) rig would do. > > The higher order stuff will though mix in when done right. It will > spread ought to a lower order rig, even if the solution is rather > difficult to find. E.g. on-list we've talked about many numerical > solutions to the problems, like Wiggins's Tabu search. But if you try to > apply the higher order optimization problem to the lower orders, it > doesn't pan out. > > My long term problem is how to at least partially, blindly, tell > arbitrary order decompositions/additions apart from each other, at > least in part. I'm not there, even yet. :/ > -- > Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front > +358-40-3751464 <http://decoy.iki.fi/front+358-40-3751464>, 025E D175 > ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2 > _______________________________________________ > Sursound mailing list > Sursound@music.vt.edu > https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, > edit account or options, view archives and so on. > -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20230612/8e3591a7/attachment.htm> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit account or options, view archives and so on.
