On 2011-07-21, Dave Hunt wrote:
There is certainly no consideration of values outside the unit sphere. [...]
Correct, and we've been here before. As BLaH points out, even the first order decoder handles distance as well as it possibly can. So does the SoundField mic on the encoding side. But the classical synthetic encoding equation is for infinitely far away sources only, that is, plane waves. Running the result through a proper, BLaH compliant decoder then reconstructs a simulacrum of such a plane wave, with first order directional blurring, spatial aliasing caused by the discrete rig, and the purposely imposed psychoacoustic optimizations overlaid on top of the original, extended soundfield. So in fact it's wrong to say that the source is produced at the distance of the rig: instead it's produced infinitely far away, modulo the above three complications. (That is bound to be one part of why even synthetically panned sources localise so nicely even when listening from outside the rig.)
If you want to synthetically encode a near-field source so to speak "by the book", you'll have to lift the source term from Daniel, Nicol and Moreau's NFC work. I seem to remember it amounts to a first order filter on the first order part of the source signal in the continuous domain, which you'll then have to discretize. (But don't take my word for it, it's been a while since I went through DN&R.) That not only affects the relative amplitude of the zeroth and first order components, but also leads to a (potentially nasty) bass boost (because of the proximity effect; this is why NFC-HOA went from an infinite encoding/transmission radius to a fixed, finite one), and a definite phase difference between the orders (because near fields of even freely radiating sources are reactive due to spatial curvature of the pressure wavefront). Simply manipulating the relative amplitude or even the spectral contour doesn't in theory cut it, though it's a cheap way to get some of the psychoacoustic effects of a nearby source.
As for the rig term in the NFC work, that's already there in POA's distance compensation circuitry, and introduces similar (but opposing) effects, based on the finite distance of the rig. (BTW, in Gerzon's opinion it was the phase shift which mattered the most here, so by extension it's likely highly significant for the encoding part as well.) That is, a correctly setup (i.e. BLaH compliant) classical Ambisonics system is precisely equivalent to a first order NFC-HOA one, and absent the three complications I mentioned, is 100% a holophonic reproduction system, just isotropically spatially blurred to the to the degree dictated by the first order basis functions (when thought of as spherical convolution kernels). The only minor nit is that synthetic panning needs a bit more refinement for near sources that wasn't being handled by the older literature.
-- Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front +358-50-5756111, 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
