On Sun, Jun 02, 2013 at 02:55:28AM +0300, Sampo Syreeni wrote: > But if you then record even the ideal infinitely distant point source > in the horizontal plane using the same setup, and try to reproduce it > using a pantophonic/2D rig, what you get is the same problem WFS gets. > The directionality is right, but the distance-wise attenuation is > wrong by a constant factor in distance, because energy is escaping > from the horizontal plane into the Z-direction as well. But if you > don't have such speakers, then even the basic horizontal point source > attenuates wrong over distance.
What you get in high order 2D AMB (using numbers of speakers similar to typical WFS systems) is that 'plane waves' do indeed attenuate with distance over the (limited) area they are reproduced. It remains to be seen if that is a problem in practice. > This doesn't much matter if you're aiming at recreation at a single > point. Then you could compensate for the effect much like the WFS > folks do. All it takes is a bank of simple filters. You're mixing up two things here. The 3dB/oct filters used in 2D WFS do not correct the attenuation of a plane wave in function of distance, nor are they meant to. They are required to get the frequency response right. It's easy to make this mistake as both the incorrect attenuation in function of distance, and the frequency response error without those filters have the same origin: when deriving the 2D WFS driving functions starting from the Helmholtz integral, at one point in the process vertical line sources are replaced by point sources. HOA systems do not require such filters, not even when pushed to the number of speakers typically used for WFS. Just this shows that the two system operate in fundamentally different ways, even in the limit. You can't simply assume that the same rules apply, or the same effects occur. > and if I'm not mistaken, jointly optimizing the distance compensation > filters with a BHJ decoder already does the job. In fact, maybe it's > already in those kinds of equations from the beginning? Can anybody > see whether it is? As long as 'energy escapes in the Z direction' there is no way avoid attenuation of what is meant to be a plane wave. And more, BHJ is first order, and no first order system will ever generate a credible plane wave over any significant area and frequency range (just try to do that with only 4 or even 8 speakers), so all this is probably rather irrelevant. As regards Faller, claiming that AMB 'doesn't work because in its 2D form it can't reproduce a real plane wave' seems like smashing an open door. No 2D system will ever do that. And even 'purist' stereo recording techniques do rather arbitrary things for sounds originating outside the horizontal plane, nobody ever claimed they don't work because of that. Ciao, -- FA A world of exhaustive, reliable metadata would be an utopia. It's also a pipe-dream, founded on self-delusion, nerd hubris and hysterically inflated market opportunities. (Cory Doctorow) _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound
