How come whenever I go away for a few days (just come back from a trip to
China) something really interesting comes up on sursound? :-)
A couple of very limited comments on this.
1) This has been covered, particularly in some of Fons' remarks, but when I
read Sampo's comments "How Christof saw it was much from the WTF point of
view. There, if you reproduce a point source in the horizontal plane only
using a horizontal array of speakers, you will get the angle of arrival
right, but the normal attenuation suddenly acquires an extra 3dB/per
normalized distance factor. In WFS they purposely compensate for that with
their linear and rectangular arrays. But very few analyses really go into
where that factor comes from, or how it could be avoided, or what it's
really about. The pantophonic analysis of ambisonic doesn't go there
either, even if it really, *really* should." I immediately thought why? WFS
needs to because it is trying to minimise errors over a large area but
Ambisonics (at least POA) is trying to get it right at a point and so,
apart from getting the volume correct and correcting for curved wavefronts
if they are too close, Ambisonic decodes just don't care how far the
speakers are away from the listener, so long as they are acoustically
equidistant.
2) I actually don't understand some of what has been said about wanting
somehow to not reproduce off-horizontal sounds on a pantophonic array. Why
on earth (other than for artistic reasons) would you not want to? Okay, the
sounds become increasingly unfocused as they go off horizontal but they
are at least there. If they weren't it would sound wrong - or rather ,
_more_ wrong. Not just from the point of view of, say, missing reflections,
but also in terms of the timbre of sources that aren't exactly point
sources (or horilzontal line sources) that are not entirely on the horizon,
since some components of the sound will be attenuated by the divergence of
the source from the horizontal.
...or, have I missed something (if so, I'm going to blame jet lag)
Dave
On 5 June 2013 13:13, Fons Adriaensen <[email protected]> wrote:
> On Wed, Jun 05, 2013 at 12:00:28PM +0100, Richard Furse wrote:
> > > -----Original Message-----
> > > From: [email protected] [mailto:sursound-
> > > [email protected]] On Behalf Of Fons Adriaensen
> > > Sent: 04 June 2013 10:54
> > > To: [email protected]
> > > Subject: Re: [Sursound] Nevaton microphones
> > [...]
> > > Imagine a circular array. For a single distant source, AMB
> > > will use only two or three speakers, those closest to the
> > > direction of the source (the contribution of the others will
> > > be *very* small at high order). WFS will use half of the
> > > circle, with a cos(angle_to_source) distrubution. Now show
> > > me how those two converge to the same... They simply don't.
> > [...]
> >
> > This is true if the AMB decoder is making the classic assumption that the
> > speakers are a long way away (i.e. essentially producing plane waves at
> the
> > listener) and for energy and in-phase decodes.
> >
> > For clarity though, it's not true for decoders that do the Hankel/Bessel
> > dance to model speaker distance, e.g. NFC on symmetric rigs, or
> Rapture3D's
> > soundfield reconstruction on general rigs where the speaker distances are
> > provided. In these cases, typically you should see a "bloom" effect where
> > the sound appears in speakers in the virtual source direction first and
> then
> > more quietly in the speakers around them etc. Using only the speakers
> > closest to the source direction results in a curved wavefront at the
> > listener, which obviously isn't right when simulating a distant source.
> > Without detailed study, the shape of the bloom doesn't seem entirely
> unlike
> > the WFS distribution...
>
> You're absolutely right about that, and it's even possible to say more.
>
> Consider again the circular array, and assume the number of speakers N
> corresponds to the order of the AMB system (i.e. N = 2 * order + 1, or
> at most a few more). If NFC is used (and you really *have* to use it for
> the high orders we're considering), then for low frequencies (as long as
> the reconstruction radius is larger than the array radius) the two systems
> will do the same, and the AMB speaker signals will be almost identical to
> the WFS ones. As frequency goes up, at some point the reconstruction area
> of the AMB system will become smaller than the rig, and you get the typical
> interference patterns near the speakers while the center stays OK.
>
> This happens exactly at the same frequency at which a WFS system using
> the same array will start aliasing, i.e. when the speakers are separated
> by half a wavelength.
>
> Above that frequency the two systems diverge. WFS will produce aliased
> images, and for AMB primary sources will start to be projected onto the
> circle of speakers. I wonder if some 'halfway' solution could be found
> in this frequency range, one that would combine but mitigate the defects
> of the two systems...
>
> 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)
>
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>
--
As of 1st October 2012, I have retired from the University, so this
disclaimer is redundant....
These are my own views and may or may not be shared by my employer
Dave Malham
Ex-Music Research Centre
Department of Music
The University of York
Heslington
York YO10 5DD
UK
'Ambisonics - Component Imaging for Audio'
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