[LAU] placement.

[Fons Adriaensen]

On Tue, Jan 20, 2009 at 09:32:35PM +0300, alex stone wrote:

> Fons, you've described my crude experiment with this. Left, Centre, Right,
> and a complete IR for 'global' as the tail. I'll have to experiment some
> more, and possibly truncate the 3 short irs further, but i'm getting the
> idea in relation to a more realistic representation, however crude it may
> be.
> 
> A question here gentlemen.
> 
> With an WXYZ setup, what do those represent? Left/Right/Front/Rear?
> So any signal in would be a composite of position based on the strength of
> gain between 2 or more points?

No, quite different.

Suppose you have four mics, one omni and three
figure-of-eights, all having the same on-axis
sensitivity. Somehow you manage to get all four
of them at exactly the same point in space.

Then

W is the output of the omni mic, attenuated by 3 dB.
X is the output of a fig-8 mic pointing forward.
Y is the output of a fig-8 mic pointing left.
Z is the output of a fig-8 mic pointing up.

This set of four signals is called (first order)
B-format in Ambisonics lingo. It can be decoded
to any surround speaker setup, but is itself
independent of any particular speaker layout.

You can also combine the signals to form 'virtual'
mics pointing in any direction, and having any
first order directivity (from omni over cardioid
to fig-of-eight), for stereo playback.

In practice a different mic configuration is used
to capture the B-format IRs (or the music), usually
a tetrahedral set of 4 near cardioids, but these
signals are then processed to produce WXYZ as
defined above.

Ciao,

-- 
FA

Laboratorio di Acustica ed Elettroacustica
Parma, Italia

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