[LAD] ambisonics UHJ encoder

fons at kokkinizita.net fons at kokkinizita.net
Wed Feb 24 14:29:46 UTC 2010

On Wed, Feb 24, 2010 at 05:01:20PM +0300, alex stone wrote:
> Jorn, Fons,
> I'm getting deeper into a setup now, and trialling a few different configs.
> A further question.
> Given that an amb sphere is equal on all "sides", and the sweet spot
> is the centre (roughly speaking), should i be configuring my dry sound
> orchestral instruments/sections to use the upper-rear/rear quadrant of
> the sphere, assuming the centre is the listener, in a total space? In
> a real world setup, my orchestra is on the stage, and the listener may
> well be (at my choice) sitting in a spot equidistant from the
> back/front/left/right of the total space. Should i assume this as a
> point of reference to begin with?

I get a bit confused. Usually 'the stage' is considered to be
'front', the positive X direction. You can decide otherwise
when mixing Ambisonics, but in a normal concert hall the
orchestra would be in the front direction. Upper-rear would
be a balcony area, or just some walls. 

> My "speakers" will be not only on stage, but as ambient
> sound/reflections in the rest of the space as well. The
> space isn't exactly spherical.

Don't think in terms of a 'spherical space'. Ambisonics as
we use it here does not consider distance. Every point on
the sphere just corresponds to a direction as seen from
the center. It's just a mathematical abstraction, the set of
directions in 3-D space is equivalent to the set of points
on a sphere.

Distance and the shape of the space are encoded in the
time patterns of direct sound and early reflections.

Concerning the virtual stereo microphone you asked about
earlier maybe this can be explained as follows. 
An Ambisionic (B-format) signal, no matter how it was
generated, can be considered as the output of a B-format
microphone placed at a specific place in some sound field. 
You can use the B-format signals to compute the output
that any normal mic would produce when placed in the same
spot. Using two of such 'virtual microphones' pointing in
different directions produces a stereo signal. It is the
classic way to obtain a stereo version of an Ambisonic
recording or mix. One of the plugins in the AMB set will
do this. 



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