[Vorbis-dev] Ambisonics in Ogg Vorbis

[Martin Leese]

"Gregory Maxwell" <gmaxwell at gmail.com> wrote:

> On 4/27/07, xiphmont at xiph.org <xiphmont at xiph.org> wrote:
> > (eg, the FAQ doesn't answer a few simple questions like 'should I be
> > using monopole speakers, or are bipoles better like in other
> > systems?')
>
> Yech. Other than a few (possibly crazy?) people, no one recommends
> dipole speaker for ambisonic playback... they don't work, at least
> mathematically.  There has been some argument based on a few people's
> experience that in some rooms they might work okay, but it's not
> entirely clear why.
>
> Small, full range, flat monitors, are probably best. Small, because
> big is the enemy of being able to place them correctly.   Consistent
> phase response is important. (i.e. reversing phase on a driver breaks
> it completely). Because the goal is to reconstruct the soundfield,
> consistent and predictable speaker behavior is useful.

Gregory's advice is sound.  The work that
suggested using dipole speakers used all
dipoles, front as well as back.  One pole of
each speaker faced the centre of the room and
the other the wall, on which was stuff to absorb
or disperse the reflections.  This is entirely
different from the use of dipoles in conventional
surround sound.

So, in brief, use monopoles as Gregory
suggests.  The key is that all speakers
cooperate to localise a sound, so all speakers
are equally important.  They must be phased
matched; the easiest way of doing this is to use
identical units.

As Gregory suggests, take care with wiring; if
one speaker phase is reversed then all is lost.
For the same reason, if you use different
power-amps, make sure all are non-inverting
(or all inverting).  Placement in the room can
also matter.  Left-right symmetry of the room is
more important than front-back.

When it works you will know.  The soundfield
will "gel" and your ears will relax.

Regards,
Martin
-- 
Martin J Leese
E-mail: martin.leese at stanfordalumni.org
Web: http://members.tripod.com/martin_leese/