[vorbis-dev] AMBISONIC critique

[Thomas Marshall Eubanks]

Gregory Maxwell wrote:

> On Tue, 15 Aug 2000, Thomas Marshall Eubanks wrote:
>
> [snip] - lots of them
>
> > at 200 Hz < f < 10 kHz, you need W, X and Y to obtain the same functionality
> > which you get from right and left stereo. The location of the W speaker is
> > problematic at these frequencies (where you place it DOES count). The phase
> > matters a lot here, as here is where we use phase to localize.
> >
> > at f > 10 kHz, it is not clear how you are to implement "joint stereo" and save bits
> > accounting for the ear's intensity interferometry. The location of the W speaker
> > is still problematic, but not as badly.
>
> W speaker?!? Okay. I understand your problem with ambisonics. *Full STOP*
>
> You wouldn't have a 'w speaker' in a proper ambisonic system. Ambisonic's
> main advantage is that is describes a speaker independant sound field!
>
> I.e. the same W,X,Y,Z B format ambisonics can be decoded to a set of 4
> speakers, 8 speakers, or a set of headphones and still perserve most of
> the intended 'sound scape'.
>

BUT, that's my basic problem here. The ambisonics developers chose a
mathematical representation which does NOT map well into what speakers
actually do (or microphones, either) IN GENERAL. I have the strong feeling
that they did this because it happened that the dipole terms were similar
to the response of certain actual microphones that they happened to use. BUT,
to reproduce this sound, you have to do a fair amount of processing even
to get stereo. When I was at MIT, this would have been called a "kludge."

I have a question :

   What is the intended audience ? People who will simply placed speakers in a room
(or next to their computers), move them around a little until it "sounds good", and
never touch them for the next 5 years,
or people who will fiddle with "decoding into a set of 4 speakers, 8 speakers"
until they match the intended performance in a certain region, maybe for each
separate recording.

Please don't make Vorbis into something only a audiophile could  love !

In the best open source tradition, I would say that what we are trying to do
here is develop towards an end to end system, so that artists could be given
a FAQ file and software on how to MAKE multi-dimensional recordings,
and then the sound system would try its best to REPRODUCE those mutli-
dimensional recordings, all using openly available software.
This could be a very good thing, and I applaud it and
intend to do what little I can to support it. I just worry that the chosen multi-
dimensional platform may
cause unnecessary difficulties.

> I think you need to actually read the ambisonics paper before commenting
> on the system. Keep in mind that alot of the older ambisonics stuff was
> pre-digital processing, so their speaker placement was limited by what
> they could achieve by simple analog dematrixing. Powerful microprocessing
> now allows us to use virtually any speaker placment scheme that provides
> sufficent 'fill'.
>

Actually, I have read all of the papers that I could find. If you would like to
provide a proper bibliography, I'll read those too :)


--

                                   Regards
                                   Marshall Eubanks

   T.M. Eubanks
   Multicast Technologies, Inc
   10301 Democracy Lane, Suite 410
   Fairfax, Virginia 22030
   Phone : 703-293-9624
   Fax     : 703-293-9609

   e-mail : tme at on-the-i.com     tme at multicasttech.com

 http://www.on-the-i.com         http://www.buzzwaves.com

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