Sampo Syreeni wrote:

On 2017-01-09, Stefan Schreiber wrote:

The critique I'd have for such panning laws is that they don't really respect the ambisonic/Gerzon theory, especially at the low frequencies.


Stereophonic panning laws are based on Blumlein's stereo theory, which in Wittek's opinion is pretty close to sound fields anyway.


Correct, but only in the high order, dense limit. In the low order, sparse array case, which especially four speaker POA deals with, you can do better. That's why POA decoders don't go in-phase but max energy even at HF, and especially why we have shelf filters which cut the decode down to velocity coherence at the low end.

But Blumlein developped his "physical cue" stereo just for 2 speakers.  ;-)
Forget about any "high order" re-interpretations. It's all about ITD, ILD and say summing location.

It is pretty obvious that Ambisonics theory is influenced by Blumlein stereo...

Now, 3-channel stereo (+ extensions) seems to be some early version of WFS...

http://www.hauptmikrofon.de/HW/Wittek_thesis_201207.pdf

p. 28:

In America Steinberg, Snow and Fletcher (Steinberg and Snow, 1934) from Bell Laboratories
explored the 'acoustic curtain', see Figure 3-3.

Snow described their ideas in
this way: "The myriad loudspeakers of the screen, acting as point sources of sound identical with the sound heard by the microphones, would project a true copy of the original sound into the listening area. The observer would then employ ordinary binaural listening, and his ears would be stimulated by sounds identical to those he would have heard coming from the original
sound source." (Snow, 1953)

(= WFS!)

These scientists quickly noticed that, due to technical constraints, it would not be feasible to put their ideas into practice. As a compromise, they limited the practical system to three channels, accepting that the original aim of recreating the real sound field would no longer be ful

filled. The three-channel stereophony produced in this way was therefore not created as a result of a mathematical analysis of the sound field, but rather as an engineering compromise. Its directional effect is based on perceptual phenomena such as the precedence effect and
level and time difference stereophony.
In contrast, Blumlein (1933) aimed at a proportional reproduction of the directional image of
the recorded scene by recreating the original physical auditory cues.

Snow (1953) pointed out, regarding the basic difference between the n-channel acoustic curtain and 3-channel stereophony: "This arrangement [3-channel stereophony, see Figure 3-3] does indeed give good auditory perspective, but what has not been generally appreciated is that conditions are now so different from the impractical <infinite screen> setup that a different
hearing mechanism is used by the brain."
Researchers who observe contradictions in the generally accepted summing localisation theory
quote this statement by Snow.


Let's say the creators of stereo (including binaural stereo, developped even earlier in the 19th century) thought all in terms of physical acoustics and psychoacoustics...


Obviously all of those decoding principles converge to holophony in the high order, dense array limit, so that Wittek is correct in that case.

Well, look to the title of Wittek's dissertation...

Switching guru mode on:
< Every reasonable stereophonic system will become some copy of real-world acoustics >, as long as you can use infinite ressources to prove your point! (Do you agree?!)

Example:
HOA DirAC...       :-)

However I'd argue that the whole point of POA is to optimally deal with the low order, sparse array case, where each of the decoding principles are pretty far from convergence, and in very different ways -- intensity panning pretty much corresponding to an in-phase decode, which we already know is *not* always optimal.

1st order Ambisonics is "just" the 2nd most simple implementation of a sound field. Should we not start interpretation in this way, not with some "sparse array" we should use somehow?? The "sparse" decoder array is an implementation issue. (Order 0 was already obsolete when Ambisonics was developped. But quadraphonic systems already existed. "Some believe that Ambisonics is a kind of improved quadraphonic system"... 8-) )

Anyway: I can't quite follow! You do an in-phase decode if you want to enlarge some (usable) listening area. It is not some optimal decoding strategy for POA, but so what?

You could also say that your argument is some "proof" that intensity panning/VBAP has some bigger sweet spot than POA. (Maybe true, but this is not really the motivation behind some in-phase area decoder.)

I would not treat stereophonic systems as some "deformed" implementation of Ambisonics. They are not...

Secondly, "theoretically perfect" systems are mostly "not really" perfect in reality:

http://www.linkwitzlab.com/Recording/record-play-map.htm

The array has equal sensitivity front and back but low pickup from the sides. I doubt that the Blumlein array is optimal for mapping an auditory scene for stereo loudspeaker playback.

So, does the Blumlein microphone pick up too much reverberation or not? And do we have some side problems?

This might all depend on the situation.


Best,

Stefan
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