Robert, Lots to comment on here. I seem to be compelled to address your negative or "not so good" observations:
> Not so Good 2) Because one- point miking ignores transient time > of arrival differences as such , one of the basic cues of sonic perception > is suppressed explicitly That's not really true. I'm assuming that when you speak of time of arrival differences that you are referring to ITDs. The thing to remember here is that ITDs are a function of our presence in the acoustic field, and as such aren't present in the recording environment and thus shouldn't be recorded. In a recording and reproduction scenario the ITDs happen in the reproduction of the recording, and as it happens ITDs are reproduced very well by Ambisonics, even first order Ambisonics. I showed this quite clearly (I hope) in AES preprint 8242. > 3) Impractical number of speakers needed really to work But one of the really cool things about Ambisonics is that it scales extremely well so that it works well with one speaker or two, although not creating surround with so few speakers. And it works quite well with only four speakers. And nowadays there are quite good decoders that work well with ITU 5-channel arrays. If higher order sources are available then they can be decoded in such a way that the directional resolution is high in the forward direction where there are relatively many loudspeakers and not so well to the rear where there are relatively few loudspeakers. > 4) Impractical number of channels needed to really work Again, that's not really true. Most common audio carriers have the capability to carry many channels, DVD, BluRay. And many systems are file-based and as such aren't really limited at all. With a system that is inherently hierarchical, as Ambisonics is, a broadcast or distrubution system can transmit as many or as few channels as is wished. > 5) In practice, keeping noise low enough is difficult I'm not entirely sure where this comment comes from. In terms of natural recording, which is what you and I would do but not most of the rest of the audio world, the Soundfield microphone as embodied in the Soundfield MkIV and MkV microphones, is really quite quiet. Not as quiet as some modern microphones, some of which have self-noise in single digits, but somewhere in the mid-teens of dB SPL. I can't bring to mind any instance when listening to the recordings of my colleague Aaron Heller that I was ever aware of the presence of noise. And there's no reason why higher order systems can't be made very quiet indeed. Gary Elko mentioned, during the discussion of the MH acoustics Eigenmike, that the self noise of the zero order (omni) output is about 0 dBA. So what does my list look like? Good: 1) Isotropic behavior. Ambisonics is really good at capturing and reproducing ambient sounds. These are the sounds that inform me that the sound scene had some real origin. 2) Reproduction of correct timbre. While it is relatively easy (but not frequently done!) to capture sound with the correct spectrum, 2-channel stereo distorts that spectral accuracy in reproduction. Ambisonics is much better although it still suffers from some of the same problems. 3) Requires lots less speakers than Wave Field Synthesis. Not so good: 1) I frequently find that I have front/back confusion. Let the debate continue. ----- Original Message ---- From: Robert Greene <[email protected]> To: Surround Sound discussion group <[email protected]> Sent: Sun, April 1, 2012 8:03:44 PM Subject: Re: [Sursound] Can anyone help with my dissertation please? OK I thought that was a good idea, for people to say what they thought was good and not good about Ambisonics. So here I go(first I guess but my mother always said Act in haste, repent at leisure. I think she meant it as cautionary but I have always taken it as advisory!). Good 1 Elegant as mathematics 2 Forces people to use one point miking which in itself is already a HUGE thing because it eliminates the absurd manipulativeness of much of commercial recording practice. 3 In principle, has the capability of reconstructing the complete soundfield. 4 Puts height in the picture and gets rid of the sound through a horizontal slit of stereo(which is ironically more like that the better it is done!) 5 In practice, more robust than one might have expected at working over a large listening area (if that matters). 6 In principle, the timbre errors of stereo arising from around the head summation are eliminated. Not so good 1 Emphasis on homogeneity makes it inefficient when not high order. (Everyone knows that perception to the side of a listener is quite different from perception frontally, but this is ignored) 2 (related to 1) Because one- point miking ignores transient time of arrival differences as such , one of the basic cues of sonic perception is suppressed explicitly and is only returned to the picture with higher order. 3 Impractical number of speakers needed really to work(cf point 2). 4 Impractical number of channels needed to really work(because higher order is needed). 5 In practice, keeping noise low enough is difficult. 6 Nearly oomplete lack of demo material, which makes it all but impossible to interest the public. One somewhat incidental issue 7 Mathematics is too tricky for most people in audio to appreciate (I know this is so because I have tried to write about Ambisonics for the general audio public--no dice, people did not get it even though I thought what I wrote was clear as crystal) About point 2: the same issue arises in Blumlein stereo, which is why some people like ORTF better. Ideal would be Blumlein up to around 700 Hz and switch to ORTF above that, it seems, or something along those lines. (Blumlein the man had ideas on this, of course). I could go on, but perhaps that is enough to get the ball rolling. Good suggestion, I think, that people should make such lists. I am curious to see what others have to say. Robert _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound _______________________________________________ Sursound mailing list [email protected] https://mail.music.vt.edu/mailman/listinfo/sursound
