On Sun, Jan 11, 2009 at 12:56:38PM +0300, alex stone wrote: > Fons, just a follow up to the discussion about an orchestral placement > template, is there a definitive resource somewhere i can go to, and learn > more about ambisonics, and how this might help me to achieve the Holy Grail > of true placement recording? The simple answer to this is 'no' :-( You can google Ambisonics of course, and you'll find some sites. Then you'll find out that information is fragmented, often contradictory, frequently outdated, and in some cases plain wrong. One of the reasons is that the technology is now well over 30 years old, at the time it was invented it was only possible to use it in its simplest forms which requires many compromises deviating from the theory, and some of these compromises have over the years established themselves as 'the thruth', confusing anyone who tries to understand things from a theoretical perspective. And of course at that time we didn't have the web. The last ten years or so have seen a renaissance of AMB research, and of course today many of the technical barriers have gone. Some people are now using up to 4th order systems routinely, and some are researching much higher orders. Today the AMB community seems to be divided in two camps, the old guards that were part of the original wave in the 70's, and a new generation of researchers that almost restarted from scratch, and is not bothered by the same legacy wisdom. Their publications (mostly PhD theses and AES papers) today provide the best information, but their approach is highly mathematical, something that can't be avoided except for the most simple cases. In between those two is part of the electro-acoustic music community which has been developing its own toolsets, most of which are incompatible with each other. I've been involved during the last half year in some 'AMB standards wars', and it has not been a very happy experience. All this should not stop you from learning about AMB, but you will find conflicting info and have to sort out the mess - you have been warned :-). First question of course is if you need it or not. If your end result is high quality surround or periphonic reproduc- tion you definitely do. If it is stereo you don't really need it, but even in that case an AMB based production chain can do some things that would be difficult to achieve in any other way. Ambisonics can be understood on many different levels, from 'intuitive' to purely mathematical. On the intuitive level, it's a way of representing the directional (not distance) distribution of sound in a 'canonical' way that is really independent of the way the sound it is captured or will be reproduced. In other words, AMB signals are not speaker feeds, but a something that can be 'decoded' into speaker signals. In its simplest form you could compare it to representing colour video not as RGB, but as (intensity, colour, saturation) which can be decoded to RGB, or to any other colour representation. On a first mathematical level, you can see this as a sort of 'spectral' representation. Any cyclic function can be Fourier transformed into a set of harmonic frequencies, each having its level and phase. In similar way, the horizontal distribution of sound directions is a cyclic function, not of time but of the horizontal angle (azimuth) of the sources, and you can apply the same Fourier transform to it, which is how horizontal AMB works. The 'order' of an AMB system refers to how many 'harmonics' are used. For a periphonic (3D) sound distribution the 'function on a circle' becomes a 'function on the sphere', depending on two variables, azimuth and elevation. Because a sphere is not the same as a 2D hyperplane the corresponding spectral transform is not the 2D FT, but is defined by the set of 'spherical harmonics'. And here of course the more difficult maths start... > I've read back through your comments about > routing to A and B to achieve 'local' and overall > placement, re convolution, but i will admit i don;t > understand all of it. I'll comment on this later. Ciao, -- FA Laboratorio di Acustica ed Elettroacustica Parma, Italia O tu, che porte, correndo si ? E guerra e morte ! _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
