Re: [Sursound] time variance...
On 2013-05-20, Fons Adriaensen wrote: So it seems that a stronger definition of TI is not necesssary. At the same time there is a definite point to compressors being "kind of time-variant" and only weakly nonlinear. They certainly don't behave like a distortion pedal or anything like that. I guess one way to look at it would be to consider the difference between the whole system as a mathematical operator on the one hand, and the constituent parts of its implementation on the other. In the case of LTI systems the difference is easily neglected because of the strong circuit invariants, including commutativity. Numerical effects like noise accumulation, representable range and coefficient quantization are pretty much the only thing fundamentally separating the two, there, and they have little to do with time. But as soon as you go to time-variant and especially (memoryful) nonlinear systems, any invariants you might have to aid in the analysis are much weaker, they don't compose easily, and so you can't factor out the internal dynamics of the system the way we do with LTI circuits. Suddenly it does matter whether parts of the system can be locally approximated as, say, slowly time-variant linear systems, like all dynamics processing can. That sort of thing is especially important when there's nonlinear feedback involved, because then you'll pretty much always be relying on such properties to prove stability and convergence. That goes for Dolby A decoders and Pro Logic II type active steering alike, to mention just two recent topics. Or the other way around, it'll bite you even in the case of fully linear but time-variant circuits with feedback: it's a well known DSP nit that the stability of such filters even under well-behaved coefficient modulation cannot be straight-forwardly deduced from the steady state system function(s), but is intimately tied to the actual circuit topology implementing the filter. So, once you contrast the system and its implementation, suddenly it's no longer generally the case that (approximate) time-invariance of (some of) the parts implies the same of the whole, or the other way around, both properties are still very important for the analysis even if only to quantify how much they're lacking (cf. the analysis of modulation artifacts in compressors), and though the two concepts aren't fully comparable, for the most part applying approximate linearity and/or time-invariance to the exploded circuit constitutes a more fine grained, or stronger, approach. -- Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front +358-50-5756111, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2 ___ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
Re: [Sursound] [allowed] Re: Recreating a 3d soundfield with lots of mics.....
At 00:50 18-05-13, Robert Greene wrote: Of course in those live versus canned experiments(also with AR) reverberation tended to make things sound pretty much the same to smooth out errors and so on. Reverberation in the RFH pre 1966? David ___ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
Re: [Sursound] time variance...
On Sun, May 19, 2013 at 10:18:35AM +0200, Jörn Nettingsmeier wrote: > On 05/02/2013 01:26 AM, Richard Dobson wrote: > > > I have always understood it to mean that the behaviour is not dependent > > upon ~when~ the signal is injected. Thus, a plain delay is TI because > > everything is always delayed the same way; while a modulated effect such > > as a flanger (maybe using a variable delay) is not TI as exactly what > > comes out depends in the time something goes in. > > ... > > for practical purposes, i guess fons' definition is more useful, > because then the term "LTI" system is strictly limited to something > that can be fully described with an impulse response. Despite what I wrote before, I tend to agree with Richard. If I interpret his formulation correctly, a system is time-invariant iff, when the output for input x(t) is y(t), then the output for x(t + T) is y(t + T), for any T. It's actually quite difficult to formulate a stronger version *unless* you assume that the system is linear as well. A linear time-invariant (LTI) system is fully defined by an impulse response, or by a transfer function in the frequency domain. Now consider three cases: 1. A filter, 2. A tremolo effect, 3. A compressor. The filter is LTI, while the tremolo and compressor are not. Do they fail to be LTI because they are not linear, or because they are not time-invariant ? The tremolo fails Richard's TI criterion. But it *is* linear in a very strong sense: for any a(t) and b(t) Tremolo (a(t) + b(t)) == Tremolo (a(t)) + Tremolo (b(t)). The compressor is time-invariant according to Richard's criterion. But it isn't linear in the way the tremolo is. It could be said to be linear 'at any instant', assuming attack and release times are non-zero. But that is a somewhat problematic definition of linearity, since apart from trivial cases (pure gain) linear processes depend on the input's or output's history, and are not defined by some relation at a single instant. So it seems that a stronger definition of TI is not necesssary. Ciao, -- FA A world of exhaustive, reliable metadata would be an utopia. It's also a pipe-dream, founded on self-delusion, nerd hubris and hysterically inflated market opportunities. (Cory Doctorow) ___ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
Re: [Sursound] [allowed] Re: Recreating a 3d soundfield with lots of mics.....
Hi That's not the RFH - nor RAH! The stage & organ are wrong for both. Every Blessing Tony > -Original Message- > From: sursound-boun...@music.vt.edu [mailto:sursound-boun...@music.vt.edu] On > Behalf Of Aaron Heller > Sent: 19 May 2013 20:18 > To: Surround Sound discussion group > Subject: Re: [Sursound] [allowed] Re: Recreating a 3d soundfield with lots of > mics. > > > There's a photo of the set up at Royal Festival Hall, about 1/3 down on this > page > > http://www.gearplus.com.au/products/wharfedale/history/0-history- > wharfedale.htm ___ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
Re: [Sursound] Recreating a 3d soundfield with lots of mics.....
Thanks for the info ,. the discussion about instrumentation recording, and the WFS example are interesting but not really what I am trying to find examples of - I want to find examples of mapping sounds moving in 3d space etc. The window one most closely resembles what I am talking about . I would still like to find some examples where people have miced up a 3d soundscapes by placing the mics close to the target sound source and then placing speakers in the same place if there are any (I know the technology has been around for a hundred years - but havent found any examples as yet). The technique works nicely because the listener can also walk around sound sources - so there can be speakers surrounding the listener but also inside the listening space that the listener can walk around, sit next to etc - you can then pan between these speakers inside the listening space and those surrounding the listener artificially. Obviously its useless for listening to music at home - rather its for sound installations - also when you record moving sources the natural panning info is preserved So far I have only managed to find too examples which are similar, though different - the window probably being the closest Anyone know of any examples ? -- next part -- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20130520/1eec8bcc/attachment.html> ___ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
