when should one perform fft based filtering?
in [DSP]
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From: robert bristow-johnson on 28 Jul 2010 02:11 On Jul 28, 1:34 am, spop...(a)speedymail.org (Steve Pope) wrote: > robert bristow-johnson <r...(a)audioimagination.com> wrote: > > >i would > >expect a plate reverb to be describable as an LTI system. > >i know a plate 'verb can sorta "resonate" on some frequencies, > >depending on its dimensions, but does a plate reverb create new > >frequency components? if so, is that not just due to the transducers > >going nonlinear? > > If different frequencies travel at different velocities through > the plate medium, is that not a nonlinearity (at least in terms > of behavior of the medium)? it's the same as non-linear phase shift. or a non-constant delay (as a function of frequency). what electrical engineers (and others) mean using the semantic "linear" is that what the system does to two signals summed to its input is the sum of what it does to each signal separately. that's what i meant by "linear". filters that have minimum phase, linear phase, whatever, in their frequency response are all considered to be linear systems and usually time-invariant. a simple little RC low-pass filter doesn't have linear phase throughout the whole frequency range (it's sorta linear at low frequencies), but it's still a linear, time-invariant system. r b-j
From: Nasser M. Abbasi on 28 Jul 2010 02:26 On 7/27/2010 10:34 PM, Steve Pope wrote: > > If different frequencies travel at different velocities through > the plate medium, is that not a nonlinearity (at least in terms > of behavior of the medium)? > > Steve How can a frequency travel at some "velocity"? Frequency is not a material object that can "travel" at any velocity? It is a property of an object, not the object itself? Do you mean 2 waves or signals traveling with different frequencies? The term "frequency" and "travel" combined as you have it, is something new to me, and never heard any of my teachers at school mention this term before. Is this something that engineers use out in the real world and that is why I never heard it before? When I leave school one day, and go to the "real world", will this term be a common term used by DSP engineers? --Nasser
From: Les Cargill on 28 Jul 2010 03:05 Steve Pope wrote: > Les Cargill<lcargill99(a)yahoo.com> wrote: > >> Steve Pope wrote: > >>> The really interesting question, totally unrelated to the above, is >>> whether you can produce a suitable (for your market) reverb purely with >>> linear filtering. It's pretty easy to demonstrate, for example, >>> that a plate reverb is nonlinear. > >> Purely convolution based plate reverb sims are pretty compelling >> and widely used. > > Thanks for this information. I've been away from this market > segment for so long, this conversation is one of my first > glimpses at even remotely recent thinking on the subject. > > > Steve You can download Reaper as a VST host and play with it. It's at a very good price - the architect is one of the people behind Winamp, so it's somewhat subsidized by his fortune from that. Matlab/Octave works too, but there's something about realtime that's fun. At various times people would offer deconvolutions of various reverbs, rooms, guitar cabinets and other stuff for free download. I haven't looked in a long time, and you can't stick your hand in the same river twice, so no clue as to availability. -- Les Cargill
From: Richard Dobson on 28 Jul 2010 05:15 On 28/07/2010 07:26, Nasser M. Abbasi wrote: > On 7/27/2010 10:34 PM, Steve Pope wrote: > >> >> If different frequencies travel at different velocities through >> the plate medium, is that not a nonlinearity (at least in terms >> of behavior of the medium)? >> >> Steve > > How can a frequency travel at some "velocity"? > > Frequency is not a material object that can "travel" at any velocity? It > is a property of an object, not the object itself? > > Do you mean 2 waves or signals traveling with different frequencies? > > The term "frequency" and "travel" combined as you have it, is something > new to me, and never heard any of my teachers at school mention this > term before. > > Is this something that engineers use out in the real world and that is > why I never heard it before? > > When I leave school one day, and go to the "real world", will this term > be a common term used by DSP engineers? > > --Nasser > > > I am not an acoustician, so caveat lector: the speed of sound in metal is (apart from being much faster than in air) dependent on (among other things) density and stiffness. While we make the generally reasonable assumption that air is at equal density within any contained space, this cannot be assumed for metal bars and plates. A plate reverb is a species of metallophone (which as a type ranges from the piano string to a vibraphone bar and a tam-tam), with some sometimes very dominant vibrational modes determined by the shape; all together they contribute to the special character of a plate reverb. Whether in a signal processing sense it can be called non-linear I am not sure, but we certainly get the sense that a stimulus at one frequency may activate a dominant mode at a different one. And or course for a hyper-complex material such as a hand-hammered cymbal or bell, where both density and stiffness may vary from one square mm to another, the modal behaviour is so complex that it can scarcely be modelled at all. My undestanding is that this definitely comes within the domain of "nonlinear acoustics", and in that sense at least, one can reasonably talk about locally varying speeds of propagation with respect to frequency. This may all be rubbish of course; one of the many subjects I have yet to get around to studying properly. Richard Dobson
From: Rune Allnor on 28 Jul 2010 06:11
On 28 Jul, 00:16, robert bristow-johnson <r...(a)audioimagination.com> wrote: > that said, Rune, doing this frequency-domain convolution in real time > *does* entail some delay (could be a half second, i dunno, depends on > the size of the FFT), but it's seldom the entire algorithm. Sure. Agreed on both points. I suppose we are left with the question what the term 'real-time processing' means: Is it sufficient that the processing keeps up with the data source for arbitrary long time, or is it necessary to also minimize the latency? I suppose opinions differ, depending on applictions. Rune |