Resonance is just delay with feedback. Resonance occurs when you delay a signal and then feed it back with some gain to the input of the delay "in phase" with the original input, which means the delayed signal adds together and boosts the input level to the delay. If you use a normal digital delay line you get what is called a linear phase delay, so each frequency is delayed by the same amount. If you use an IIR filter to delay the signal you get what is called a non-linear phase delay, so each frequency is delayed by a different amount. The particular arrangement of multiple stages of delays and how the feedback is arranged both around individual stages and globally all determine the structure of the filter, and the type of filtering achieved.
Most synth filters use IIR filters. The amount of non-linear phase delay is typically referred to in terms of degrees, as this is usually more useful for filter design. When the signal has been delayed 360 degrees at a particular frequency, if you add this to the input signal you get a boost in amplitude of that frequency since it is back in phase with the input, which is called constructive interference. With enough feedback gain a resonant peak will form as the constructive interference is in a feedback loop with itself. Most resonant filter designs use the fact that taking the negative of a signal actually changes the phase by 180 degrees at all frequencies. So most filters delay the phase of the signal by 180 degrees somehow, then subtract this from the input with some gain (negative feedback) and so form a resonant peak. Many people have spent a lot of time forming many types of low pass resonant filter structures. In synths it is mostly the non-linear properties that very between different filters, there are actually only 3 structures typically used to form low pass resonant filters: multiple IIR one pole integrators with feedback (svf) or multiple IIR one pole low pass filters with feedback (sallen key, cascade). Andy On Sat, 21 Jul 2018 at 01:29, Spencer Jackson <ssjackso...@gmail.com> wrote: > Resonance is the characteristic of some systems to store and release > energy at particular frequencies. It's not limited to filters, mechanical > systems like springs or pendulums have resonance (get on a swing at the > park and try to change your frequency and you'll feel the effects of > resonance). > > In an electrical system, a minimal passive resonant circuit would be one > with 2 capacitors and 2 resistors. Selecting the values of the components > determines the frequency, but what happens is that at a certain frequency > the energy gets stored and passed back and forth between the capacitors, > like the swing going back and forth. This storage and energy swapping > emphasizes that frequency. Depending on the "Quality factor" or amount of > resonance that frequency can become much more apparent than the other > frequencies in the signal even if the input is wide-band. > > When you are talking about delay and feedback, you are creating a digital > filter, but I think it is worthwhile to spend some time understanding the > theoretical concept and think in terms of energy and frequency. Your > feedback delay becomes the storage and certain frequencies will resonate > with that system. > > > But it is having the resonance in parralel to a dry sound that bothers > me; but may be that's the only way to do ? > > I'm not sure what you mean but I think you need some study in filter > design, because a single feedback delay causes comb filtering but its not a > classical lowpass. Is that what you are trying to achieve? Digital filters > are almost universally combinations of short delays (typically 1 sample) > placed in different patterns and fed back in different amounts (e.g. > https://en.wikipedia.org/wiki/Digital_filter#Filter_realization). > > Here are some resources that may help you: > http://www.dspguide.com/ch14.htm > > https://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/VAFilterDesign_2.0.0a.pdf > > That first one is a book that can help you more with the fundamentals in > the early chapters as well. I hope this is somewhat helpful, if not perhaps > I need to understand better specifically what you are trying to achieve. > _Spencer > > > On Fri, Jul 20, 2018 at 10:13 AM, Mehdi Touzani <mehdi.touz...@gmail.com> > wrote: > >> Hi all, >> I follow the llist for a while, but I am not a DSP programmer, I do DSP >> audio apps for about 20 years now, for sonic core plateform "Scope" and >> Xite". I begin with other things like juce or flowstone, but so far, >> scope is still far superior in terms of sound results. Too bad there is no >> scripting tool for it (well there is but it is not available to me). >> >> My question is probably weird for you - like super noob - , because i am >> NOT looking for math or codes, but hints about a general >> design/architecture. >> >> So... how do you do a resonance in a lowpass circuit? :-) not the >> math, not the code, just the architecture. >> >> Personnally, i came to the conslusion that some kind of very short >> delay with feedback, mixed in parallel with the non resonance low pass >> sound, could do the trick. . The more feedback, the more resonance, and >> with feedback over -6dB, it would begin to self resonate. >> >> But it is having the resonance in parralel to a dry sound that bothers >> me; but may be that's the only way to do ? >> >> I have read about hardware filters and ok, there is math, but there is >> also the effect of the hardware components. Some articles even suggest that >> a bandpass in parralel could do the trick : or more exactly, a bandpass >> fonction is created after the resonance, to remove frequencies that would >> overlap with the cutoff. >> >> >> So, any hints that could help me improve the process etc would be very >> welcome. >> >> Thanks >> >> Mehdi >> >> _______________________________________________ >> dupswapdrop: music-dsp mailing list >> music-dsp@music.columbia.edu >> https://lists.columbia.edu/mailman/listinfo/music-dsp >> > > _______________________________________________ > dupswapdrop: music-dsp mailing list > music-dsp@music.columbia.edu > https://lists.columbia.edu/mailman/listinfo/music-dsp
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