That sounds like a sensible reason why a karplus-strong resonator could not alias. That's good news, although i suppose it isn't new at all to most people on the list... By the way i had read the FLOSS pages before... Thanks Derek!
Pierre 2010/3/31 Derek Holzer <de...@umatic.nl> > Correct, nothing played back at original sampling rate will alias. If you > speed that sample up, then some of the recorded harmonics will go over the > Nyquist number and alias. > > Please read the page I sent you on aliasing: > > > http://en.flossmanuals.net/PureData/Antialiasing > > and also > > http://en.flossmanuals.net/PureData/WhatIsDigitalAudio > > Aliasing happens when you try to synthesize or play back a frequency higher > than 1/2 the sampling rate (this is called the Nyquist number). In non-sine > wave oscillators, it often comes from the highest harmonics. In samples, it > comes from playing them back faster than the original sampling rate. It > happens at the moment those frequencies are synthesized, and cannot be > removed later. Thus the oversampling approach documented in the FLOSS Manual > (and taken directly from Miller's Pd manual patches). > > A Karplus-Strong resonator is a delay line and as I understand it, so long > as no pitch shifting is going on then it can't alias. You would not be able > to create a Karplus-Strong resonator at 30KHz unless you have a sampling > rate of 60KHz, because the shortest delay time you can get is still one > sample (1/44100 of one second at normal sampling rate). Again, math gurus > are welcome to correct my calculations. > > D. > > > > On 3/31/10 6:39 PM, Pierre Massat wrote: > >> I m not sure i understand aliasing well... So anything that's sampled >> and played back without altering the pitch would not suffer from >> aliasing? When exactly does aliasing occur? during the DAC conversion, >> or before that? Let's say i set a karplus-strong resonator to a >> frequency of 30 KHz (assuming i'm a dog and i can hear a pitch that >> high), at a 44.1 KHz sampling rate, than what happens? No aliasing at all? >> >> >> >> 2010/3/31 Derek Holzer <de...@umatic.nl <mailto:de...@umatic.nl>> >> >> >> I was thinking about this the other day.... is it possible to have >> aliasing with Karplus-Strong? Because it's a delay line, nothing is >> being played back at any higher rate than it was sampled at, so no >> aliasing should be possible. Right? Math-gurus correct me if I'm wrong. >> >> Otherwise, any signal generator needs to be bandlimited or oversampled: >> >> http://en.flossmanuals.net/PureData/Antialiasing >> http://en.flossmanuals.net/PureData/GeneratingWaveforms >> >> Frank Barknecht has some spliced-transition trick he uses as well, >> I'm sure it will come up in a reply or two on this thread as well... >> >> D. >> >> >> On 3/31/10 6:27 PM, Pierre Massat wrote: >> >> Hi! >> >> I ve been reading the on-going debate about interpolation for a few >> days, and it just occured to me that i don't how go about avoiding >> aliasing more generally than with band-limited wavetables. If i >> wanted >> to play a sample at a pitch higher than the original, or if i >> wanted to >> use a karplus-strong resonator to generate notes, what would be the >> proper way of ensuring that no aliasing occurs? Do people >> generally use >> low-pass filters with a cut-off somewhere below the Nyquist >> frequency? >> Or is there a trick that one can use earlier on in the signal >> path of a >> patch? >> >> >> -- >> ::: derek holzer ::: http://macumbista.net ::: >> ---Oblique Strategy # 139: >> "Revaluation (a warm feeling)" >> >> >> > -- > ::: derek holzer ::: http://macumbista.net ::: > ---Oblique Strategy # 151: > "Take away the important parts" >
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