1/3T allpass design
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From: jungledmnc on 20 Jan 2010 20:40 Hey folks, I'm not really a filter expert :). Is it possible to design an allpass filter (with zero latency, so IIR ideally) with phase response 1/3T? (so that each frequency would be shifted of exactly 1/3 of its period) If so where should I start? Thanks a lot! dmnc
From: Vladimir Vassilevsky on 20 Jan 2010 21:48 jungledmnc wrote: > Hey folks, Hey stupident, > I'm not really a filter expert :). Is this 3-phase motor control? > Is it possible to design an allpass > filter (with zero latency, so IIR ideally) with phase response 1/3T? (so > that each frequency would be shifted of exactly 1/3 of its period) There is approximately 1000 methods to do that. > If so where should I start? Start with explanation of what you really want. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Jerry Avins on 20 Jan 2010 22:22 jungledmnc wrote: > Hey folks, > I'm not really a filter expert :). Clearly. > Is it possible to design an allpass > filter (with zero latency, so IIR ideally) with phase response 1/3T? (so > that each frequency would be shifted of exactly 1/3 of its period) Huh? Only a wire has zero latency (if by latency you mean delay). Corollary: IIR filters have delay. > If so where should I start? Figure out how long does 120 degrees lasts at 1 Hz. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: steveu on 20 Jan 2010 22:45 >jungledmnc wrote: >> Hey folks, >> I'm not really a filter expert :). > >Clearly. > >> Is it possible to design an allpass >> filter (with zero latency, so IIR ideally) with phase response 1/3T? (so >> that each frequency would be shifted of exactly 1/3 of its period) > >Huh? Only a wire has zero latency (if by latency you mean delay). >Corollary: IIR filters have delay. It has to be a very short wire. :-) >> If so where should I start? > >Figure out how long does 120 degrees lasts at 1 Hz. Steve
From: Tim Wescott on 21 Jan 2010 00:14
On Wed, 20 Jan 2010 19:40:29 -0600, jungledmnc wrote: > Hey folks, > I'm not really a filter expert :). Is it possible to design an allpass > filter (with zero latency, so IIR ideally) with phase response 1/3T? (so > that each frequency would be shifted of exactly 1/3 of its period) If so > where should I start? Maybe, no, Hilbert transform. Maybe you can design an allpass filter with a phase response of 1/3T, _if_ you restrict the frequency range in which it'll work (low frequencies will _kill_ you). No, you cannot have zero latency, at least not with real-time input. Filters give a result that's a weighted sum of prior inputs; allpass filters do, too. Dictate the phase and magnitude responses and you dictate the delay -- you really don't have a choice in this. A Hilbert transform will give you a filter output of 1/4T (and cost you a lot of taps if you approximate it with an FIR). (your input) * cos (2*pi/3) + (Hilbert of your input) * sin(2*pi/3) = your input shifted by 2*pi/3, i.e. your "1/3T delay". I haven't done this, but I dimly recall some threads about getting better performance vs. complexity out of the Hilbert transform using IIR filters. You _won't_ get better delay characteristics -- you'll just get a more compact (and obscure) filter. -- www.wescottdesign.com |