I have looked at P.P. Vaidayanthan's "Multirate Systems and Filter Banks" on the topic of the monotonicity of the unwrapped phase response of an allpass digital filter. The proof is based on the observation that the group delay is always positive and hence slope of the phase response is always negative, ergo phi(w) is a decreasing function. The group delay > 0 is shown for a first order section, with the statement that the group delay for an Nth order system is the addition of N monotonically decreasing functions, and hence the result follows. Can someone suggest a reference (in DSP literature or math) to an alternative proof that is perhaps more rigorous? (without getting sidetracked by whether or not the the proof in PPV's text is rigorous enough :->). Thanks! All pass is, of course, H(z) = z^{-N} A(z^-1)/A(z), where A(z) is 1+ a1 z^{-1} + ... +aN z^{-N} and z \in C. -vv

# Monotonicity of allpass phase function

Started by ●March 5, 2010

Reply by ●March 5, 20102010-03-05

On 5 Mar, 10:50, vv <vanam...@netzero.net> wrote:> I have looked at P.P. Vaidayanthan's "Multirate Systems and Filter > Banks" on the topic of the monotonicity of the unwrapped phase > response of an allpass digital filter. =A0The proof is based on the > observation that the group delay is always positive and hence slope of > the phase response is always negative, ergo phi(w) is a decreasing > function.There was a discussion here a few years ago where somebody (I can't remember who - RBJ? Andor?) demonstrated that a causal filter might in fact have negative group delay in parts of the frequency band. The effect showed up as a very short rise time in the impulse response of the filter. Rune

Reply by ●March 5, 20102010-03-05

On 5 Mar, 10:59, Rune Allnor <all...@tele.ntnu.no> wrote:> On 5 Mar, 10:50, vv <vanam...@netzero.net> wrote: > > > I have looked at P.P. Vaidayanthan's "Multirate Systems and Filter > > Banks" on the topic of the monotonicity of the unwrapped phase > > response of an allpass digital filter. =A0The proof is based on the > > observation that the group delay is always positive and hence slope of > > the phase response is always negative, ergo phi(w) is a decreasing > > function. > > There was a discussion here a few years ago where somebody > (I can't remember who - RBJ? Andor?) demonstrated that a > causal filter might in fact have negative group delay in > parts of the frequency band. The effect showed up as a very > short rise time in the impulse response of the filter. > > RuneFound it: http://groups.google.no/group/comp.dsp/msg/c820aea7bdaf4cb2?hl=3Dno It was Andor who saw the flaw in the argment that a causal system must necessarily have positive group delay everywhere. He found a filter fundtion that was *both* causal *and* had negative group delay over substantial parts of the frequency band. So if the proof in the book is based on the supposition that a causal filter response must have positive group delay everywhere, the proof is wrong. Rune

Reply by ●March 5, 20102010-03-05

On Mar 5, 3:42=A0pm, Rune Allnor <all...@tele.ntnu.no> wrote:> So if the proof in the book is based on the supposition > that a causal filter response must have positive group > delay everywhere, the proof is wrong.The allpass is filter is causal and stable and its group delay is always positive. There is nothing wrong with the proof. Assume the pole is at r exp(j x), where r < 1. The group delay for a first order allpass is (1-r^2)/|1-r exp(j(x-w))|^2, which is always positive. If the filter is not allpass, causal and stable filters can give rise to an expression for group delay that goes negative for some w, which is also well-know, I guess. (On negative group delay, it may be of interest to some people to see Morgan Mitchell and Raymond Y. Chiao: =93Causality and Negative Group Delays in a simple band-pass amplifier=94, American Journal of Physics, Vol. 66 no. 1, January 1998). -vv

Reply by ●March 5, 20102010-03-05

Rune Allnor wrote:> On 5 Mar, 10:59, Rune Allnor <all...@tele.ntnu.no> wrote: >> On 5 Mar, 10:50, vv <vanam...@netzero.net> wrote: >> >>> I have looked at P.P. Vaidayanthan's "Multirate Systems and Filter >>> Banks" on the topic of the monotonicity of the unwrapped phase >>> response of an allpass digital filter. The proof is based on the >>> observation that the group delay is always positive and hence slope of >>> the phase response is always negative, ergo phi(w) is a decreasing >>> function. >> There was a discussion here a few years ago where somebody >> (I can't remember who - RBJ? Andor?) demonstrated that a >> causal filter might in fact have negative group delay in >> parts of the frequency band. The effect showed up as a very >> short rise time in the impulse response of the filter. >> >> Rune > > Found it: > > http://groups.google.no/group/comp.dsp/msg/c820aea7bdaf4cb2?hl=no > > It was Andor who saw the flaw in the argument that a > causal system must necessarily have positive group delay > everywhere. He found a filter function that was *both* > causal *and* had negative group delay over substantial > parts of the frequency band. > > So if the proof in the book is based on the supposition > that a causal filter response must have positive group > delay everywhere, the proof is wrong.From what I read here, the proof in the book assumes that an allpass filter has positive group delay everywhere. I don't have the book, so I can't check that. Jerry -- Blaise Pascal: Men never do evil so completely and cheerfully as when they do it from religious conviction. ��������������������������������������������������������������

Reply by ●March 5, 20102010-03-05

On Mar 5, 6:35=A0am, VV <vanam...@netzero.net> wrote:> On Mar 5, 3:42=A0pm, Rune Allnor <all...@tele.ntnu.no> wrote: > > > So if the proof in the book is based on the supposition > > that a causal filter response must have positive group > > delay everywhere, the proof is wrong.APF adds another condition.> The allpass is filter is causal and stable and its group delay is > always positive. =A0There is nothing wrong with the proof. =A0Assume the > pole is at r exp(j x), where r < 1. =A0The group delay for a first order > allpass is (1-r^2)/|1-r exp(j(x-w))|^2, which is always positive. =A0If > the filter is not allpass, causal and stable filters can give rise to > an expression for group delay that goes negative for some w, which is > also well-know, I guess. =A0(On negative group delay, it may be of > interest to some people to see Morgan Mitchell and Raymond Y. Chiao: > =93Causality and Negative Group Delays in a simple band-pass amplifier=94=,> American Journal of Physics, Vol. 66 no. 1, January 1998).is your problem with rigor that it's only a 1st-order APF? if so, imagine two 1st-order APFs in series, one with the pole/zero rotated by w0 and the other rotated by -w0. because the two poles and two zeros are complex conjugate, it's still a real APF (and 2nd-order). otherwise, i do not understand your problem with the rigor. the normal way that i ever prove that some function is monotonic is to show that the derivative of that function never changes sign - always non-negative for monotonically increasing, always non-positive for monotonically decreasing. after Newton and Leibniz, how else do we prove monotonicity? r b-j

Reply by ●March 5, 20102010-03-05

On Mar 5, 9:54=A0am, Jerry Avins <j...@ieee.org> wrote:> > =A0From what I read here, the proof in the book assumes that an allpass > filter has positive group delay everywhere. I don't have the book, so I > can't check that. >Jerry, it's pretty easy to see in the s-plane, why APFs have monotonically decreasing phase. you can see that geometrically, but you can prove it analytically for a single pole and zero. extending it for higher order APFs is a matter of translating where "f=3D0" goes (and adding the phase results). extending that to the z- plane can be done by using the bilinear transform and recognizing that the frequency warping that results is also monotonic. r b-j

Reply by ●March 5, 20102010-03-05

robert bristow-johnson wrote:> On Mar 5, 9:54 am, Jerry Avins <j...@ieee.org> wrote: >> From what I read here, the proof in the book assumes that an allpass >> filter has positive group delay everywhere. I don't have the book, so I >> can't check that. >> > > Jerry, it's pretty easy to see in the s-plane, why APFs have > monotonically decreasing phase. you can see that geometrically, but > you can prove it analytically for a single pole and zero. > > extending it for higher order APFs is a matter of translating where > "f=0" goes (and adding the phase results). extending that to the z- > plane can be done by using the bilinear transform and recognizing that > the frequency warping that results is also monotonic.Sure. What I don't know about Vaidayanthan is whether or not he limits his proof to APFs, or allows it to be overextended to all filters Jerry -- It matters little to a goat whether it be dedicated to God or consigned to Azazel. The critical turning was having been chosen to participate. �����������������������������������������������������������������������