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# Prony's Method

Module by: Douglas L. Jones. E-mail the author

Prony's Method is a quasi-least-squares time-domain IIR filter design method.

First, assume Hz H z is an "all-pole" system:

Hz= b 0 1+ k =1M a k zk H z b 0 1 k 1 M a k z k
(1)
and hn= k =1M a k hnk+ b 0 δn h n k 1 M a k h n k b 0 δ n where hn=0 h n 0 , n<0 n 0 for a causal system.

## Note:

For h=0 h 0 , h0= b 0 h 0 b 0 .
Let's attempt to fit a desired impulse response (let it be causal, although one can extend this technique when it isn't) h d n h d n .

A true least-squares solution would attempt to minimize ε2= n =0| h d nhn|2 ε 2 n 0 h d n h n 2 where Hz H z takes the form in Equation 1. This is a difficult non-linear optimization problem which is known to be plagued by local minima in the error surface. So instead of solving this difficult non-linear problem, we solve the deterministic linear prediction problem, which is related to, but not the same as, the true least-squares optimization.

The deterministic linear prediction problem is a linear least-squares optimization, which is easy to solve, but it minimizes the prediction error, not the |desiredactual|2 desired actual 2 response error.

Notice that for n>0 n 0 , with the all-pole filter

hn= k =1M a k hnk hn k 1 M a k h n k
(2)
the right hand side of this equation is a linear predictor of hn h n in terms of the M M previous samples of hn h n .

For the desired reponse h d n h d n , one can choose the recursive filter coefficients a k a k to minimize the squared prediction error ε p 2= n =1| h d n+ k =1M a k h d nk|2 ε p 2 n 1 h d n k 1 M a k h d n k 2 where, in practice, the is replaced by an N N.

In matrix form, that's ( h d 00...0 h d 1 h d 0...0 h d N1 h d N2... h d NM ) a 1 a 2 a M h d 1 h d 2 h d N h d 0 0 ... 0 h d 1 h d 0 ... 0 h d N 1 h d N 2 ... h d N M a 1 a 2 a M h d 1 h d 2 h d N or H d a h d H d a h d The optimal solution is a lp =( H d H H d -1 H d H h d ) a lp H d H d -1 H d h d Now suppose Hz H z is an M th M th -order IIR (ARMA) system, Hz= k =0M b k zk1+ k =1M a k zk H z k 0 M b k z k 1 k 1 M a k z k or

hn= k =1M a k hnk+ k =0M b k δnk={ k =1M a k hnk+ b n   if  0nM k =1M a k hnk  if  n>M h n k 1 M a k h n k k 0 M b k δ n k k 1 M a k h n k b n 0 n M k 1 M a k h n k n M
(3)
For n>M n M , this is just like the all-pole case, so we can solve for the best predictor coefficients as before: ( h d M h d M1... h d 1 h d M+1 h d M... h d 2 h d N1 h d N2... h d NM ) a 1 a 2 a M h d M+1 h d M+2 h d N h d M h d M 1 ... h d 1 h d M 1 h d M ... h d 2 h d N 1 h d N 2 ... h d N M a 1 a 2 a M h d M 1 h d M 2 h d N or H^dah^d H d a h d and a opt =H^dH H d -1 H d Hh^d a opt H d H d -1 H d h d Having determined the a a's, we can use them in Equation 3 to obtain the b n b n 's: b n = k =1M a k h d nk b n k 1 M a k h d n k where h d nk=0 h d n k 0 for nk<0 n k 0 .

For N=2M N 2 M , H^d H d is square, and we can solve exactly for the a k a k 's with no error. The b k b k 's are also chosen such that there is no error in the first M+1 M 1 samples of hn h n . Thus for N=2M N 2 M , the first 2M+1 2 M 1 points of hn h n exactly equal h d n h d n . This is called Prony's Method. Baron de Prony invented this in 1795.

For N>2M N 2 M , h d n=hn h d n h n for 0nM 0 n M , the prediction error is minimized for M+1<nN M 1 n N , and whatever for nN+1 n N 1 . This is called the Extended Prony Method.

One might prefer a method which tries to minimize an overall error with the numerator coefficients, rather than just using them to exactly fit h d 0 h d 0 to h d M h d M .

## Shank's Method

1. Assume an all-pole model and fit h d n h d n by minimizing the prediction error 1nN 1 n N .
2. Compute vn v n , the impulse response of this all-pole filter.
3. Design an all-zero (MA, FIR) filter which fits vn* h z n h d n v n h z n h d n optimally in a least-squares sense (Figure 1).

The final IIR filter is the cascade of the all-pole and all-zero filter.

This is is solved by min b k b k n =0N| h d n k =0M b k vnk|2 b k n 0 N h d n k 0 M b k v n k 2 or in matrix form ( v000...0 v1v00...0 v2v1v0...0 vNvN1vN2...vNM ) b 0 b 1 b 2 b M h d 0 h d 1 h d 2 h d N v 0 0 0 ... 0 v 1 v 0 0 ... 0 v 2 v 1 v 0 ... 0 v N v N 1 v N 2 ... v N M b 0 b 1 b 2 b M h d 0 h d 1 h d 2 h d N Which has solution: b opt =VHV-1VHh b opt V V -1 V h

Notice that none of these methods solve the true least-squares problem: min a , b a , b n =0| h d nhn|2 a b n 0 h d n h n 2 which is a difficult non-linear optimization problem. The true least-squares problem can be written as: min α , β α , β n =0| h d n i =1M α i e β i n|2 α β n 0 h d n i 1 M α i β i n 2 since the impulse response of an IIR filter is a sum of exponentials, and non-linear optimization is then used to solve for the α i α i and β i β i .

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