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# syredi

Design of iir filters, syredi code interface

### Syntax

[fact,b2,b1,b0,c1,c0,zzeros,zpoles]=syredi(ityp,iapro,om,deltap,deltas)

### Arguments

- itype
integer, the filter type: 1 stands for low-pass, 2 for high-pass, 3 for band-pass, 4 for stop-band.

- iappro
integer, the design approximation type: 1 stands for butterworth, 2 for elliptic, 3 for Chebytchev1, 4 for Chebytchev2.

- om
4-vector of cutoff frequencies (in radians)

`om=[om1,om2,om3,om4]`

,`0<= om1 <= om2 <= om3 <= om4 <= pi`

.When

`ftype`

='lp' or`'hp'`

,`om3`

and`om4`

are not used and may be set to 0.- deltap
a real scalar, the ripple in the passband.

`0< deltap <1`

- deltas
a real scalar, the ripple in the stopband.

`0< deltas <1`

- gain
scalar, the filter gain

- b2
real row vector, degree 2 coefficients of numerators.

- b1
real row vector, degree 1 coefficients of numerators.

- b0
real row vector, degree 0 coefficients of numerators.

- c1
real row vector, degree 1 coefficients of denominators.

- c0
real row vector, degree 0 coefficients of denominators.

- zzeros
complex row vector, filter zeros in the z-domain

- zpoles
complex row vector, filter poles in the z-domain

### Description

Computes iir filter approximation. The result is given as a set of
second order transfer functions
`Hi=(b0(i)+b1(i)*z+b2(i)*z^2)/(c0(i)+c1(i)*z+z^2)`

and
also as a poles, zeros, gain representation.

The filter obtained is `h=fact*H1*...*Hn`

.

### Remark

This built-in function is mainly intended to be used by the eqiir function.

### References

The syredi code is derived from doredi package written by Guenter F. Dehner, Institut fuer Nachrichtentechnik Universitaet Erlangen-Nuernberg, Germany.

Dehner,G.F. 1979, DOREDI: Program for Design and Optimization of REcursive DIgital filters-Programs for Digital Signal Processing, ed:Digital Signal Processing committee of IEEE Acoustics, Speech and Signal Processing Society.

For DOREDI.f source code see http://michaelgellis.tripod.com/dsp/pgm25.html

### Examples

[fact,b2,b1,b0,c1,c0,zzeros,zpoles]=syredi(1,4,[2*%pi/10,4*%pi/10,0,0],0.02,0.001); h=fact*(b0+b1*%z+b2*%z^2)./(c0+c1*%z+%z^2)

### See also

- eqiir — Design of iir filters

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