# ellip

Create an Elliptic filter.

## Syntax

[b,a] = ellip(n,Rp,Rs,Wp)

[b,a] = ellip(n,Rp,Rs,Wp,band)

[b,a] = ellip(n,Rp,Rs,Wp,domain)

[b,a] = ellip(n,Rp,Rs,Wp,band,domain)

## Inputs

`n`- The filter order.
`Rp`- The maximum attenuation in decibels in the passband,
`Wp`. `Rs`- The minimum attenuation in decibels in the stop band.
`Wp`- A scalar specifying the cutoff frequency of a low or high pass filter, or a two element vector specifying the cutoff frequencies of a bandpass or bandstop filter. For a digital filter the values (in Hz) are normalized relative to the Nyquist frequency. For an analog filter the values are in radians/sec.
`band`- The band type of the filter. Omit for low pass or bandpass. Use 'high' for high pass, and 'stop' for bandstop.
`domain`- Omit for digital filters. Use 's' for analog filters.

## Outputs

- b
- The numerator polynomial coefficients of the filter.
- a
- The denominator polynomial coefficients of the filter.

## Example

Create a third order Elliptic low pass digital filter with a 300 Hz cutoff, a 1000 Hz sampling frequency, and a maximum passband attenuation of 1 dB.

`[b,a] = ellip(3,1,20,300/500)`

```
b = [Matrix] 1 x 4
0.33901 0.74185 0.74185 0.33901
a = [Matrix] 1 x 4
1.00000 0.50193 0.70451 -0.04473
```

## Comments

Filters can become unstable for high orders, and more easily so for bandpass or stopband filters.