Programmer Guide/Command Reference/EVAL/iir1

Create and/or apply an IIR filter.

---

Usage 1

iir1(f1, f2 {, m, rp, rs, inv, type})

f1, f2

the lower and upper cutoff frequency; both values must be specified as relative frequencies (f / sampling-rate); 0 <= f1 < f2 <= 0.5

m

the filter order (default=5)

rp

the desired passband ripple in dB; 0 < rp (default=0.5)

rs

the desired stopband damping level in dB; 0 < rs (default=40)

inv

invert filter response (0=no, 1=yes); if this value is set to 1, the filter frequency response is inverted and f1/f2 are the boundaries of the stopband

type: the type of the filter

type=1

... Butterworth filter

type=2

... Chebyshev filter

type=3

... elliptic filter (this is the default)

Result

The function computes the filter coefficients for the IIR defined by the arguments and returns a matrix c with 2 columns. The first column (c[*,0]) contains the denominator coefficients (poles) and the second (c[*,1]) the nominator coefficients (zeros). The matrix c can be used to apply the filter to a signal (see Usage 3).

---

Usage 2

iir1(f1, f2, m, rp, rs, inv, type, x)

f1, f2, m, rp, rs, inv, type

see Usage 1

x

a vector containing the source signal

Result

The function computes the filter coefficients for the IIR defined by the arguments and applies it to the signal x. The result has the same type as the input signal x and containes the filtered signal.

---

Usage 3

iir1(c, z, x{, s})

c

IIR filter coefficients (see Usage 1)

z

the filter state (delay); must be a matrix with the same dimensions as c

notes:

• The matrix should be initialized with zeros (e.g.: #z := eval init($#c[!nrow],$#c[!ncol],0))
• The values of z are changed (z must be a reference to a table item

x

the signal sample (scalar) or the signal vector to be filtered

s

the downsampling factor; must be an integer in the range 0<s<ncol(x) (default=1)

note:

• If s>1, only the filter output values y[0], y[s], y[2s], ... are stored in the result.
• If s>1, nrow(x) should be a multiple of s.

Result

The filtered signal (sample or vector). The argument (reference) z is updated with the new filter state.

---

See also

fir1

<function list>