infinite impulse response decimator (iirdecim)

API Keywords: iirdecim decimate infinite impulse response IIR resample downsample recursive filter

The iirdecim object family implements a basic interpolator with an integer output-to-input resampling ratio \(M\) . It is similar to the firdecim family but with an infinite impulse response (recursive) filter performing the alias rejection. In essense, the iirdecim family is essentially just a iirfilt object which operates on a block of samples at a time. An example of the iirdecimator can be seen in [ref:fig-filter-iirdecim-crcf] .


Figure [fig-filter-iirdecim-crcf]. iirdecim_crcf (iirdecimator) example with \(M=4\) , compensating for filter delay.

Listed below is the full interface to the iirdecim family of objects. While each method is listed for iirdecim_crcf , the same functionality applies to iirdecim_rrrf and iirdecim_cccf .

  • iirdecim_crcf_create(M,*b,nb,*a,na) creates a iirdecim object with a decimation factor \(M\) using the filter coefficients defined by \(\vec{a}\) and \(\vec{b}\) .
  • iirdecim_crcf_create_default(M,order) creates a iirdecim object from a prototype filter with a decimation factor \(M\) and a filter order \(order\) . The default filter is a Butterworth design.
  • iirdecim_crcf_create_prototype(M,ftype,btype,format,order,fc,f0,Ap,As) creates a iirdecim object from a prototype with a decimation factor \(M\) . See iirdes for details on the filter design parameters.
  • iirdecim_crcf_destroy(q) destroys a iirdecim object, freeing all internally-allocated memory.
  • iirdecim_crcf_print(q) prints the parameters of a iirdecim object to the standard output.
  • iirdecim_crcf_reset(q) clears the internal buffer of a iirdecim object.
  • iirdecim_crcf_execute(q,*x,*y) computes the output decimation of the input sequence \(\vec{x}\) (which is \(M\) samples in size) and stores the single output sample result in \(y\) .
  • iirdecim_crcf_execute_block(q,*x,n,*y) computes the output decimation of the input sequence \(\vec{x}\) (which is \(nM\) samples in size) and stores the resulting \(n\) samples in \(y\) .
  • iirdecim_crcf_groupdelay(q,fc) computes the filter group delay at frequency \(f_c\) .

An example of the iirdecim interface is listed below.

#include <liquid/liquid.h>

int main() {
    // options
    unsigned int M = 4;     // decimation factor
    unsigned int n = 21;    // filter length

    // design filter and create decimator object
    iirdecim_crcf q = iirdecim_crcf_create_default(M,n);

    // generate input signal and decimate
    float complex x[M];         // input samples
    float complex y;            // output sample

    // run decimator (repeat as necessary)
        iirdecim_crcf_execute(q, x, &y);

    // destroy decimator object

A more detailed example is given in examples/iirdecim_crcf_example.c in the main liquid project directory.