Infinite Impulse Response Interpolator (iirinterp)

API Keywords: iirinterp interpolate infinite impulse response IIR resample upsample recursive filter

The iirinterp object implements an infinite impulse response (IIR) interpolator with an integer output-to-input resampling ratio. An example of the iirinterp 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

Listed below is the full interface to the iirinterp family of objects. While each method is listed for iirinterp_crcf , the same functionality applies to iirinterp_rrrf and iirinterp_cccf .

  • iirinterp_crcf_create(M,*b,nb,*a,na) creates a iirinterp object with a interpolation factor \(M\) using the filter coefficients defined by \(\vec{a}\) and \(\vec{b}\) .
  • iirinterp_crcf_create_default(M,order) creates a iirinterp object from a prototype filter with a interpolation factor \(M\) and a filter order \(order\) . The default filter is a Butterworth design.
  • iirinterp_crcf_create_prototype(M,ftype,btype,format,order,fc,f0,Ap,As) creates a iirinterp object from a prototype with a interpolation factor \(M\) . See iirdes for details on the filter design parameters.
  • iirinterp_crcf_destroy(q) destroys a iirinterp object, freeing all internally-allocated memory.
  • iirinterp_crcf_print(q) prints the parameters of a iirinterp object to the standard output.
  • iirinterp_crcf_reset(q) clears the internal buffer of a iirinterp object.
  • iirinterp_crcf_execute(q,x,*y) computes the output interpolation of the input sample \(x\) and stores the output in the \(M\) -sample buffer \(\vec{y}\) .
  • iirinterp_crcf_execute_block(q,*x,n,*y) computes the output interpolation of the input sequence \(\vec{x}\) (which is \(n\) samples in size) and stores the resulting \(nM\) samples in \(y\) .
  • iirinterp_crcf_groupdelay(q,fc) computes the filter group delay at frequency \(f_c\) .

Figure [fig-filter-iirinterp]. iirinterp_crcf (interpolator) example with \(M=4\) , compensating for filter delay.

A graphical example of the interpolator can be seen in[ref:fig-filter-iirinterp] . A detailed example program is given in examples/iirinterp_crcf_example.c , located under the main liquid project directory.