Finite Impulse Reseponse Interpolator (firinterp)

The interp object implements a basic interpolator with an integer output-to-input resampling ratio. An example of the interp interface is listed below.


#include <liquid/liquid.h>

int main() {
    unsigned int M=4;       // interpolation factor
    unsigned int h_len;     // interpolation filter length

    // design filter and create interpolator
    float h[h_len];         // filter coefficients
    firinterp_crcf q = firinterp_crcf_create(M,h,h_len);

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

    // run interpolator (repeat as necessary)
    {
        firinterp_crcf_execute(q, x, y);
    }

    // destroy the interpolator object
    firinterp_crcf_destroy(q);
}

Listed below is the full interface to the interp family of objects. While each method is listed for firinterp_crcf , the same functionality applies to interp_rrrf and interp_cccf .

  • firinterp_crcf_create(M,*h,N) creates an interp object with an interpolation factor \(M\) using \(N\) filter coefficients \(\vec{h}\) .
  • firinterp_crcf_create_prototype(M,m,As) create an interp object using a filter prototype designed using the firdes_kaiser_window() method (see [section-filter-firdes] ) with a normalized cut-off frequency \(1/2M\) , a filter length of \(2Mm\) coefficients, and a stop-band attenuation of \(A_s\) dB.
  • firinterp_crcf_create_rnyquist(type,k,m,beta,dt) creates an interp object from a square-root Nyquist filter prototype with \(k\) samples per symbol (interpolation factor), \(m\) symbols of delay, \(\beta\) excess bandwidth, and a fractional sampling interval \(\Delta t\) . [section-filter-firdes-rnyquist] provides a detailed description of the available square-root Nyquist filter prototypes available in liquid.
  • firinterp_crcf_destroy(q) destroys the interpolator, freeing all internally-allocated memory.
  • firinterp_crcf_print(q) prints the internal properties of the interpolator to the standard output.
  • firinterp_crcf_clear(q) clears the internal interpolator buffers.
  • firinterp_crcf_execute(q,x,*y) executes the interpolator for an input \(x\) , storing the result in the output array \(y\) (which has a length of \(M\) samples).
filter_firinterp_crcf.png

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

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