Finite Impulse Reseponse Interpolator (``firinterp``)
=====================================================

.. author   jgaeddert Joseph D. Gaeddert <joseph@liquidsdr.org>
.. date     January 1, 1975
.. location Boston, MA
.. header   /doc/firinterp/banner.png
.. keywords firinterp, interpolation, resample
.. api      firinterp_crcf

The ``interp`` object implements a basic interpolator with an integer
output-to-input resampling ratio.

Operation
---------

Interpolation in |liquid| is accomplished with a polyphase filter-bank channelizer.

An example of the ``interp`` interface is listed below.

.. code-block:: c

    #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);
    }

Examples
--------

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``.

.. qplot:: series filter/firinterp_example.c
    :kwargs: {
        "format":
        {
            "xlabel" : "Time [samples]",
            "xrange" : [-5, 40],
            "yrange" : [-1.6, 1.6],
            "legend" : ["interp","symbols"]
        },
        "plots":[
            {"ylabel":"Real",
             "series":[
                ["ty","y-real",".-",{"color":"#aaa"}],
                ["tx","x-real",".", {"color":"#004080"}]
            ]},
            {"ylabel":"Imag",
             "series":[
                ["ty","y-imag",".-",{"color":"#aaa"}],
                ["tx","x-imag",".", {"color":"#008040"}]
            ]}
        ]}
    :width: 75%
    :name: fig-filter-firinterp_crcf
    :caption: ``firinterp_crcf`` (interpolator) example with :math:`M=4`, compensating for filter delay.

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

Interface
---------

Listed below is the full interface to the ``firinterp`` family of
objects.

.. api:: firinterp