quadratic or cubic interpolator

On Dec 13, 2006, at 1:16 PM, Tim wrote:
> I don't know if I'll have time to make an external, but the code
> may be helpful. How can I get it from you?

Here you go. Complete with reference on where I found the technique.

/
************************************************************************
********
float rbb_allpassInterp(float *in_array, float index, long bufferLength,
        float last_out, float *out_ptr)

inputs:            in_array -- name of array of input values
                index -- floating point index value to interpolate
                bufferLength -- length of in_array
                last_out -- value of last output from buffer
                out_ptr -- pointer to output location
description:    performs allpass interpolation on an input array and
returns the
    results to a location specified by a pointer; implements filter as
specified
    in Dattorro 2: J. Audio Eng. Soc., Vol 45, No 10, 1997 October
returns:        interpolated output
************************************************************************
********/
void rbb_allpassInterp(float *in_array, float index, long bufferLength,
        float last_out, float *out_ptr)
{
    // index = i.frac
    long index_i = (long)index;                    // i
    long index_iP1 = index_i + 1;                // i + 1
    float index_frac = index - (float)index_i;    // frac

    // make sure that index_iP1 is not out of range
    while (index_iP1 >= bufferLength) index_iP1 -= bufferLength;

    // formula as on bottom of page 765 of above Dattorro article
    *out_ptr = in_array[index_i] + index_frac * (in_array[index_iP1] -
last_out);
}

-----
Nathan Wolek
nw@nathanwolek.com
http://www.nathanwolek.com