Thread: Interpolation in C - Help

Hi Members,

I have received C source code function. The function carries out interpolation of a speech siganl. But what is the name of this type of interpolation??? There are a number of different types of interpolation and I need to know the actual type of interpolation it performs:

Code:

float interpolate (float    freq,double   filter_curve_db [][2],int      number_of_points) {
    double  result;
    int     i;
    double  freqLow, freqHigh;
    double  curveLow, curveHigh;
    
    if (freq <= filter_curve_db [0][0]) {
        freqLow = filter_curve_db [0][0];
        curveLow = filter_curve_db [0][1];
        freqHigh = filter_curve_db [1][0];
        curveHigh = filter_curve_db [1][1];

        result = ((freq - freqLow) * curveHigh + (freqHigh - freq) * curveLow)/ (freqHigh - freqLow);
    
        return (float) result;
    }

    if (freq >= filter_curve_db [number_of_points-1][0]) {
        freqLow = filter_curve_db [number_of_points-2][0];
        curveLow = filter_curve_db [number_of_points-2][1];
        freqHigh = filter_curve_db [number_of_points-1][0];
        curveHigh = filter_curve_db [number_of_points-1][1];

        result = ((freq - freqLow) * curveHigh + (freqHigh - freq) * curveLow)/ (freqHigh - freqLow);
    
        return (float) result;
    }
        
    i = 1;
    freqHigh = filter_curve_db [i][0];
    while (freqHigh < freq) {
        i++;
        freqHigh = filter_curve_db [i][0];    
    }
    curveHigh = filter_curve_db [i][1];

    freqLow = filter_curve_db [i-1][0];
    curveLow = filter_curve_db [i-1][1];

    result = ((freq - freqLow) * curveHigh + (freqHigh - freq) * curveLow)/ (freqHigh - freqLow);

    return (float) result;
}

Many Thanks if you can help.

I cannot ask the person I received as this is code taken from an ITU P.862 standard. It is open source to researchers.

There are no comments within the code . What Im trying to do is convert the C to MATLAB for future use and implementations of the algorithms. I want to do my project through MATLAB but it based on the source code I received.

I think it is Linear Interpolation.

It looks like an extended form of linear interpolation.

Linear is simply this:

Value=v1+interp*(v2-v1);

Bilinear is this:

Value1=v1+interp1*(v2-v1);
Value2=v3+interp1*(v4-v3);
FinalValue=Value1+interp2*(Value4-Value3);