Fir interpolator with zero order hold and fir-spline interpolation combination

A FIR interpolator which uses a zero order hold to add samples in-between original samples of a digital input stream. The use of zero order hold with a FIR interpolator reduces output harmonics as compared to the use of conventional zero fill. Moreover, a computation method has been developed to greatly reduce the amount of computation needed by combining the zero order hold and the FIR filter. The output of the FIR interpolator can then be interpolated by a Spline interpolator to further interpolate the input stream. The combination of zero order hold FIR interpolation with Spline interpolation provides an efficient, cost effective technique of interpolating a digital input stream while producing relatively small output harmonics.