Read FilterLength after the PrepareFilter method was called, or after the first call the Update method was made to obtain the length of FIR filter used for interpolation. 

Filter length affects the following parameters: *the width of transition region of the lowpass filter. *the level of attenuation in the stopband of the lowpass filter. *the quality of interpolation. 

The filter must be long enough to meet the most strict specification. The automatic FIR length estimation takes in to account only the width of the transition region and the level of attenuation, but not the quality of interpolation. The longer the filter, the higher is the interpolation quality. 

The interpolation quality is also affected by the value of the ImpulseFactor property which defines the factor by which the FIR impulse response is oversampled (linear interpolation grid density) and the amount of allowed Ripple in the passband. Smaller ripple forces the edges of the impulse response quicker to zero thus making the "truncated" impulse response less sensitive to the assumption that taps are zero outside the given impulse response. 

Another important issue is the aliasing. If the resamplingFactor is smaller than 1 (the sampling frequency is being lowered), then the lowpass filter must attenuate all frequencies above the FS*ResamplingFactor frequency. (FS = original sampling frequency.) 

Usually it is best to use multirate FIR filters as antialiasing filters and then use this component only to apply interpolation where the interpolation filter is very short (7,9,11 or 13 taps). Such a setup can achieve SNR (Signal to noise ratio) up to 85dB. With more taps SNR of 180dB (double precision) can be achieved. 

The value of the allowed ripple which defines the beta parameter of the Kaiser window has a significant impact on the quality of the interpolation. If lowpass antialiasing filter is applied with a separate filter, then the lowpass interpolation filter can concentrate on the quality of interpolation only.

TBandlimitedInterpolator Properties