Low order sinc interpolation vs. polynomial interpolation for variable fractional delay I'm implementing a variable fractional delay element for use in online audio processing. Applications include ie. Karplus-Strong synthesis, flanger, chorus, echo, vibrato. I'm not oversampling, so drop-sample and linear interpolation are not really acceptable for interpolation.

Could a low order sinc FIR work as an alternative to 2:nd+ order polynomial interpolation? By low order I mean on the order of just a few zero crossings.

I really like sinc interpolation because it offers two advantages over polynomial interpolation.


Coefficients (table of windowed sinc values) are independent of the delay amount, whereas in polynomial interpolation the filter coefficients are an N:th order polynomial of the delay amount. This complicates the code when the amount of delay is variable.
Coefficients are independent of interpolation order. For polynomial interpolation you need precomputed polynomials in the delay amount for all the orders you might need.


The main drawback I see with higher order sinc interpolation is the amount of latency it imposes. N+1 point polynomial interpolation of order N gives just N/2 samples of latency.

I have also considered 1 pole allpass interpolation, but I suspect it will have bandwidth problems similar to linear interpolation. (Is multi pole allpass interpolation a thing?)
If you don't need matching points to come thru the resample interpolation unchanged, then you can use a different low-pass filter kernel.  A minimum phase equivalent to a linear phase windowed Sinc will have an identical frequency response, but less delay, especially when using a longer window.