Here's a wonderful tutorial on how to do digital resampling and interpolation in general.

You'll also find, within the rtl directory, examples of a nearest neighbour interpolator, a linear upsampling interpolator, and a quadratic upsampling interpolator.
All three have been discussed on the ZipCPU blog: the nearest neighbour interpolator here, the linear interpolator here, and the quadratic interpolator here.

If time permits, I look forward to adding some higher order interpolation solutions and/or splines to this repository.

The tutorial is offered here under the Creative Commons, Attribution-NoDerivations 4.0 International license.

The Verilog source code and software (if any) are released under the GPLv3.

If these conditions are not sufficient for you, other licenses terms are available for purchase.