Filter design with moving cutoff frequency
I have been reading about filter design. I am aware that it is possible to create delay lines to achieve filtering for a fixed cutoff frequency. But, when someone designs a filter for a virtual oscillator / sampler, the filter isn't fixed, and is changeable by the user.
What is the best type of filter design to start with if you want to make a filter in C, where the resultant filter has a changeable cutoff frequency? I tried google for this, but didn't find a good start...