The Chorus and Flanger effect use a low frequency oscillator to modify the length of the delay in real time.

If the chorus and flanger have analogue inputs and on-board A/D and D/A converters, the delay length can be modulated by modulating the sampling rate.

In a pure digital device, modulating the delay length is a lot more difficult. The circular buffer which implements the delay can only provide samples delayed by N samples where N is an integer.

To get the samples between the any two provided by the circular buffer we need to interpolate between them.