Linearly interpolated fractional delay is equivalent to filtering and resampling a weighted impulse train (the input signal samples) with a continuous-time filter having the simple triangular impulse response
Convolution of the weighted impulse train with produces a continuous-time linearly interpolated signal
This continuous result can then be resampled at the desired fractional delay.
In discrete time processing, the operation Eq.(4.5) can be approximated arbitrarily closely by digital upsampling by a large integer factor , delaying by samples (an integer), then finally downsampling by , as depicted in Fig.4.7 . The integers and are chosen so that , where the desired fractional delay.
Frequency Response of Linear Interpolation
Minimizing First-Order Allpass Transient Response