The ideal anti-aliasing filter, applied to a continuous-time signal before sampling, consists of a rectangular window applied in the frequency domain with a cutoff at half the sample rate.
The inverse Fourier transform of this filter is a sinc function with a zero-crossing interval of one sample:
The ideal unit-amplitude impulse train with period seconds is given by
In order to bandlimit the ideal impulse train, we can apply the ideal anti-aliasing filter to it as:
where is the period in samples.
Since is now bandlimited, it can be sampled without aliasing as
This infinite summation can be reduced to an expression of the form (Stilson and Smith, 1996)
(2)
where the digital sinc function is given by
is the number of harmonics and is always odd. It cannot exceed the period in samples.
Equation (2) is a closed-form expression that can be used to generate a bandlimited impulse train in a manner similar to the use of DSFs.
Figure 10:
The time and frequency magnitude response of an impulse train waveform (440 Hz fundamental) created using the BLIT sinc function of Stilson & Smith.
Note that similar numerical instabilities will arise with this expression as was found with the DSFs. L'Hopital's rule can be used to evaluate the limit of the function as its denominator approaches a value of zero, resulting in