The sampling theorem describes why the input of an ADC requires a low-pass analog electronic filter, called the anti-aliasing filter: the sampled input signal must be bandlimited to prevent aliasing (here meaning waves of higher frequency being recorded as a lower frequency).
Thus, the reconstruction filter smooths the waveform to remove image frequencies (copies) above the Nyquist limit.
Practical filters have non-flat frequency or phase response in the pass band and incomplete suppression of the signal elsewhere.
For this reason, oversampling may be used to ensure that frequencies of interest are accurately reproduced without excess energy being emitted out of band.
Alternatively, a system may have no reconstruction filter and simply tolerate some energy being wasted reproducing higher frequency images of the primary signal spectrum.
However, images are frequently gamma encoded, notably in the sRGB color space, so luminance is not linear.
The most common day-to-day filters are:[5] These are in increasing order of stopband suppression (anti-aliasing), and decreasing speed For reconstruction purposes, a variety of kernels are used, many of which can be interpreted as approximating the sinc function,[4] either by windowing or by giving a spline approximation, either by cubics or higher order splines.