An image or signal that has been digitized according to the Nyquist Sampling Theorem contains all the information necessary to reconstruct the original. It is not however an exact duplicate of the original until it has been passed through a reconstruction filter. In the case of a stereo system, the reconstruction filter is a low-pass filter on the output of the digital-to-analog converter. In the case of a digital image, the only practical way to reconstruct the image is to upsample it to a higher sampling rate and then low-pass filter it digitally. This produces a very good approximation to the original image.