If one inserts N zeros in between samples, as stv014 and xnor described, one can use a pair of comb filters to obtain the same result of just holding the samples. First use a feedforward comb filter with delay set to N+1 and gain set to -1. Follow this by a feedback comb filter with delay and gain set to 1 (otherwise known as an accumulator.) And voila! Insertion of the N zeros scales the frequency axis by a factor of N and introduces some images in the spectrum, but otherwise does nothing to the magnitude and phase of the original sampled signal. Holding the value instead of inserting the zeros, effectively passes the N zero inserted signal through the pair of comb filters. These filters may attenuate the images introduced by the zero insertion process. However, the comb filters will color the signal with their own frequency response.