I recently encountered a DSP system which did some internal upsampling via zero padding. Expecting zero-order-hold, I was surprised to find that a DC signal did not produce a DC output--the harmonics of the internal sampling frequency were quite bad, indicating, I suppose, the need for a better anti-aliasing filter.

This leads to my question: What upsampling techniques are commonly used, and what are their relative merits? Why would I choose zero padding, zero order hold, or first order hold, and what other techniques are available?