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?

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; many harmonics of the internal (lower) sampling frequency were also present in the output.

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?

Some clarifications:

• The system is real-time, so the upsampling scheme must be causal.
• The upsampler is followed by a an anti-aliasing filter that can also be specified.