# A reference request for zero-order hold (ZOH)

From Wikipedia:

A zero-order hold reconstructs the following continuous-time waveform from a sample sequence $x[n]$, assuming one sample per time interval $T$: $$x_{\mathrm{ZOH}}(t)\,= \sum_{n=-\infty}^{\infty} x[n]\cdot \mathrm{rect} \left(\frac{t-T/2 -nT}{T} \right) \$$ where $\mathrm{rect}()$ is the rectangular function.

I'd like to find a good bibliographic reference for it.