Notes on Digital Signal Processing: Overview of Sampling Techniques
Digitization of an analog signal is the one process above all others that makes DSP such a useful technology. If it were limited to working with only those signals that orginate in digital form, DSP would be just an academic curiosity. Digitization actually comprises two distinct operations: sampling and quantization, which are usually analyzed separately.
2.1 Implicit Sampling Techniques
In implicit sampling, a sample measurement is triggered by the signal attaining some specified value or crossing some specified threshold. Recording the times at which zero-crossings occur in a bipolar signal is an example of implict sampling.