Principles of digital sampling of a physiologic signal

The practice of clinical neurophysiology requires fast, reliable and accurate assessment of a variety of biologic signals. Appropriate filters and rates of analog-to-digital sampling must be used to avoid distorting the signal. Using principles of sampling theory and examples, we describe the frequency content of signals encountered in clinical neurophysiology laboratories, offer guidelines for band-limiting frequencies, and give rules for determining minimal sampling frequencies. Errors introduced by undersampling (aliasing) are illustrated. When sophisticated computational techniques, such as discrete Fourier transform, are used to reconstruct the original wave form, a sampling frequency just above the double of the highest frequency content of the signal is adequate. Sampling neurophysiologic wave forms for direct display, however, requires a sampling frequency at least 4 times as high as that of the upper frequency filter.