The A/D converter observes the signal at regular intervals and assigns a number to each observation. The resulting sequence can be stored, processed and displayed, but it preserves the information correctly only under specific conditions.

Sampling theorem

A band-limited signal can be reconstructed when the sample rate exceeds twice the highest frequency present. Real systems require margin and anti-alias filters because practical filters are not ideal.

Aliasing

A component above the permitted band may appear as a lower frequency that was not present in the original signal. Once acquired, aliasing cannot be reliably removed by software.

Quantisation

Amplitude is represented by a finite number of levels. Bit depth affects the smallest step, but real noise and analogue quality can reduce the number of effectively useful bits.

Sample rate and sample count have different roles. The former determines theoretical bandwidth; the latter determines buffer duration and FFT resolution.

Reconstruction

The D/A converter and output filter turn the sequence into a continuous signal. On screen, VA may also interpolate samples to represent the waveform more clearly, without creating information beyond the acquired bandwidth.

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Read more in the Visual Analyser Handbook

This page is an operational introduction. Chapter 1 and 5 — Sampling and reconstruction of the forthcoming book covers theory, controls, algorithms, examples and measurement notes in much greater depth.