An audio interface returns numbers proportional to the signal, but it does not automatically know volts, pascals or probe and attenuator gain. Calibration establishes the relationship between numeric readings and physical quantities.

Amplitude calibration

By applying a known value with known uncertainty, VA determines a conversion coefficient used by the oscilloscope, voltmeter and analyser. The procedure assumes sufficiently linear behaviour over the selected range.

Frequency compensation

A correction curve can compensate for the non-flat response of the interface, microphone or complete chain. VA can combine custom curves and standard weightings when appropriate.

Calibration is not full characterisation. A single point corrects scale but does not remove nonlinearity, noise, distortion, drift or phase error. Metrological work requires a broader evaluation of the entire chain.

Good practice

  • Use a reliable reference
  • Record configuration and levels
  • Repeat after hardware changes
  • Store curves with date and device information
  • Check clipping before and after correction
  • Associate uncertainty with the reference
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Read more in the Visual Analyser Handbook

This page is an operational introduction. Chapter 2 and 5 — Calibration and uncertainty of the forthcoming book covers theory, controls, algorithms, examples and measurement notes in much greater depth.