How distortion is measured by a distortion meter?

A distortion meter measures audio signal distortion by comparing the output of an audio device to its input. It typically analyzes the waveform of the signal and calculates the percentage of distortion, which can include harmonic distortion and intermodulation distortion. By using fast Fourier transform (FFT) techniques, the meter identifies and quantifies unwanted frequency components introduced by the device under test. The results help assess the fidelity and performance of audio equipment.