On the other hand, when it comes to recording voice or music and reproducing it as close to the original as possible, then distortion is always undesirable. Classification and measurement of these unwanted distortions are discussed below.
Harmonic vs. no harmonic distortion
Non-linear distortions are further divided into harmonic or non-harmonic distortion. In harmonic distortion, all integer multiples of the fundamental frequency are considered. This is a classic distortion measurement called THD (Total Harmonic Distortion). In special applications, the total number of specific harmonics is considered. Non-harmonic distortions are those frequencies that are not multiples of the fundamental frequency, as is the case with IMD (intermodulation distortion).
Devices that measure THD are widely available and easy to use. There is one major drawback for devices with low bandwidth range; for test signals with high frequencies, the harmonic distortions go out of range. For accurate readings, you need either a measurement system with a high bandwidth range, or you need to measure the IMD. For IMD measurements, two or more pure tones with independent frequency and amplitude are used as test signals. Non-linear distortions will then produce sums and differences of multiples of these input frequencies. The distortions are thus distributed throughout the entire analysis bandwidth.