Any change in the waveform or harmonic content of an original signal as it passes through a device. The result of nonlinearity within the device.
- Linear distortion = magnitude and phase distortion
- Nonlinear distortion = new spectral components are produced
In a nonlinear system a sine wave generates harmonics:
If total rms level is:
Then the harmonic distortion:
- Harmonic distortion is not a particularly good measure from a perceptual point of view
- Low-order harmonic distortion may improve perceived quality
- JND~1% for 2nd, ~0.3% for 3rd, ~0.1-0.3% for 4th harmonic
Other distortion mechanisms:
Intermodulation distortion (IM)
- Sine waves, of f1 and f2 generate f1-f2 , f1+f2 etc.
- IM describes perceived distortion better than HM
Transient intermodulation distortion (TIM)
- Distortion that is created in fast transients but not in steady state signals
Quantization noise in digital signal processing
- Perceived as distortion if correlation with signal
- Perceived as noise if not correlated
Pre-echo in audio coding
- Temporal spreading of a signal in time "backwards"
Perceptual criteria needed in digital audio instead of simple distortion and SNR measures.
See also: Intermodulation Distortion.