Audio Precision: Nonlinear Audio Distortion
Dr. Richard C. Cabot, Bruce Hofer, Robert Metzler
Distortion is a measure of signal impurity. It is usually expressed as a percentage or decibel ratio of the undesired components to the desired components of a signal. Distortion of a device is measured by feeding it one or more sine waves of various amplitudes and frequencies. In simplistic terms, any frequencies at the output which were not present at the input are distortion. However, strictly speaking, components due to power line interference or other spurious signals are not distortion. There are many methods of measuring distortion in common use: harmonic distortion and at least three different types of intermodulation distortion. These are different test procedures rather than different forms of distortion in the device under test.
The transfer characteristic of a typical device is shown in Figure 13.3.1. This represents the output voltage at any point in the signal waveform for a given input voltage; ideally this is a straight line. The output waveform is the projection of the input sine wave on the device transfer characteristic. A change in the input produces a proportional change in the output. Because the actual transfer characteristic is nonlinear, a distorted version of the input waveshape appears at the output. Harmonic distortion measurements excite the device under test with a sine wave and measure the spectrum of the output.
Because of the nonlinearity of the transfer characteristic, the output is not sinusoidal. By using Fourier series, it can be shown that the output waveform consists of the original input sine wave plus sine waves at integer multiples (harmonics) of the input frequency. The spectrum of the distorted signal is shown in Figure 13.3.2. For a l-kHz input, the output consists of 1 kHz, 2 kHz, 3 kHz, etc. The harmonic amplitudes are proportional to the amount of distortion in the device under test. The percentage harmonic distortion is the rms sum of the harmonic amplitudes divided by the rms amplitude of the fundamental.