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Sound Card Distortion - Theory And Measurement
Distortion is classified as either linear or nonlinear. Linear distortion includes processes that may change the level or phase of a signal or its individual frequency components, but not add any new components. Ordinarily these processes would be described by their effects, such as "frequency response" or "phase shift", rather than "linear distortion".
A nonlinear system (or portion of a system) is one whose output is not simply proportional to its input, but instead is related by some nonlinear equation that may contain squared or higher power terms. All systems exhibit nonlinear behavior at their limits (or those wouldn't be the "limits"). For example, an amplifier has a maximum output voltage determined by its power supply, and a loudspeaker has a maximum displacement determined by its construction.
Most modern solid-state amplifiers exhibit "hard" limiting, where they behave linearly up to some maximum voltage, then can't go beyond that limit... they simply "clip" there.
Loudspeaker limiting behavior is similar but "softer": The cone encounters increasing stiffness as it nears its suspension limits, and decreasing driving force as the voice coil moves out of its magnetic field, but no abrupt "brick wall" limit as with amplifier clipping, until the voice coil actually bottoms out.
In addition to these limiting or "saturation" effects, some systems have particular problems with very small signals. Many power amplifiers encounter a problem called "crossover" distortion. They use "push-pull" circuitry with separate portions for handling positive and negative parts of the signal. This improves efficiency, since only one portion needs to be active at a time.
As the signal passes through zero, the amplifier must switch between the positive circuits and the negative circuits. If this hand-off is not done smoothly, there may be a small region where neither portion is working and the output jumps to zero prematurely.
Since distortion is typically reported (and perceived) as a percentage of the output, an amplifier may produce very high output power with very low distortion when it is below its clipping point, yet at low output power that same amplifier may produce an intolerable amount of distortion due to crossover problems: At low power, the signal is spending most of its time near zero, so the distortion there is a bigger percentage of the total output.
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