Clipping

Overview

The term "clipping" is used to describe a type of waveform distortion with the characteristic of a horizontal "flat" section on the top or bottom section of the waveform. There can be symmetrical and asymmetrical clipping, depending on factors such as whether it occurs in the digital or analog domain and the type of audio circuitry in the analog domain.

Basics

Audio waveforms have two domains- the time domain which is the horizontal scale, and the amplitude domain, which is the vertical scale. Clipping is by definition an amplitude issue and will occur at a specific amplitude which depends on the cause.

For example, in an "ideal" audio amplifier operating on a "bipolar" power supply voltage, when the output level would need to exceed the supply voltage to properly reproduce the input waveform; the section of the waveform that "exceeds" the supply voltage is replaced by a "flat line" at the supply voltage; because the amplifier output cannot exceed that voltage. In reality, the output cannot exceed a voltage somewhat below the supply voltage because there must be circuitry "between" the supply voltage and the output, for the amplifier to function. The difference is typically in the range of 1-2 volts; depending on the design of the amplifier, with special designs being able to amplify to a voltage less than 1 volt less than the supply voltage.

The resulting "clipped" waveform will have the top and bottom sections equally affected (at very nearly the same voltage positive and negative), and this is referred to as "symmetrical clipping."

In digital audio, a form of encoding is commonly used that employs a "sign bit." The other bits determine the amplitude of the waveform, and the sign bit determines whether the amplitude is positive or negative. A positive amplitude means the section of the waveform above "zero amplitude" and a negative amplitude means the section of the waveform below. In the analog example (above); this would correspond to a positive or negative voltage. Because of this, digital clipping is typically perfectly symmetrical in nature (it occurs at exactly the same amplitude positive and negative).