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.
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 below 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).
Asymmetrical clipping can occur in audio circuits that operate on single-ended power supplies (typically only a positive voltage and ground) and in circuits with "AC coupling." Single-ended audio circuits must employ AC coupling to allow the audio waveform to be "re-centered" approximately half-way between the supply voltage and ground. There are a number of reasons why the waveform may not be perfectly centered at one-half the supply voltage:
- The method of centering is either not precise or not centered by design.
- AC coupling in conjunction with asymmetric audio waveforms introduces "DC error" in the waveform. DC error is basically a shifting of the entire waveform upwards or downwards from the original position in the amplitude domain and is typically as a result of the tendency for AC coupling to cause the power of the negative and positive sections of the waveform to be equal (or in other words to "cancel each other").
DC error can also be present in bipolar amplifiers with AC coupling, and the end effect is the same for clipping- it will be asymmetrical.