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Clipping (audio)

From Wikipedia, the free encyclopedia

The picture shows an oscilloscope screen of an amplifier "clipping."  The amplifier should be outputting a clean sine wave with rounded tops and bottoms, but instead they are cut off flat, or "clipped", as if they had been cut with a pair of scissors or shears.
The picture shows an oscilloscope screen of an amplifier "clipping." The amplifier should be outputting a clean sine wave with rounded tops and bottoms, but instead they are cut off flat, or "clipped", as if they had been cut with a pair of scissors or shears.
This PCM waveform is clipped between the red lines
This PCM waveform is clipped between the red lines

Clipping is a form of distortion that occurs when an amplifier is overdriven, which happens when it attempts to increase voltage or current beyond its limits.

Overview of clipping

When an amplifier is pushed to create a signal with more power than it can support, it will amplify the signal only up to its maximum capacity, at which point the signal will be amplified no further. As the signal simply "cuts" or "clips" at the maximum capacity of the amplifier, the signal is said to be "clipping." The extra signal which is beyond the capability of the amplifier is simply cut off, resulting in a distorted waveform.

Many electric guitar players intentionally overdrive their guitar amplifiers to cause clipping in order to get a desired sound. See Guitar distortion.

All amplifiers have a maximum amount of power, so theoretically they all can clip. However, many amplifiers will not allow a user to turn gain up beyond that point, so that the amplifier never clips. This is sometimes to reduce accidental distortion, or more importantly, to keep from damaging the amplifier as some amplifiers cannot withstand the heat generated by some forms of clipping.

Effects of clipping

In power amplifiers, the signal from an amplifier operating in clipping has two characteristics that could damage a connected loudspeaker:

  • Because the clipped waveform has more area underneath it than the smaller unclipped waveform, the amplifier produces more power when it is clipping. This extra power can damage any part of the loudspeaker, including the woofer or the tweeter, by causing overexcursion, or by overheating the voice coil.
  • In the frequency domain, clipping produces strong harmonics in the high frequency range. The extra high frequency weighting of the signal could make tweeter damage more likely than if the signal was not clipped. However most loudspeakers are designed to handle signals like cymbal crashes that have even more high frequency weighting than amplifier clipping produces, so damage attributable to this characteristic is rare.

Digital clipping

In digital signal processing, clipping occurs when the signal is restricted by the range of a chosen representation. For example in a system using 16-bit signed integers, 32767 is the largest positive value that can be represented, and if during processing the amplitude of the signal is doubled, sample values of 32000 should become 64000, but instead they are truncated to the maximum, 32767. Clipping is preferable to the alternative in digital systems wrapping which occurs if the digital hardware is allowed to "overflow", ignoring the most significant bits of the magnitude, and sometimes even the sign of the sample value, resulting in terrible modification of the signal.

Sources of clipping

In analogue audio equipment, there are three common causes of clipping.

  • An integrated circuit or discrete solid state amplifier cannot give an output voltage larger than the voltage it is powered by (commonly a 24- or 30-volt spread for operational amplifiers used in line level equipment).
  • A vacuum tube can only move a limited number of electrons in an amount of time, dependent on its size, temperature, and metals.
  • A transformer (most commonly used between stages in tube equipment) will clip when its ferromagnetic core becomes electromagnetically saturated.

Some audiophiles believe that the clipping behavior of vacuum tubes is superior to that of transistors, in that vacuum tubes clip more gradually than transistors, resulting in harmonic distortion that is generally less objectionable.

See also

  • Loudness war
  • Valve sound
  • Clipper
  • Overdrive (music)
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