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Clipping (audio)
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Clipping is a form of waveform distortion that occurs when an amplifier is overdriven, which happens through attempts to increase the voltage or current beyond its maximum power capability. Driving an amplifier into clipping causes it to put out power in excess of its published ratings, which are customarily done with a "clean" sine wave signal just at the onset of clipping.
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Clipping is a form of waveform distortion that occurs when an amplifier is overdriven, which happens through attempts to increase the voltage or current beyond its maximum power capability. Driving an amplifier into clipping causes it to put out power in excess of its published ratings, which are customarily done with a "clean" sine wave signal just at the onset of clipping. This makes it a sometimes unexpected cause of loudspeaker failure.
Overview of clipping
When an amplifier is pushed to create a signal with more power than its power supply can produce, 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 sine wave becoming a distorted squarewave type waveform.
Many electric guitar players will intentionally overdrive their guitar amplifiers to cause clipping in order to get a desired sound (see guitar distortion).
All amplifiers have voltage and current limits. Some amplifiers will use creative techniques to increase these limits or decrease them as needed. If an amplifier needs to output 20 V, but is limited to +10 V to -10 V, then the amplifier will output +10 V. It is best not to speak of a power limit outside of specific applications where the amplifier has an output impedance nearly equal to the load. For instance, a modern solid state amplifier may achieve maximum power output into a load that is ten times smaller than it can thermally handle over a long period of time.
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 maximum unclipped waveform, the amplifier produces more output power. (See the waveform to the right for an example.) This extra power can cause damage to any part of the loudspeaker, including the woofer, tweeter, or crossover, via overheating.
- In the frequency domain, clipping produces harmonics at higher frequencies than the input signal. Extra high-frequency weighting of a signal is more likely to damage tweeters than a signal that was not clipped as higher high frequency power levels are delivered to the tweeter. However, most loudspeakers are designed to handle signals with abundant high frequencies, like cymbal crashes, which have a greater high-pitch frequency weighting than amplifier clipping could produce. Therefore damage attributable to this characteristic is rare.
Other effects of clipping include:
- When applied to a musical signal, the clipping may prevent a note from decaying in a normal amount of time. This can cause rapidly played notes to blend together.
- Music which is clipped experiences amplitude compression, whereby all notes begin to sound equally loud because loud notes are being clipped to the same output level as softer notes.
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 gross distortion of the signal.
Avoiding clipping
As seen on the oscilloscope, the wave resulting from the clipping is not a full sine wave. To avoid this, the overall level of a mix can be lowered, or a limiter can be used to dynamically bring the levels of the loud parts down (for example, bass and snare drums).
It is not simple to eliminate all clipping, as filtering (e.g. a high-pass filter) can align various frequencies in such a way as to create excessive peak outputs. The excessive peaks may become clipped even though the amplifier can play any single sine wave without clipping. As such, some audiophiles will use amplifiers that are rated for power outputs over twice the speaker's ratings.
Repairing a clipped signal
It is impossible to restore a clipped signal to its original state because part of the original signal was lost. It is preferable to avoid clipping, but if a recording has clipped, and cannot be re-recorded, repair is an option. The goal of repair is to make up a plausible replacement for the clipped part of the signal.
Several methods can partially restore a clipped signal. Once the clipped portion is known, one can attempt partial recovery. One such method is interpolation or extrapolation of known samples. While this approximation is rarely close to the original, the subjective quality may be improved.
Other methods may also be used. One of the methods in CuteStudio Declip, for example, works by copying the signal directly from one stereo channel to another, as it may be the case that only one channel is clipped.
Several software solutions of varying results and methods exist to counteract this problem: Sony Sound Forge, Adobe Audition, Nero Wave Editor, and a plugin in the Audacity LADSPA package come with clip restoration software. There is also a Nyquist plugin called for Audacity.
Sources of clipping
In analogue audio equipment, there are six general 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).
- If the power supply capacitor is no longer able to keep the voltage "flat" due to a massive current draw, the positive and negative voltage supply of the amplifier will fluctuate resulting in sort of a clipped signal (AC line frequency harmonics).
- 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.
- While less common, an amplifier can limit the current output for a variety of reasons both intentional or not. The result of this form of clipping may not create a flat top to the Voltage wavefrom, but rather a flat top to the current waveform.
- Certain signal processing elements can produce a unique form of phase-inverted clipping when the input signal exceeds the common-mode input range of an opamp. The result is that the voltage waveform clips, but in the wrong direction.
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.
Length clipping
The term clipping may also refer to the shortening of voice snippets due to failures in voice activity detection equipment. It is unrelated to amplifier saturation.
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