Clipping (audio) - AbsoluteAstronomy.com

Clipping is a form of waveform

Waveform

Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...

distortion that occurs when an amplifier

Amplifier

Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

is overdriven and attempts to deliver an output voltage or current beyond its maximum capability. Driving an amplifier into clipping may cause it to output power in excess of its published ratings.

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 can 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 square-wave-type waveform.

Many electric guitar

Electric guitar

An electric guitar is a guitar that uses the principle of direct electromagnetic induction to convert vibrations of its metal strings into electric audio signals. The signal generated by an electric guitar is too weak to drive a loudspeaker, so it is amplified before sending it to a loudspeaker...

players

Guitarist

A guitarist is a musician who plays the guitar. Guitarists may play a variety of instruments such as classical guitars, acoustic guitars, electric guitars, and bass guitars. Some guitarists accompany themselves on the guitar while singing.- Versatility :The guitarist controls an extremely...

intentionally overdrive their guitar amplifier

Guitar amplifier

A guitar amplifier is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker...

s to cause clipping in order to get a desired sound (see guitar distortion

Distortion (guitar)

Distortion effects create "warm", "dirty" and "fuzzy" sounds by compressing the peaks of a musical instrument's sound wave and adding overtones. The three principal types of distortion effects are overdrive, distortion and fuzz. Distortion effects are sometimes called “gain” effects, as distorted...

).

Amplifiers have voltage, current and thermal limits. Clipping may occur due to limitations in the power supply or the output stage. Some amplifiers are able to deliver peak power without clipping for short durations before energy stored in the power supply is depleted or the amplifier begins to overheat.

Amplifier power rating

Power rating

In electrical and electronic engineering, the power rating of a device is a guideline set by the manufacturer as a maximum power to be used with that device. This limit is usually set somewhat lower than the level where the device will be damaged, to allow a margin of safety.The power rating can...

s are typically established by driving the device-under-test to the onset of clipping, to a predetermined distortion level, variable per manufacturer or per product line. Driving an amplifier to 1% distortion levels will yield a higher rating than driving it to 0.01% distortion levels. Similarly, testing an amplifier at a single mid-range frequency, or testing just one of two channels, will yield a higher rating than if it is tested throughout its intended frequency range with both channels working. Manufacturers may use these methods to market amplifiers whose published maximum power output includes some amount of clipping in order to show higher numbers. For instance, the Federal Trade Commission

Federal Trade Commission

The Federal Trade Commission is an independent agency of the United States government, established in 1914 by the Federal Trade Commission Act...

(FTC) established an amplifier rating system in which the device is tested with both channels driven throughout its advertised frequency range, at no more than its published distortion level. The Electronic Industries Association (EIA) rating system, however, determines amplifier power by measuring a single channel at 1,000 Hz, with a 1% distortion level—1% clipping. Using the EIA method rates an amplifier 10 to 20% higher than the FTC method, at the cost of audio fidelity.

Effects of clipping

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

Loudspeaker

A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

Because the clipped waveform has more area underneath it than the smaller maximum unclipped waveform, the amplifier produces more output power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

. (See the waveform to the right for an example.) This extra power can cause damage to loudspeaker components, including the woofer
Woofer
Woofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...

, tweeter
Tweeter
A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...

, or crossover
Audio crossover
Audio crossovers are a class of electronic filter used in audio applications. Most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and lack of distortion so most hi-fi speaker systems use a...

, via overheating.
In the frequency domain
Frequency domain
In electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....

, clipping produces harmonics at higher frequencies than the unclipped signal. This additional high frequency energy has the potential to damage a loudspeaker's tweeter
Tweeter
A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...

via overheating.

Other effects of clipping include:

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

Digital signal processing

Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of 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

Signed-digit representation

Signed-digit representation of numbers indicates that digits can be prefixed with a − sign to indicate that they are negative.Signed-digit representation can be used in low-level software and hardware to accomplish fast addition of integers because it can eliminate carries...

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, for instance, 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

Arithmetic overflow

The term arithmetic overflow or simply overflow has the following meanings.# In a computer, the condition that occurs when a calculation produces a result that is greater in magnitude than that which a given register or storage location can store or represent.# In a computer, the amount by which a...

", ignoring the most significant bit

Most significant bit

In computing, the most significant bit is the bit position in a binary number having the greatest value...

s 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

Sine wave

The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...

. To avoid this, the overall level of a mix can be lowered, or a limiter

Limiter

In electronics, a limiter is a circuit that allows signals below a specified input power to pass unaffected while attenuating the peaks of stronger signals that exceed this input power....

can be used to dynamically bring the levels of the loud parts down (for example, bass

Bass drum

Bass drums are percussion instruments that can vary in size and are used in several musical genres. Three major types of bass drums can be distinguished. The type usually seen or heard in orchestral, ensemble or concert band music is the orchestral, or concert bass drum . It is the largest drum of...

and snare drum

Snare drum

The snare drum or side drum is a melodic percussion instrument with strands of snares made of curled metal wire, metal cable, plastic cable, or gut cords stretched across the drumhead, typically the bottom. Pipe and tabor and some military snare drums often have a second set of snares on the bottom...

s).

It is not simple to eliminate all clipping, as filtering (e.g. a high-pass filter

High-pass filter

A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...

) 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

Complex hard-clipped signals (recorded at CD resolution or less) cannot be restored to their original state because the information contained in the peaks that are clipped is completely eliminated. Soft-clipped signals can be restored to their original state to within a case-dependent tolerance because no part of the original signal is completely eliminated. In this case, the degree of information loss is proportional to the degree of compression caused by the clipping. Lightly clipped bandwidth-limited signals that are highly oversampled have a high likelihood of perfect repair.

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

Interpolation

In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....

or extrapolation

Extrapolation

In mathematics, extrapolation is the process of constructing new data points. It is similar to the process of interpolation, which constructs new points between known points, but the results of extrapolations are often less meaningful, and are subject to greater uncertainty. It may also mean...

of known samples. While this is only an approximation of the original, the subjective quality is usually improved, sometimes with no audible difference.

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

Sound Forge

Sony Sound Forge is a digital audio editing suite by Sony Creative Software which is aimed at the professional and semi-professional markets....

, Adobe Audition

Adobe Audition

Adobe Audition is a digital audio workstation from Adobe Systems featuring both a multitrack, non-destructive mix/edit environment and a destructive-approach waveform editing view.-Origins:...

, Nero Wave Editor, and a plugin in the Audacity

Audacity

Audacity is a free software, cross-platform digital audio editor and recording application. It is available for Windows, Mac OS X, Linux and BSD.Audacity was created by Dominic Mazzoni while he was a graduate student at Carnegie Mellon University...

LADSPA package come with clip restoration software. There is also an Audacity plugin called Clip Fix that uses cubic splines to attempt to restore a continuously differentiable signal.

Sources of clipping

In analog

Analog signal

An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

audio equipment, there are several causes of clipping:

The peak-to-peak of a solid-state transformerless amplifier (most integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

and discrete solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

circuits) is limited to the power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...

voltage less a small amount that depends on the design of the circuit (especially the driver configuration) and the saturation voltage (V_ce(sat) for bipolar transistors, or R_ds(on) for Field Effect Transistor
Fet
Fet is a municipality in Akershus county, Norway. It is part of the Romerike traditional region. The administrative centre of the municipality is the village of Fetsund.Fet was established as a municipality on 1 January 1838...

s), and further reduced if the output stage does not have a quiescent DC output voltage set to half the supply voltage. For example, with a typical operational amplifier the Absolute Maximum Rating for the supply voltage is 36 volts, but a safe operating design supply voltage is 30 volts; if this was supplied as a perfectly balanced +15V and -15V then the theoretical peak output for an ideal rail-to-rail output opamp would be 15 Volts peak (10.6V RMS
RMS
-Science and technology:* Root mean square, a concept encapsulating the "average", in some sense, of a quantity. Frequently encountered in statistics, the physical sciences and electronics...

, 30V peak-to-peak), but a real-world opamp such as the 741
741
Year 741 was a common year starting on Sunday of the Julian calendar. The denomination 741 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.- Europe :* Carloman, Pippin the Short and Grifo succeed...

is likely to only be able to drive about 10 volts peak into loads above 2 kilohms, i.e. about 7.1V RMS).
An amplifier may have an asymmetrical output swing, perhaps because a transistor is biased so its collector voltage is not half the supply voltage (or the "balanced" power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...

rails aren't perfectly balanced); clipping may begin earlier on one half of the output waveform. Bootstrapping
Bootstrapping (electronics)
In the field of electronics, a bootstrap circuit is one where part of the output of an amplifier stage is applied to the input, so as to increase the input impedance of the amplifier...

or a redesign of the circuit may alleviate this when it is caused by difficulties in driving emitter follower output stages.
If the power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...

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 that contains some AC line frequency harmonics.
A vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

can only move a limited number of electrons in an amount of time, dependent on its size, temperature, and metals. Usually fall-off in amplification with increasing output current results in "soft clipping".
Amplifying devices may also have limits on their inputs, for example the maximum base current a bipolar transistor can take, and a vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

may have problems with grid current if the input signal becomes too positive. These factors can distort (clip) the input signal, if it comes from a high enough impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

source, or destroy the device (and so the designer will probably employ other limiting circuits; see below).
A transformer (most commonly used between stages in tube equipment) will clip when its ferromagnetic core becomes electromagnetically saturated
Saturation (magnetic)
Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off...

.
An amplifier can limit the current output, or the input voltage, for a variety of reasons both intentional or not. Intentional limiting circuits would not be expected to come into effect in normal operation, but when the output load resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

is too low or the system is connected to an exceptionally high signal level, for example. The result of this form of clipping might not create a flat top to the Voltage waveform, 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 tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

s (especially when used with little or no negative feedback

Negative feedback

Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

) is superior to that of transistors, in that vacuum tubes clip more gradually than transistors, resulting in harmonic distortion that is generally less objectionable. This gradual onset of clipping is known as gain compression

Gain compression

Gain compression is a reduction in 'differential' or 'slope' gain caused by nonlinearity of the transfer function of the amplifying device. This nonlinearity may be caused by heat due to power dissipation, or by overdriving the active device beyond its linear region...

or "soft clipping". Circuits can be designed using either tubes or transistors to achieve this effect, and the behavior can be simulated with digital processing.

Clipping detection

Clipping in a circuit can be detected by comparing the original input signal with an output signal that has been adjusted for changes in applied gain. For instance, if a circuit has 10 dB of applied gain, it can be tested for clipping by attenuating the output signal's gain by 10 dB and comparing it to the input signal. If the circuit is driven into clipping, the attenuated output signal will show less voltage in the comparison. The electrical offset between the two signals can be used to illuminate clipping detection indicators, such as a red LED

LEd

LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

, and can be used to decrease the gain of a preceding circuit so that the level of clipping distortion can be limited.

In the late 1990s, Sony introduced VACS (Variable Attenuation Control System) on most stereo/component systems in the market. This feature prevents clipping which commonly occurs during high volume playback.