Clipping is a form of waveform distortion that occurs when an amplifier
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 players intentionally overdrive their guitar
amplifiers to cause clipping in order to get a desired sound.
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 ratings 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 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 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:
Because the clipped waveform has more area underneath it than the smaller
maximum unclipped waveform, the amplifier produces more output power.
This extra power can cause damage to loudspeaker components, including the
woofer, tweeter, or crossover, via overheating.
In the frequency domain, clipping produces harmonics at higher frequencies
than the unclipped signal. This additional high frequency energy has the
potential to damage a loudspeaker via overheating.
As a result, many amplifier designers have incorporated circuits to prevent
clipping. Such circuits compare the input signal to the output signal to
detect the presence of extra power caused by distortion. The simplest
circuits act like a fast limiter, which engages after about one decibel of
clipping is detected. A more complex circuit, called "soft-clip", was used
from the 1980s onward to limit the signal at the input stage. The soft-clip
feature begins to engage prior to clipping, for instance starting at 10 dB
below maximum output power. The output waveform retains a rounded
characteristic even in the presence of an overload input signal as much as 10
dB higher than maximum specified. 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, 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", 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.
It is not simple to eliminate all clipping, as filtering 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 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. Complex hard-clipped signals 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.
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 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, iZotope
Rx3, Adobe Audition, Nero Wave Editor, and a plugin in the Audacity 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 audio equipment, there are several causes of clipping:
The peak-to-peak of a solid-state transformerless amplifier is limited to
the power supply voltage less a small amount that depends on the design of the
circuit and the saturation voltage for bipolar transistors, or Rds(on) for
Field Effect Transistors), 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, but a real-world opamp such as the 741 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;
clipping may begin earlier on one half of the output waveform. Bootstrapping 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 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 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 may have problems with grid current if the input
signal becomes too positive. These factors can distort the input signal, if
it comes from a high enough impedance source, or destroy the device.
A transformer will clip when its ferromagnetic core becomes
electromagnetically saturated. 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 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 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. This gradual onset of clipping is known as gain compression 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, and can be used to decrease the gain of a
preceding circuit so that the level of clipping distortion can be limited.
See also Clipper
Clipping Distortion
Dynamic range compression Loudness war
Tube sound References