Have you ever heard of this phenomenon in audio? Nonetheless, if you have been involved in audio engineering for a while, you will surely have experienced it before. It is a problem that almost all beginner sound engineers will struggle with during their early years. Hence, the golden question we have for today will be – What is Audio Clipping?
I won’t go through everything there is to know about today’s subject (If I do, I’ll probably have an aneurysm by the end). There are a lot of technical concepts that only an electrical engineer could fathom. But don’t worry, there will still be enough information for an audio practitioner to understand the causes, effects and solutions to this problem!
Audio Clipping – What is it?
When an audio power amplifier (simply called power amp) is being overdriven and forced to produce an output voltage or current beyond its maximum capability, a type of waveform distortion called audio clipping will occur as a result. If you drive an amplifier into clipping, you may cause it to output power at a level that is above its power rating.
It is pointless to try and push an amplifier to create a signal that requires more power than the amp’s power supply can produce. The signal will only be amplified up to the amp’s maximum capacity, at which point the signal cannot be further amplified. The signal simply “cuts” or “clips” at the maximum capacity of the amplifier, hence the term “clipping”.
Any extra signal that is beyond the capability of the power amp will simply be cut off, resulting in a sine wave becoming a distorted square-wave-type waveform. In music production, electric guitar players intentionally overdrive their guitar amplifiers to cause clipping and other forms of distortion to creatively produce the desired sound.
Every audio power amplifier in the market have voltage, current and thermal limits. Audio clipping may be caused by limitations in the power supply or the output stage. However, there are some amplifiers that are able to produce peak power without clipping for a short time, before it begins to overheat or the energy stored in the power supply is depleted.
Let’s look at the topics we will be covering today:
- Damaging effects
- Digital clipping
- Preventing clipping
- Repairing clipped signals
When an audio power amplifier is operating in clipping, the electrical audio signal that is produced has two characteristics that could damage a connected loudspeaker:
- A waveform that is clipped has more area underneath, as compared to a smaller maximum unclipped waveform, resulting in the amplifier producing more output power. This extra power can cause loudspeaker components such as the woofer, tweeter or crossover to overheat and eventually be damaged.
- In the frequency domain, a clipped waveform produces harmonics at higher frequencies as compared to an unclipped signal. This extra high frequency energy could potentially damage a loudspeaker via overheating.
To prevent clipping, most power amps have circuits that compare the input signal to the output signal to detect the presence of extra power due to distortion. The simplest circuits are basically fast limiters, that engages after detecting about one decibel clipping. A more complex circuit, known as “soft-clip”, is used to limit the signal at the input stage.
The soft-clip circuit is designed to engage before clipping. As an example, it may start engaging the audio signal at 10 dB below maximum output power. This results in the output waveform maintaining a rounded characteristic even when it is affected by an overloaded input signal, as much as 10 dB higher than the maximum specified.
A digital audio signal will be clipped if it is restricted by the range of a chosen representation. For instance, in a 16-bit system, 32767 is the highest positive value that can be represented. So, if the signal’s amplitude is doubled during processing, let’s say, the sample value of 32000 should become 64000, but instead they are truncated to the maximum (32767), then clipping will occur.
A sound wave that is clipping is not a full sine wave. To prevent this, you should lower the overall level of the mix or use a limiter to dynamically bring down the levels of the loud audio elements in a mix (for example, bass and snare drums).
It is hard to eliminate all clipping, as filtering (e.g. a low-pass filter) can create excessive peak outputs by aligning frequencies in a certain way. Even if the amplifier can deliver any sine wave without clipping, the excessive peaks may still be clipped. Hence, some audiophiles will use amplifiers that are rated for power outputs over twice the speaker’s ratings.
Repairing Clipped Signals
It would be ideal to avoid clipping at all costs. However, if a recording has clipped and cannot be re-recorded, you can consider repairing it. The aim of repairing is to produce an acceptable replacement for the signal’s clipped section. Hard-clipped signals cannot be fully recovered as the information contained in the peaks that are clipped is entirely removed.
It is possible to restore soft-clipped signals to their original state (depends on the severity of the case) because no part of the original signal is completely removed. In this case, the degree of information loss is proportional to the degree of compression (as a result of clipping). Bandwidth-limited signals that are clipped, but highly oversampled, have a high chance of perfect repair.
You can find software applications that are designed to recover clipped recordings. Some examples include Sony Sound Forge, iZotope Rx3, Adobe Audition, Nero Wave Editor, and a clip restoration plugin that comes with the Audacity LADSPA package.
This is all I have for you guys today. I hope all of you budding audio engineers can now produce clean, and clear mixes without audio clipping!
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