Sixty years on from the invention of Rock 'n' Roll, distortion still hasn't lost its appeal. What was originally considered an unpleasant and undesirable side effect of overloading aud1o equipment has become a key element of countless genres of modern music. Although it might conjure up images of metal guitarists shredding away, distortion shouldn't just be thought of as a brutal effect for smashing the hell out of a sound. Subtle applications of distortion can be used to great effect on any instrument.

Let's take a look at the history of distortion and then look at some of the different types of distortion effect. From analogue to digital, valve to solid state, we'll show how they affect the sound and offer some examples of how you can use distortion effects in your music. We'll also take a look at bitcrushing and sample rate reduction, two digital techniques which might not strictly be considered distortion but can be used for similar purposes.

The history of distortion truly begins in the 1950s. Before the proto-rock rhythm and blues recordings of the decade, distortion was considered unwanted noise that occurred as a result of pushing equipment too hard. Rock 'n' Roll pioneers changed everything with their deliberate application of distortion as a special effect. By the late 1960s, distortion was firmly established as part of the musical lexicon. The invention of dedicated overdrive pedals, fuzzboxes and distortion units made it easier to achieve sizzling distorted tones without relying solely on amp distortion. Guitarists including Jimi Hendrix and Eric Clapton helped to ensure that the fuzzbox became an essential weapon in the guitarist's armory, while amplifiers were prized as much for their dirty distorted crunch as for their clean tones.

Distortion can be introduced by a number of elements of your signal chain, including speakers, transformers and various smaller electronic components. However, the sound we now usually know as overdrive or mild distortion primarily originates from overloading valve amplifiers. As valve amplifiers are pushed closer to their limits, they start to behave in a less linear fashion, and as a result the signal is compressed. As the signal continues to exceed the amplifier's maximum level, rt starts to be clipped, meaning that the peaks are compressed and flattened. The waveform is re-shaped, tending towards a squarer wave. The characteristics of different waveforms have a major effect on their sound. The squared off peaks of the distorted signal mean that its sound becomes more like a square wave, rich in pleasant sounding odd harmonics (see FM222 and FM223 for our explanation of harmonics). Anyone who's talked to a guitarist about tone will know there are hundreds if not thousands of ways of describing the different varieties of distorted signal, from creamy to harsh and everything in between.

Rather than thinking about distortion in terms of wave-shaping, we can also consider the effect it has on the harmonic content of a signal. Remembering back to our early discussions of subtractive synthesis (FM213J, you should recall that different wave shapes (square, sine, sawtooth and so on) contain different patterns of harmonics. A square wave, for example, contains the fundamental frequency and all odd-numbered integer harmonics. Therefore, as we clip the waveform we start to introduce more and more odd-numbered harmonics as the wave moves closer and closer to a square shape. Not only is this a good way of understanding how distortion affects a signal, it also allows us to think about why certain types of distortion are considered more pleasing than others.

Looking at harmonics also raises an interesting point that is often overlooked. By distorting a signal, we're adding a lot of content to its frequency range. Take for example a sine wave sub bass line. Without any distortion, the sine wave contains only the fundamental frequency of the note being played. If we distort the signal, this is no longer the case. Distorting the signal adds harmonics, and Increasing the level of distortion also increases the amplitude of these harmonic frequencies. The consequences of this are significant. Firstly, the perceived volume of the sub bass will change as we hear the harmonics and our brain fills in the gaps about the fundamental frequencies. This is particularly apparent on smaller speaker systems - a 40Hz sine wave may be almost inaudible on a small speaker or cheap pair of headphones, but the harmonics induced by distortion will make the sound stand out Secondly, as producers, we need to consider the overall balance of frequencies as we mix. Distorting a part will have knock-on effects on our mix thanks to frequency clashes wherever the harmonics are introduced. If you find yourself having to correct a mix by cutting frequencies out of a distorted part, you may be better off winding back the distortion a touch to avoid going round in circles.

As with most other electronic music equipment, valve distortion was soon followed by solid-state equivalents. A quick note: 'solid-state' refers to electronic circuits which contain components built from solid materials. Whereas valves contain vacuums and occasionally gases, components such as transistors and semiconductors are built entirely from solid materials. These days, if a piece of equipment doesn't contain any valves it's a pretty safe bet that you can call it solid-state. In the 1960s, new transistor and diode technology made it possible to build small pedal-based distortion effects. The principle is similar: force an overly hot signal into a circuit in order to push it beyond its working lits and induce clipping of the waveform. However, since solid-state components behave differently to their valve counterparts, the resulting sound is subtly different Typically, the break-up of a solid-state circuit is more sudden and more aggressive than tube distortion, characterized by higher levels of odd-order harmonics. Although the difference between valve and solid·state is easily identified, things get a little more complex when you try to define different types of solid-state distortion effect, such as fuzzboxes and overdrives. Overdrive is often used to describe valve distortion (or solid-state distortion units which attempt to mimic the sound of valves), whereas fuzzboxes are usually high gain units which square off a signal dramatically.

For many people, distortion can get confusing when we consider digital signals. Digital emulations of analogue distortion effects are now commonly available in hardware or software form. Given that they tend to imitate the sound of valve and solid-state distortion units, similar sounds are usually achieved. However, there's a separate phenomenon also known as digital distortion which is infinitely less desirable.

The clipping which occurs when a signal above 0 dBFS is fed into a digital input is a sudden, harsh squaring off of the waveform as the amplitude tries and falls to exceed its maximum value. This hard clipping is typically characterized by a very abrasive and harsh sound, since the clipping is instantaneous rather than slow and progressive like soft clipping in analogue equipment. Once digital clipping has made it onto a recording it's practically impossible to remove, so it's crucial that these 'overs' are avoided by maintaining good practice and leaving plenty of headroom on digital inputs when recording. Digital distortion can also occur occasionally in other processing so it's important to learn how it sounds and keep an ear out for it just in case something does go awry.

Although it's sometimes not considered a form of distortion in the same sense as a cranked valve amp or analogue fuzzbox, bitcrushing and downsampling a digital signal creates a distorted effect which is also worth considering here. Bitcrushing refers to the process of reducing the resolution of a signal, while downsampling involves reducing the sampling rate. Depending on how far you take them, both processes help to create a low fidelity signal riddled with aliasing and digital imprecision. Many bitcrushing effects also allow you to control the input gain of the signal.

Bitcrushing and sample reduction really seem to be coming into fashion right now as a way of achieving a slightly different distorted sound without turning to the usual valve warmers and distortion effects. Whether you keep it subtle for a bit of authentic '80s fuzziness or crush the hell out of a sound, bitcrushing and downsampling should be considered alongside overdrives and tube effects as a different flavor of distortion.