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HTG Explains: How Do Guitar Distortion and Overdrive Work?

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With so many musical genres, it’s no surprise there are plenty distortion pedals out there. But what makes them so different? Let’s take a closer look at what happens to audio signals as they pass through these relatively simple devices.

Distortion is a general term for any modification to an audio signal that provides a significant alteration. The world of music indeed has quite a few different types. But how does it all work? To answer that, we need to look at how sine waves are affected by volume.

Clipping and Distortion

Basic overdrive and guitar distortion can be visualized by the effect of clipping. We mentioned clipping in a previous article, HTG Explains: How Does Dynamic Range Compression Change Audio? Compression helps prevent clipping, but in this case, we want to emphasize it.

Clipping_waveform

(Image credit: Wikimedia Commons)

In the original signal, you can see that the sine wave exceeds the threshold of the device. Normal waves that are within the proper threshold sound smooth. As the playback devices can’t really exceed the threshold, what happens is the crests and troughs of the wave start to square off. This changes the quality of the sound. Why? Well it has to do with math.

Let’s zoom in on a sine wave.

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Now, imagine we play another tone alongside this one, something with a higher frequency but that matches at the peaks. We’ll only introduce it at a low amplitude. Here’s what the result looks like.

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You can see that it starts to take the shape of that square-cornered wave from the clipping section. When you introduce an odd-numbered overtone, you’ll start to see this type of shape. If we increase the amplitude of that same overtone, you’ll see a more particular shape.

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So you can see those sharp corners form a little more prominently. We can exaggerate this further with the addition of yet another odd-numbered overtone.

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Having a lot of clipping changes the shape of the sine wave in a way that is mathematically represented by a different equation entirely, shown above as the addition of two sine waves. The harder the clipping, the greater the resemblance to a increasingly complex waves. Softer clipping won’t really affect the sound too much.

Let’s take a look at what a close up of some distorted waves in Audacity.

waveforms

Here, I’ve highlighted a portion of the waves that match up. The second wave is a distorted sine wave, something that looks like it was clipped and then compressed down. It’s a square wave. Here’s a sample of a 440 Hz – middle A – sine wave, and a 440 Hz square wave.

A 440Hz Sine (No Clipping) Wave

A 440Hz Square (Clipped) Wave

We’ve seen what happens with odd-numbered overtones. Even-numbered overtones do something different.

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Compare this to the third wave in the Audacity screenshot above. This is referred to as a sawtooth wave, and sounds very different.

A 440Hz Sawtooth Wave

While we’ve skipped the math, we hope you see how wave addition simulates the effects of clipping in different fashions. Differently shaped waves change the quality of the sound in some very important ways. This is partially why distorted guitars have such a rich set of overtones and why there are so many kinds of distortion pedals out there.

Overdrive

There are many different types of distortion, one of the most common being overdrive. It works by applying an increase in gain, at specific outputs. Softer playing doesn’t really cause the telltale distortion to occur, but harder playing or a higher signal volume to the overdrive processor will cause the telltale clipping patterns to come through. Overdrive offers softer clipping, which helps keep the original timbre of the instrument more or less in tact, or else tries to make up for some of the loss.

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Overdrive was originally found with tube amplifiers where increasing a voltage gain would “overdrive” the amp and produce the desired effect. Modern overdrive processors, such as those found in pedals, try to replicate this for amps that aren’t tube based. They require a higher volume from the amp to help create the effect in addition to some “color mixing” to help simulate the effect well. This last function is most easily seen in the tone dial. Overdrive preserves a good deal of dynamic range and can still produce some clean sounds, but can let some of those overtones come out shining with some push.

Distortion

Overdrive, while still technically distortion, is grouped separately because of its mild effect and it’s primary reliance on controlled clipping. More common distortion pedals, such as the grunge and metal stompboxes that are so common today, are more bold about their fluctuation. Instead of relying on gain fluctuations, they alter the shape of the wave in distinct patterns and do it in a way that isn’t dependent on the amount of gain. Overdrive’s “warmer” overtones are lost here, as well as a significant amount of the original timbre.

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Outright distortion really cuts out the dynamic range and adds some equalizer effects. Usually, the middle range is what we can hear best, so to make up for that the equalizer settings are set up to boost the high and low end. This is why the lower notes really drive metal, and why the pinch-harmonics which are barely audible normally really screech with distortion. Each type of distortion pedal has a particular shape it pushes its signal towards as well as specific EQ settings and some in-house special mixing thrown in, so it’s easy to be overwhelmed when looking at which to buy. Be sure to give each a listen and play with their settings to get a full grasp of what it can do.

Fuzz

Another really popular and specific type of effect is fuzz, used widely in the industrial  and metal genres and is often used for vocals as well as instruments. Fuzzboxes add a particular type of distortion that sounds just as its name implies. The original signal is wholeheartedly obliterated and turned into a square-waveform. It’s almost as if it hits a brick wall before continuing in a completely transformed shape.

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Fuzzboxes also add extra harmonic overtones to help give an artificially rounded out and warmer sound. This is done by an adjustable frequency multiplier, and if a harsher sound is desired, can yield inharmonic overtones instead. Actually, these artificially added overtones add a lot to string melodies and provide a good backdrop. Sitars bank on these same harmonics, and if you’ve ever heard one plugged into a regular distortion pedal, you’d swear it was in a fuzzbox instead.


Now that you know why distortion does what it does, you should be able to alter it to help make your particular playing style more pronounced. You can even use your knowledge of equalizers to help the process. And, while we primarily discussed these effects in light of guitars, they can be applied to vocals and other instruments as well. Experiment and you break the ever-dissolving genre barriers present today!

 

Yatri Trivedi is a monk-like geek. When he's not overdosing on meditation and geek news of all kinds, he's hacking and tweaking something, often while mumbling in 4 or 5 other languages.

  • Published 05/20/11

Comments (3)

  1. Hatryst

    I don’t have an electric guitar, but this would surely be helpful in some guitar synths that I use ;)

  2. Snert

    I’ve been a guitar affects geek since ’73 and still have some several bread-board units I made. Over 30 some years later and they STILL work. They look like junk but they work and that’s the whole idea. And I feel a certain sense of pride when I use them. “I made this meself!!!”

  3. BoxunloX

    Great article. I have been playing guitar for around 12 years and until recently was playing professionally in a rock band and I truly appreciate the great work in this article. HTG rocks!

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