What is Harmonic Distortion

An evil monster to avoid, keeping levels out of reach from the reds, or a warm familiar companion adding body and soul to our tracks? Distortion can be both. But what actually is it?

Distortion happens when a source sound gets modified during any of the stages of the signal chain. Audio equipment specs include a THD (Total Harmonic Distortion) value, showing the amount of distortion happening to the signal when it passes through the device (the smaller, the better). Harmonic distortion is a tone distortion – it affects the tone quality of the sound – the quality and quantity of harmonic content. What happens to the sound waves? For example, if the amount of signal going through a preamp is too hot (the actual volume, which in the process is transformed in voltage) then the sound waves begin to square off – their shape gets modified and the top and bottom of the waveforms gets chopped off. This creates even and odd harmonics that where not present in the original sound. The signal gets modified – distorted.

Different perception of valves and transistor distortion

When overdriven, analog circuits built with transistors tend to create odd harmonics – this apparently is perceived by our brain as not pleasant, harsh. Instead, valve circuits produce even harmonics of sounds – and our brain response tends to associate this with a warmer, fuller sound enrichment. Why does this happen? There is some very complicated literature about that, so we’ll just accept the fact that – generally speaking – odd harmonics produce ear fatigue, while their even “counterparts” not only are tolerated, but pleasant and musically perceived as a sort of “chanting choir”, coloring the fundamental.

Analog Tape Distortion/Saturation

Saturation is a type of distortion. It generates the distinctive sound that we hear and love on many productions (and lets not forget that everything before approx 1992 had to go throught tape…) Generaly speaking, saturation occurs when a signal is printed hot to tape, in the range of soft clipping. But the sound that comes out of it is a sum of different all tape-related factors, consisting of random and imperceptiple changes on signal caused by the variations in tape speed (eg. dirty tape heads). These variation (wow and flutter) affected the whole spectrum, and because of the recording and mixing procedure – tracks passed through several stages of tape recording before making it to the final master tape – they where still barely audible, but – combined with the original hot signal to tape, definitely part of the whole sound.

Analog distortion/saturation plugins

Here’s some suggestions to give analog feel to single tracks or mixes. The plugins below (apart from the Roundtone) are either free or available in demo version so you can try them out:

TapeHead Saturator by Massey Plugins. From subtle to extreme settings, a versatile, easy to use tool that you can use to get your tape machine sound. The plugin features three controls: Drive and Trim and and Normal/Bright switch. You can download a free demo version on the Massey Plugins site.

SoftTube Saturation Knob: this little amazing plugin has three controls – modes: Keep High, Neutral and Keep Low, for three different kinds of characters of distortion. Available as insert for your DAW or as module for the Eurorack Modular platform. You can download it here for free.

TDR Slick EQ is easy to use, flexible and precise. The additional harmonic content is created through the switchable non-linearity EQ and 4 differe saturation models: “American”, “British”, “German” and “Soviet”. You can download for free here

FerricTDS by Variety of Sound. If you want to try out the dynamics of high end tape recorders, then this plugin is the right tool. Acting on dynamics, saturation and limiting, this vintage looking tool can give you the analog feel you are looking for. It can be downloaded on the PluginBoutique website at this link.

Roundtone – Multitrack tape machine and Delay. This is a tape machine emulator, that can be used on single tracks (mono/stereo) on submixes and mixes. You can choose between several algorithms and you can use it without worrying too much about CPU, as it is light and latency free. The free download is available on the Sknote site here.

What’s the point in emulating analog sound anyway…

Apart from the obvious fact that some musical genres may benefit of vintage sound qualities, it is true that the old school studio equipment was able to create a full – glued sound, where imperfections where smoothed and sound was probably less accurate and hi-fi, but great in terms of dynamic range and overall energy and body.