Why is the curve symmetric rather than biased?

Because a symmetric curve leaves the waveform's positive and negative halves matched, which prevents a direct current offset appearing in the output. Biased, asymmetric curves add even harmonics and a thump at the start of a note, which is characteristic of some hardware but is not what this effect is for.

Why does distortion make a part sound louder?

Because the curve lifts quiet passages more than loud ones. At a drive of five a very quiet passage comes up by several times, while a full scale one is merely rounded over, so the distance between loud and quiet shrinks. That compression is as much of the effect as the added harmonics.

What does fold-back-free mean?

That the curve always increases with input, so a louder input never produces a quieter output. Curves that wrap around when they exceed their range break that rule and produce an unpleasant crackle on loud passages, which is what a strictly increasing curve avoids.

Should I put anything after the distortion?

A gentle equaliser to tame the added high frequencies is common, and a limiter if the result is now over level. Both are separate tools here, which keeps the distortion itself to one variable.

Distortion Effect

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Distortion Effect adds harmonic grit with a single drive control. The shaping curve is smooth and symmetrical, so it rounds peaks over rather than squaring them off.

What is Distortion Effect?

Set the drive from one to thirty, drop a file, and process; the footer reports how much a quiet passage is lifted at that setting.

The control and the curve:

A drive control from one to thirty, defaulting to five.
Four presets named for what they sound like: a warm setting at three, a crunch at eight, a heavy one at fifteen and a fuzz at twenty-five.
A footer reporting how far a quiet passage at a tenth of full scale is lifted by the chosen curve, which is a direct readout of how much the shaping compresses the dynamic range.

The curve is a hyperbolic tangent scaled by a normalisation factor, and its mathematical properties are the reason it sounds the way it does. It is odd symmetric, meaning that a positive input and the corresponding negative input produce exactly opposite outputs, which is why it adds no direct current offset to the signal. It maps full scale to full scale in both directions, so the peak level is preserved rather than reduced. It is strictly increasing, so a louder input always produces a louder output, and that is what keeps it free of the fold-back artefacts that a wrapped or deliberately non-monotonic curve would produce, where a louder input can produce a quieter output with unpleasant consequences. And because it compresses proportionally, quiet passages are lifted further than loud ones: at a drive of five, a passage sitting at a tenth of full scale comes out at around half of full scale, while a passage already at full scale stays there but rounded over rather than flat-topped. That asymmetry between quiet and loud is the whole character of the effect, and it is why distortion makes a part sound louder as well as grittier — what changes is not merely the harmonic content but the relationship between the loud and quiet passages.

What it does not do:

No tone control, so the added harmonics are not shaped afterwards.
No bias or asymmetry control, so the effect is a symmetric one rather than producing even harmonics.
No output level compensation, so a heavily driven file comes out louder.
No oversampling, so the harmonics generated above half the sample rate are not filtered.
One curve only, with no other shapes available.
Output is sixteen bit WAV.

Use it for the ordinary purposes of distortion: making a bass or a guitar sound driven, adding edge to a synth, or giving a drum bus some grit. When the distortion should be a lo-fi texture rather than a smooth saturation, the bitcrusher produces quantisation noise instead of harmonics, and when the goal is to make a signal louder without adding harmonics, the amplifier and the normalizer do that without colouring it. One caution is worth repeating, since it is the most common mistake with any saturation: the added harmonics raise the peak level of the material as a whole, so a part that sounds right in isolation can push a mix over when it is combined with everything else, and checking the output level after the fact is part of using the effect well.

How to use Distortion Effect

Drop an audio file onto the panel.
Set the drive with the slider or one of the four presets.
Read the footer to see how much the curve lifts quiet passages at that setting.
Process the file and download the distorted WAV.

When to use Distortion Effect vs related tools

Distortion Effect is the tool for adding harmonics and reducing the distance between quiet and loud in one step. Bitcrusher Effect produces quantisation and aliasing artefacts instead, which is a different and harsher texture. Ring Modulator moves the spectrum rather than folding it, and Audio Limiter is what to use afterwards if the added harmonics have pushed the peaks over.

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