It is not a distortion of the truth (excuse the shakespearian pun!) to state that this effect has literally changed music history. The birth of rock music in fact owes a lot to this effect, which was discovered by chance as a result of a wrong amplification. The functioning principle of the distorter is very simple. When the amplitude of a signal goes beyond the maximum threshold level of an amplifier's input, a phenomenon called saturation takes place. This means that when the input is signal below this threshold, the amplifier works correctly and reproduces an amplified waveform at the output. However, when the input signal goes beyond the threshold, the amplifier has reached its limit and thus cannot further amplify the waveform. This in practice means that the amplifier supplies constant output during the time span in which the input signal remains above the threshold. When the input signal returns below the amplifier's threshold it begins to work properly again. The following diagram illustrates the amplifying curve of an amplifier, and its action on a sinusoidal input signal whose maximum levels surpass the amplifier's threshold.
So, we have a saturated signal at the output. Saturation introduces a sudden variation of the signal, which no longer follows its natural sinusoidal behaviour, which in turn means that the new signal contains new frequencies which are higher than the original ones. Let's now explore this fact from a signal theory perspective, by referring to the notions we introduced in its pertaining section [Distortion by saturation ] .
We have often spoken about how any complex signal can be boiled down to a sum of sinusoids at different frequencies (and phases). The more sudden the transitions of the signal, the more frequencies are necessary to reproduce it in terms of sinusoids. If we take a look at the saturated sinusoid in the diagram, we can see that some very sudden transitions have been introduced to it and there must therefore have been an addition of new frequencies to the spectrum. It is these new frequencies that generate the typical distortion sound. So, distortion at its purest is obtained by lifting the gain level of a pre-amplifier to the extent that part of the signal which will then reach the final amplifier [Amplification curve ] goes beyond the pre-amplifier's threshold. A good valvular Marshall produces a marvellous distortion sound. If you don't have one, you can use an effect which simulates saturation, although the quality of the effect won't be quite the same.
The following is an example of a sound to which various distortion effects have been applied.