2 Signal processing
2.7.4 Wave form
The last group of effects are the socalled "unlinear effects". They all alter the waveform one way or another, or create a totally different one, controlled by the original waveform.
Distortion
The most obvious waveform effect is distortion. Originally distortion was created by just turning up an amplifier too far, so the power section ran out of headroom (hit the ceiling) and chopped off (clipped) the tops of the signal. Slight distortion is often described as "growl". Some bass amplifier brands are known for this sound. For electric guitars, usually a lot more than a growl is sought for, and hence clipper circuits are used. A clipper circuit deliberately chops up the signal producing sounds from rumbling to roaring to squeeking to fuzzing. Nowadays, countless amplifiers and stomp pedals exist, each claiming to have their own distinct sound, but they all rely on the same principle: clipping.
For bass amps, distortion is often introduced in the loudspeaker cabinet 
. The drivers have a limited linear excursion, and when driven beyond their linear range, they will distort gradually as volume increases. This is part of the "growl" mentioned above.
Rectifier
Another form of distortion is rectifying, a method of converting AC to DC, applied in practically every power supply. The negative half of the signal is flipped over to the positive side.
Wave shaping
When a transfer function is applied to a musical signal, sounds ranging from brass-like to ring modulation can be created. This is called wave shaping. A lot of different transfer functions exist, and these effects are mainly incorporated in small digital pedal boards. Often it creates a distortion-like sound.
Harmonic enhancer
Another variation of the distortion. The signal is high-pass filtered at a few kHz, then distorted, and mixed back to the signal. It creates the sense of increased brightness and clarity. It is often applied on female vocals to give them a breath-ey sound.
Octave doubler
By half-cycle rectifying the signal, its frequency can be divided by two, creating a sound one octave below the original pitch. This is often used to fatten up solos and other parts played high up the neck. This effect is known to overload amplifiers when used on low bass lines. Usually, the effect can't be used on it's own, but can only be mixed in.
Pitch shifter
While the octave doubler can only divide the input signal's pitch by two, this effect can change the pitch over a large range, usually -2 octaves to +2 octaves.
Ring modulator
The amplitude (envelope) of the signal is modulated (controlled) with a high pitched frequency. The resulting sound is an unmusical, bell-like ringing, hence the name. Some ring modulators feature a controllable frequency, be it from an envelope follower, or from a rocker pedal.
Synthesizer
This is the most radical form of altering the sound of a musical instrument. First, the pitch of the signal is determined. With this information, a synthesizer is controlled, creating its own sounds. Some synthesizers feature MIDI note out capability, so you can hook up MIDI syntheziser modules to create virtually every synth instrument. Ever dreamed of playing the piccolo on a bass guitar? With this kind of stuff you can.
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© Joris van den Heuvel 2001-2009