Distortion and overdrive: In the early days of electronic music, vacuum tube amplifiers were used for processing the output electrical signals. Like any amplifier, these tube amplifiers had a maximum amplification above which they began to clip the peaks of the waveforms. While the resulting distortion may have disturbed the electrical engineers, a substantial fraction of the musicians liked the distorted sound and it became part of the musical medium.
Now, in the days of solid state signal processing, outboard units are available to produce a facsimile of the old tube-type distortion. Some units actually incorporate a vacuum tube and do it the old way. Other electronics packages simulate the effect. The "tube-type" distortion is preferrable to many over the kind of distortion produced by solid-state amplifiers because the tubes just gradually rounded over the peaks as they went into distortion, whereas the solid state devices just chop off the tops of the peaks cleanly at the supply voltage point, producing a harsh distortion. One type of distortion device, employed as a distortion pedal, was called a "fuzz box".
- Compression: A "limiter" is envisioned as a circuit which prevents the output signal from exceeding a certain limit. If it did this by clipping, it would cause distortion, but if it can do it cleanly by just progressively reducing the amplification of the incoming signal, then it may properly be called a compressor. A compressor reduces the overall dynamic range by "compressing" the gain of high amplitude signals while maintaining the design gain for lower amplitude signals.
Compressors are available as footpedal controls and can be used as an effect on electric guitar signals, for example. They can be used to obtain greater sustain for a string by setting the gain high and allowing the compressor to keep the output signal at a more-or-less constant level until the natural sustain of the string drops the signal below a certain threshold.
- Ring modulation: In the context of signal reshaping, the ring modulator takes the signal from the instrument and adds a second signal from a local oscillator or signal source. The two signals are combined to produce the sum and difference frequencies, which are then the output of the device. This scheme was used in the electronic music of the 1950's. The output frequencies track the input signal frequencies, but do not equal them, so there is a shift from the original pitches. The ring modulator has been produced as a footpedal, and ring modulator type effects are included in some modern electronic effects boxes.
- Pitch Shifters: Microprocessor-based devices can take the signal from a guitar and shift its frequency base. This amounts to sampling the sound and reconstructing it at a different frequency, so a lot of computation is involved. These devices range from note-shifting foot-pedals to studio-based harmonizers.