## 3 Artificial Audio Effect Technologies
### Analogue Signal Processing
The design principles and history of equalizers are further detailed by [PERSON] [15] and [PERSON] [106]. Based on a sweeping filter, in 1966, [PERSON] and [PERSON] of Warwick Electronics created a novel effect pedal simulating the effect created by trumpet players modulating the sound by moving a mute at the bell of their instruments. The _vah-vah_ pedal produces its typical sound by moving the centre frequency of a resonant band-pass filter up and down in the spectrum depending on the pedals position. Prior to the commercial availability of the effect, self-made designs have been used since the 1950s. Guitarist [PERSON] is credited as having recorded the first song using a similar device. He modified a DeArmond volume control pedal by replacing the pedal's volume potentiometer with a control to move a tone control from low to high frequencies [107]. Since its inception, the _vah-vah_ effect has found widespread use, especially after [PERSON]'s use of the Cry-Baby pedal in the late 1960s, and remains popular to this day. While the shape of traditional filters discussed above is controlled manually by the user, adaptive filters present an example of an adaptive audio effect in the analogue domain. Adaptive filters adjust their frequency response depending on the incoming signal. They are characterised by (i) the signals being processed, (ii) the structure that defines how the output signal is derived from the input signal, (iii) the parameters that are iteratively adjusted to change the the filter's transfer function, and (iv) the circuit design (or algorithm for digital implementations) that defines the error function and how the parameters are optimised [108]. Alt'ough most current applications of adaptive filtering are in the digital domain, analogue implementations of adaptive filters have been especially relevant when digital electronics could not provide sufficient processing speed. The first applications of adaptive filters include adaptive antenna systems mitigating the effect of interfering directional noise sources [109], the equalisation, echo cancellation, and crosstalk cancellation in wired digital telecommunication [110]. In digital magnetic storage systems, analogue adaptive filters are used to provide forward equalisation for the signal received from the read head. The first adaptive noise cancelling system was designed in a 1965 student project at Stanford University [111]. A detailed summary of early applications of analogue adaptive filters can be found in Reference [112]. Controlled nonlinear distortion of audio signals, resulting in added harmonics and a compressed sound, for aesthetic purposes has long been established as a standard audio effect used in modern music production. For instance, it is an essential tool in rock music to produce the typical guitar sound. Distortion as a musical effect to shape timbre can be traced back to the introduction of the electric guitar and, as many other discoveries in the field of audio effects, can be attributed to accidental discovery and unintended use of technology. In order to compete with the loudness of brass instruments when guitars were included in dance bands in the 1920s, guitar pickups were introduced. The first commercial electric guitar was manufactured in 1931 by Rickenbacker in the form of the _Frying Pan_ lap steel guitar. To reduce manufacturing cost, early electric guitar amplifiers used output transformers which exhibited distortion levels of up to 50% when turned up beyond their specification. By the 1940s, blues guitarists discovered that overdriving amplifiers beyond their capacity enabled them to deliberately shape the sound. The first commercial distortion pedal was the _Maestro FZ-1 Fuzz Tone_ manufactured by Gibson and became available in the early 1960s. It was aimed at guitar, banjo, and bass players, and the sound was described as \"simulating other instruments such as trumpets, trombones, and tubas\" [113, 114].