## 3 Artificial Audio Effect Technologies
### Analogue Signal Processing  
A large number of sound transformation techniques based on analogue signal processing, i.e., physically altering a continuous signal by changing the voltage or current with electrical components, appeared with the introduction of early electronic musical instruments. In this context, the sound transformations were implemented in order to shape synthesised audio signals as opposed to recordings. [PERSON] [16] reviewed the history of electronic sound modification with an emphasis on sound synthesis. In some of the earliest examples of electronic instruments, the _Telefunken Trautonium_ (marketed from 1933-1935) and the _Hammond Nowachord_ (presented 1939 at the New York World's Fair) formant filters are used for the shaping of overtones. Invented by [PERSON] [94; 95], the _voder_ and _vocoder_ simulated the resonances of human voice using band-pass filters. The vocoder, originally designed to reduce the bandwidth of speech transmission, included a signal analyser of the filtered bands to control the synthesis process, an approach now referred to as _envelope following_. The early Hammond organs were capable of amplitude modulation (AM) for a _tremolo_ effect and were later able to perform frequency modulation (FM) for _vibrato_. [PERSON] [16] notes that the tremolo effect preceded the vibrato capability in the Hammond instruments due to initial difficulties in the implementation. In the 1940s, instruments equipped with _ring modulators_ appeared, for instance, the _Bode Melochord_[96]. [PERSON] [97], ofounder of the Studio for Electronic Music of the West German Radio in Cologne, which was heavily involved in the exploration of music creation based purely on electronically synthesised sounds since the 1950s, discusses the musical applications of ring modulation. Both AM and ring modulation (RM) produce side bands. AM retains the carrier frequency in the resulting spectrum due to the unipolar modulator; the modulation frequency itself on the other hand is not present. The spectrum of a ring-modulated signal consists of the same side bands; however, due to the bipolar modulator, the carrier frequency is not present. Based on the ring modulator, [PERSON] [98] presented a _frequency shifter_, which, unlike the commonly known pitch-shifting effect, alters each component of the spectrum by a fixed amount, thus changing the harmonic relationships and creating new timbres. This device has been developed for electronic musical instruments manufacturer _Moog_, a company credited for marketing one of the first modular synthesisers. [PERSON] developed a large number of techniques in the field of signal transformation for musical purposes and filed several patents, among them a phase-shift-type frequency shifter, specialised filters, and ring modulators to be integrated in electronic instruments [16]. Artificial reverberation based on delay lines and all-pass filters was described by [PERSON] and [PERSON] [99; 100]. The all-pass filters diffuse the sound by adding frequency-dependent time shifts to the output and are also referred to as _impulse expanders_ or _impulse diffusers_. The _vacuum tube_ (or _thermionic valve_), an analogue component invented in the first decade of the 20 th century for the amplification of low-level signals, has been used extensively in the design of guitar amplifiers and microphones as well as in audio effects such as equalisers and dynamics compressors. Compressors reduce the dynamic range of an audio signal. Dynamic range compression was initially developed for radio broadcast and has been used since the 1950s to compensate for the limited dynamic range of the broadcasting medium. For instance, AM radio has a dynamic range of 50 dB and FM radio has a dynamic range of 60 dB. In the 1960s, compressors found widespread use in recording studio, again, to reduce the dynamic range of a complete recording to the specifications of the recording medium, e.g., LP (long-playing) records (65 dB) or analogue tape recorder (70 dB).