## 3 Artificial Audio Effect Technologies
### Analogue Signal Processing
The effect is controlled by several parameters: The _threshold_ determines the sound level above which the the volume is reduced. The amount of volume change is governed by the _ratio_ setting. A ratio of 1:1 produces no change on the output; 10:1 results, for instance, in a +10 dB change in volume on the input in a +1 dB change on the output. Additional parameters are _gain_ to boost the output, and _attack_ and _release_, determining how quickly the compressor responds to input levels exceeding and falling below the given threshold (see Figure 6). Additionally, the _knee_ parameter may shape the response at the threshold point, i.e., how curved the transition is. Early compressors used a _hinge_ parameter instead of a threshold parameter, setting a midpoint of the approximated dynamic range of the input signal with the ratio determining the amount of dynamic range reduction (see Figure 7) [101]. Figure 6: Parameters for gate, expander, compressor, and limiter. Figure 7: Compressor/expander with hinge parameter. A compressor with a very high ratio, nearing infinite, is referred to as a limiter. While compressors have been in use since the 1930s following the invention of vacuum tubes, prior to their introduction, audio engineers often relied on manually adjusting the loudness of the incoming signal during recording [10]. In the mastering process of contemporary music production, dynamic compression is applied to the mixed signal of a recording as well as creatively on individual channels. Compressors may also be controlled by feeding a secondary signal into the sidechain, a secondary audio input for the level detection. This technique is applied for instance to attenuate music in a radio program during speech or to create a _ducking_ effect by feeding the bass drum into the sidechain, controlling the attenuation of other instruments [102]. Expanders work in a similar fashion; however they increase the dynamic range by decreasing the output level when the input level falls under the given threshold. In dynamic range processors, the input sound level is measured in a sidechain; hence, this nonlinear effect is an _adaptive_ effect, i.e., it exhibits variable behaviour dependent on sound characteristics. We discuss adaptive audio effects in more detail in Section 4.1. The filtering of audio frequencies can be traced back to experiments with frequency-division multiplexing in acoustic telegraphy in the late 19 th century [103]. Early audio filters were integrated parts of phonograph playback systems and audio receivers. These equalisers were fixed to specific frequency ranges to be amplified or attenuated. While the inventors of the moving coil loudspeaker Kellogg and Rice experimented with equalizers as early as the 1920s to enhance the loudspeakers' frequency response, [PERSON], working for RCA in the 1930s, is credited with the invention of the first equalizer designed as a stand-alone device equipped with variable frequency filters. The equalizer found widespread use in the film industry, both to enhance speech in post-production as well as to improve the sound in cinemas. Filters for the adjustment of bass and treble are described in the 1949 paper by [PERSON] [104]. By the 1950s, equalisation became a standard technique, both in record production and playback. An early example of a stand-alone equaliser with sliders to control the attenuation and amplification of a bass shelving filter and a peaking filter is the Langevin Model EQ-251A equaliser introduced in the beginning of the 1960s. While earlier designs featured 2 or 3 tone controls, later professional graphic equalizers often have gain controls for 31 bands, with the centre frequency of each band fixed at 1/3 of an octave away from the center frequency of the neighboring bands. In the early 1970s, more flexible equalisers were developed. Parametric equalizers allowed, in addition to the gain control, adjustment of the centre frequency and bandwidth of each filter [105].