## 4 Current Research and Future Directions  
### Adaptive Digital Audio Effects  
Adaptive audio effects are often considered a more recent or newer class of audio effect; however, they rely mostly on traditional signal analysis and processing principles and have been around for decades. Adaptive audio effects are typically controlled by mapping higher level _features_ to some audio effect parameters. The high-level signal flow of an adaptive effect is depicted in Figure 8. The construction of an adaptive digital audio effects includes three steps [137]:  
1. the analysis/feature extraction aspect
2. the mapping between features and effects parameters
3. the transformation and resynthesis aspect of the digital audio effect  
Adaptive digital audio effects may be classified in in the following categories [137]:  
* **Auto-adaptive effects**--control parameters are derived from features extracted from the input signal. * **External-adaptive effects**--control parameters are derived from at least one input signal other than that to which the effect is applied. * **Feedback-adaptive effects**--control parameters are derived from features extracted from the output signal. * **Cross-adaptive effects**--a combination of at least two external-adaptive effects, where the features mapped to control parameters of the effect applied to one signal are extracted from the other. One of the earliest examples of an adaptive audio effect is the dynamic range compressor (see Section 3.3), where the extracted envelope of a sound signal is used to apply an adaptive gain function [138; 139]. These adaptive effects were developed further into noise gates, expanders, limiters, companders, and upward compressors [140; 141; 142; 143]. From this point, adaptive audio effects were  
Figure 8: Diagram of an adaptive digital audio effect: The control parameters are derived from audio features extracted from an input signal. The features may be extracted from the signal that is to be transformed (input 1), from a different input signal (input 2), or from the effect output. developed where an audio feature is used to control another audio track. This is often described as sidechain and can be primarily found in side-chain compressors for ducking, though there are cases where it is used with noise gates [140]. In all these cases, the feature used to control the audio effect is a smoothed version of the input signal, often described as the envelope. Effects such as the compressor or noise gate use nonlinear functions to transform the signal according to the incoming signal. More sophisticated effects have been proposed with numerous possible features to be used as control parameters, among them spectral-, loudness-, and pitch-related features. In more recent implementations, adaptive audio effects are controlled by some descriptive audio features. These features are obtained by applying techniques drawn from the field of Music Information Retrieval (MIR). Audio features describe qualities of a given audio signal and are also referred to as audio _descriptors_[144; 145; 146]. Low-level audio features include spectral descriptors (for instance, by computing Fourier transform) and descriptors for loudness or dynamics, while high-level descriptors cover abstract and semantic concepts such as key, chords, as well as music genres or mood. Audio features in particular with respect to adaptive digital audio effects are reviewed in Reference [1]. Detailed discussion about these audio features, their extraction, and mapping to control parameters can be found in References [137; 147]. Although these adaptive digital audio effects as described by [PERSON] et al. [83] can be considered a new class of audio effects, the audio transformations themselves are, for the most part, based on established signal processing techniques and audio effect algorithms.