Feedback Systems: An Analytical Framework

The use of feedback-based systems in the music domain dates back to the 1960s. Their applications span from music composition and sound organization to audio synthesis and processing, as the interest in feedback resulted both from theoretical reflection on cybernetics and system theory, and from practical experimentation on analog circuits. The advent of computers has made possible the implementation of complex theoretical systems in audio-domain oriented applications, in some sense bridging the gap between theory and practice in the analog domain, and further increasing the range of audio and musical applications of feedback systems. In this article we first sketch a minimal history of feedback in music; second, we briefly introduce feedback systems from a theoretical point of view; then we propose a set of features that characterize them from the perspective of music applications; finally, we propose a typology targeted at feedback systems used in the audio/musical domain and discuss some relevant examples. Some Brief Historical Remarks The use of feedback systems gained popular momentum in the 1960s in relation to the success of cybernetics (Wiener 1948) and system theory (von Bertalanffy 1968), two trans-disciplinary epistemological approaches that, starting from the 1940s, strongly emphasized the relevance of closed information loops in organized structures. Nicolas Schöffer (1954) was the first in art history to advocate the use of cybernetic systems in his “spatio-dynamic” works. In 1955 he created the first physical installation implementing a self-regulating mechanism, the CYSP 1. Installed in Paris, this cybernetic sculpture included some computational capabilities thanks to the technology offered by the Philips company. Provided with photocells and a microphone, CYSP 1 was capable of sensing the environment—including itself—and of sonically reacting by playing back sounds composed by French composer Pierre Henry (Prieberg 1960). Computer Music Journal, 37:2, pp. 12–27, Summer 2013 doi:10.1162/COMJ a 00176 c © 2013 Massachusetts Institute of Technology. Another seminal experience, although scarcely recognized, was Roland Kayn’s musical output from 1964. Drawing inspiration from cybernetics, the German composer experimented extensively with auto-regulating systems based on feedback loops, both as formal models for instrumental composition and as networks of analog signal generators for electronic music (Patteson 2012). During the same decade a second suggestion came from signal theory and acoustics, through the investigation of the Larsen phenomenon discovered some decades earlier. The Larsen effect happens when—given sufficient amplification—the sound captured from a microphone connected to a speaker is reproduced and again captured, recursively, resulting in a positive feedback that produces pitched tones from the iterated amplification of a signal (Boner and Boner 1966). Since then, feedback in the audio domain has been extensively exploited by pop and rock musicians, in particular guitar players incorporating feedback in their style as a result of the interaction between the two poles of the electroacoustic chain, pickups on the one hand and, on the other, amplifiers and loudspeakers, with intermediate effects playing a fundamental role. The first case is probably I Feel Fine by The 12 Computer Music Journal Beatles in 1964, and the most significant example is Jimi Hendrix intensively exploring feedback since the first album with his band The Jimi Hendrix Experience (Are You Experienced from 1967, e.g., the opening of Foxy Lady, see Hodgson 2010, p. 118ff.). Furthermore, the empirical practices emerging in the 1960s from hardware hacking and circuit bending have also contributed to the development and spreading of that approach, considering that they can be intrinsically feedback-related. Reed Ghazala was one of the pioneers of circuit bending, a practice that, even if in a nonsystematic way, includes structural feedback as a possible outcome of empirical hacking with jumper wires on existing circuits (Ghazala 2005). Other musicians have probably discovered feedback in music simply by experimenting with their equipment, without necessarily having a complete awareness of what was occurring, while also being able to understand the charm and potential of this phenomenon. Among the first composers to incorporate feedback in their work were Robert Ashley (The Wolfman, 1964), John Cage (Electronic Music for Piano, 1964), Steve Reich (Pendulum Music, 1968), and Alvin Lucier (I Am Sitting in a Room, 1969), as well as Gordon Mumma (Hornpipe, 1967) who worked extensively with self-constructed circuits, and David Tudor (Tone Burst, 1975), who was particularly relevant, basing many of his practices exclusively on feedback (Microphone, 1973). This interest was also common among video artists, many of whom who were analogously experimenting in the same years with optical feedback (in particular Nam June Paik; Steina and Woody Vasulka; and Bill Viola), and the interest originates in both cases in the possibility of experimenting with recording technologies. The case of Bill Viola is particularly interesting: his work Information (1973) stems from a technical mistake, in which a videotape recorder tried to record its output, with Viola intervening in the loop by means of a switch (Viola 1995). As Viola himself recalls, in his formative years he was associated with David Tudor and deeply exposed to Tudor’s works and thinking (Viola 2004). Computer-based technologies have made a wider set of possibilities in terms of control over sound generation and organization available to both composers and performers. Not by chance, many composers, performers, and sound artists now work intensively with computer-based feedback systems. As an example, to name but a few from the Italian scene, and apart from Agostino Di Scipio, whom we discuss subsequently: Marco Cecotto, with his Inner Voices—A Conversation (2013), an audiovisual installation based on automated processing of Larsen tones (http://vimeo.com/62904221); Roberto Pugliese, who in 2011 created, in his Equilibrium Variant, a pair of mechanical arms interacting by listening to and producing Larsen effects (www.robertopugliese.com/page2/page22/page22 .html); Simone Pappalardo, who, through electromagnetic emitters and receivers, creates electromagnetic feedback loops that eventually turn into music systems for hybrid performance/installation works, like his Room 3327 (www.simone-pappalardo .it/Simone-Pappalardo/room3327.html); and Massimo Scamarcio, who experimented with an audio ecosystem populated by resonant filters in feedback (Scamarcio 2008). In recent decades, feedback has also played a major technological role in digital signal processing. A feedback oscillator instrument first appeared in JeanClaude Risset’s Introductory Catalog of Computer Generated Sounds in 1969. Another example is the self-modulating oscillator using amplitude modulation (where the output signal is used to control the modulation index) that was implemented in the Music V language. Subsequently, the technique (this time with feedback frequency modulation) gained wide popularity because of Yamaha’s patented application of the method in its digital synthesizers (Roads 1996). Feedback has made it possible to model filters whose behavior is representative of analog filters (infinite impulse response filters), which under specific conditions turn into sound generators such as the digital resonator, a second-order bandpass filter where stable self-oscillation occurs and allows the implementation of sample-quality sinusoidal oscillators. The notion of a recirculating wavetable in the well-known Karplus-Strong algorithm for plucked-string and drum synthesis is based on a noise source initially populating a table that is then put through a feedback mechanism that re-populates the same table with the output samples after they have been processed by a modifier element (Karplus and Strong 1983). Sanfilippo and Valle 13 In general, feedback is particularly good for modeling acoustic interactions, and, not by chance, physical model implementations and digital reverberators usually include a feedback mechanism as a crucial element (see, e.g., Cook 1992; Rocchesso and Smith 1997). In the rest of this article, we will focus on the musical application of feedback systems, even if most considerations apply to feedback in general.