CK91 - discussion and analysis
by Daniele Pozzi
CK91 is a live electronics performance for two microphones, laptop and two or more loudspeakers where the performer acts on the physical configuration of the system in order to alter its internal dynamics, therefore catalysing the emergence of different sonic forms. The system is composed of two interdependent feedback networks: an acoustic one is formed by the coupling of microphones to loudspeakers, and the environment in which the performance takes place. The second one is a digital feedback network, consisting of recursive signal processing functions that both generate and modify audio. The component which is central in the coupling of the two networks, converting signals from the analog to the digital domain and viceversa, is the audio interface. The two networks are coupled, meaning that digital transformations integrate microphones signals as part of their signal flow (the digital network is open to the environment); they have different sensitivities to these inputs and their internal processes are dependent on the sounds which are picked up by the microphones. The result of these transformations is eventually sent to the loudspeakers, bringing about the Larsen effect and thereby creating the conditions for circular causality. The causes are fed back to themselves through their effects, and the effects are the result of their combination with the causes, thus breaking the input–output linear proportion.
The interrelation of these sub systems creates an aural phase space which is always different, depending on the environment, on the spatial configuration of its physical elements and on the choice of its components (different kinds of microphones or loudspeakers affect the resulting sound quality). This phase space is considered as a territory that the performer navigates by executing various actions in the performance space. Different behaviours are achieved by exploring the room resonances, different angles and distances of the microphones from the loudspeakers, and low-level sonic interactions. The performance is articulated in three parts, each exploring different equilibria between the digital, the analog and the acoustic components.
1.1 DSP Structure
The basic structure of the work consists of two interrelated DSP networks that interface with the environment through microphones and loudspeakers. One is specifically composed to treat the audio feedback loop between microphones and loudspeakers, articulating Larsen tones and regulating their amplitude and spectral content through negative feedback dynamics. The main features of this network are described in Section 1.2.1. This first network is also responsible for generating control values (mainly through a rough analysis of its DSP chain) that reshape the internal dynamics of a second network, which synthesises sounds through non-standard digital feedback techniques. This reshaping process happens by altering the topology of the feedback network through a variation of the amplitude of the recirculating signals and changing the relations between the components by modifying their parameters. This network is discussed in Section 1.3. Since the second network works at the smallest delay possible - when the control period is equal to one sample - the two networks run on two different SuperCollider servers to avoid CPU load. The first one executes on a server having a control period of sixty four samples, while the second runs on another server with block size one. The two exchange audio through a shared Jack server, as illustrated in the figure below.