The Skipproof Virtual Turntable for High-Level Control of Scratching

Skipproof is an application that emulates a typical disc jockey (DJ) setup of turntable plus mixer and also allows high-level control of the playing style known as scratching. High-level control in this case means performing with modeled, complex DJ gestures through simplified actions: For instance, letting a single movement produce a sound that normally would require precisely synchronized right- and left-hand gestures. The performer controls Skipproof either with the software interface or through hardware devices connected to the computer. The hardware devices become alternative performance interfaces to the standard turntable, controlling the Skipproof application with both low-level gestures and high-level control actions in real time. The mapping between hardware input and Skipproof output is freely adaptable. Skipproof is in the prototype phase, but it has already been used in several projects in recent years.

Traditionally, scratching is performed through synchronized gestures: One hand controls the record speed on the turntable (thus also the pitch), and the other hand uses the audio mixer’s crossfader to turn the sound on or off. The crossfader is a slider that was originally designed for fading gradually between two turntables (or other sound sources) in order to go seamlessly from one song to the next, but in scratching it is instead used for turning the sound of a single turntable rapidly on or off (often while the other turntable is playing for instance a rhythm track). The playing gestures are commonly known as scratch techniques and constitute a common language among DJs (Hansen 2002; Smith 2006). Similarly to other instruments, traditional DJ playing skills must be acquired through dedicated practice, which is reflected in a growing market for teaching material (e.g., DJ Q-bert 2003; Sloly and Frederikse 2004; Webber 2007).

Earlier studies (Hansen 2002; Hansen and Bresin 2003) have analyzed and described DJ-performed scratch techniques, and some of the most popular techniques have been modeled and implemented in Skipproof. The modeling was based on this analysis, with additional input gained by having an active dialog with the scratch DJ community. These models can in turn be used and manipulated in real-time performance with simple control actions; this makes it possible even for non-experts to play expressively within the stylistic boundaries of DJ playing practices.

The motivations for writing Skipproof were to have a platform on which to model and simulate scratch techniques, as well as a tool for studying how scratch techniques are used in expressive performances. Also, we wanted to explore instrument-mapping strategies for scratching and to experiment with alternative performance interfaces for DJs. Skipproof can be combined with hardware and other software, and has been featured in performances with DJs using quite different control interfaces (including the Radio Baton, the Reactable, and various gesture sensors).

To the authors’ knowledge, this is the first software that implements modeled scratch techniques and provides high-level performance control of such techniques. The paper is structured as follows: The following section gives a background on scratching and DJ interfaces, and then the Skipproof application is described. Three performance situations where Skipproof has been used are presented, including results from informal user evaluation. Finally, the current implementations and possible future uses of Skipproof are discussed.

Skipproof is available under the terms of the GNU Public License (GPL) and can be downloaded from http://www.speech.kth.se/~kjetil/software/ .

Figure 1.

Three representations of the chirp scratch. (a) Spectrogram of two chirps in succession performed on the fresh sample. (b) Common graphical notation (Carluccio et al. 2000) of a chirp’s record and crossfader movements: The plot shows the phonograph needle position relative to the fresh sample (vertical axis) as a function of time; the bold part of the movement is muted by the crossfader. (c) Sensor output from the record and crossfader of a recording used in Skipproof.

Background

A recent study of DJ performances describes the playing strategies and acoustics of the instrument (Hansen, Fabiani, and Bresin submitted). The study identifies how performance parameters are used expressively in...