Diffusion refers to the practice of performing electroacoustic music by projecting it through multiple loudspeakers placed around the performance space. Traditionally, the composer or other person sits at a centrally placed mixing console and manually controls the levels and other processing of the sound.
At Simon Fraser University we have been developing and using a computer-controlled system for diffusion based on Harmonic Functions' DM-8 multi-DSP box, designed by Vancouver engineer Tim Bartoo. It was premiered at ICMC95 in Banff in Barry Truax's commissioned music theatre work, Powers of Two: The Artist, and since then it has been functional in the Sonic Research Studio and PODX system at SFU. In 1996 it was used by four visiting composers (Claude Schryer, Darren Copeland, Sabine Breitsameter, Hans Ulrich Werner) for their soundscape compositions as part of the Soundscape Vancouver 96 project, co-sponsored by the Goethe Institute and Vancouver New Music, presented at the CBC studios in Vancouver on June 7, 1996. 8-channel works whose spatialization was realized with the DM-8 are Pendlerdrøm, Sequence of Earlier Heaven, and the complete electroacoustic opera Powers of Two. The octophonic catalogue on this site lists all works realized at SFU with this system as well as the 16-channel AudioBox.
Although conventional diffusion is remarkably effective with a stereo source, both the two channel bottleneck, and the limitations of manual control and too little rehearsal time, are currently the weak links in the performance of electroacoustic music. Having 8 discrete sources available, all independently controllable, is not only acoustically richer for tape music (since detail is not lost through stereo mixing) but also challenging compositionally in order to integrate a spatial conception into the work. However, the same system can be used for live, or mixed live and tape performance, since nothing is assumed about the relation of the 8 input signals.The AudioBox system is essentially a 16 by 16 matrix which routes 8 channels of input (for us, the Tascam DA-88) and 8 onboard channels of material stored on the hard drive, to 8 channels of output, presumably going through a conventional amp and speaker configuration. The hardware is a custom designed box, external to the host Macintosh, communicating via SCSI messages to the graphic front end. The software for user control (ABControl) is written by Chris Rolfe, which can be used either in a live performance mode with mouse triggered events, or else as a pre-programmed cue list. This editable mixing score allows each of the input tracks to have its amplitude controlled, as well as projected onto independent trajectories to the output channels.
Of interest to electroacoustic composers is the ease with which a given set of speakers can be substituted for another when a new performance configuration is encountered, or when a mixdown is needed. To change to a different speaker configuration, only the speaker map needs to be edited, not each instance of its use. The map also assists the composer in dealing with particular spatial configurations independently of the often confusing lists of speaker numbers.
The nature of cross-fades between speakers is a particularly tricky subject, and the software assists the user with both graphic displays of the levels involved and real-time aural tests of the effect. Cross-fade percentage is a key variable, allowing a continuum of effects from jumping between channels to completely smooth transitions to be achieved. Spatial patterns can be controlled manually or programmed by an automated process called a "diffusion generator".
The 16-channel AudioBox designed for Richmond Sound Design in Vancouver, has been used in the Sonic Research Studio at SFU as well as the composer's studio to realize works such as Island, Temple, Prospero's Voyage and The Shaman Ascending. Those interested in the availability of the new 16, expandable to 64-channel TiMax2 matrix mixer should visit the Outboard website. This unit was used to realize my 2010 work Fire Spirits.
References: B. Truax, "Composition and diffusion: space in sound in space," Organised Sound, 3(2), 141-6, 1999.
C. Rolfe: www.naisa.ca/soundtravels/2003/difpract.html
This project was supported by a Computer-Integrated Media Research Grant from the Canada Council.