Computer Controlled Diffusion
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. Recent 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.
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 DM-8 system is essentially an 8 by 8 matrix which routes 8 channels of input (for us, the Tascam DA-88) 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, equipped with 4 Motorola 56001 chips and a 68000 controller, communicating via MIDI system exclusive messages to the graphic front end. The software for user control is a Max application, written by Chris Rolfe, which can be used either in a live performance mode with mouse triggered events, or else as a pre-programmed score synched with the MIDI timecode on the tape. Presets and an editable mixing score allow each of the input tracks to have its amplitude controlled. These mixing levels can be graphically entered, or tracked from the user's control of virtual potentiometers in real time. These recorded levels are analyzed and compressed by the program for an optimum data representation and can later be edited by the user.
A 20-page documentation of the software is available, but here are a few highlights. The 8 by 8 matrix allows manual input/output connections to be made (i.e. speakers turned on and off), preset patterns of which can be stored and implemented with variable fade times. The cross fade from one configuration, say a stereo reduction, to another, for example a multi-channel distribution over 5 - 10 seconds, is a typical operation that would be difficult to achieve manually but is aurally very attractive.
A set of 'players' extend the matrix control to either 'static' speaker lists, or to 'dynamic' trajectories. Unlike the matrix operation, they automate both the turning off of outgoing channels as well as the turning on of new channels. The dynamic assignments generate a series of cross-fades, moving an input from speaker to speaker in what we call a 'trajectory' at a specific rate with adjustable fade patterns. Pre-defined speaker patterns can be looped, cycled (forward and reverse), or randomly assigned. Since 8 such patterns can be simultaneously running, very complex movements can be easily generated. All of the player parameters transfer directly to the score method of control, hence a particular trajectory configuration can be tested in real time, then copied into the score with its precise point of implementation.
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. A speaker list is defined once and labelled (e.g. left, circle, etc.) with nothing assumed about where those speakers are located. To change to a different speaker configuration, only the list needs to be edited, not each instance of its use. The label 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. A 'sustain delay' parameter delays the fadeout of the previous speakers in a dynamic sequence to create a more 'polyphonic' effect (analogous to the vapour trail behind a jet). Finally, the 'fade increment' is a simple method to generate the cascaded entry of a speaker list, similar to the way one might bring in a set of speakers incrementally in conventional manual diffusion to create another polyphonic effect.
Although the system is designed for controlling 8 source channels, other uses are possible. For instance, a stereo source could be duplicated up to four times at the input of the matrix, and four pairs of distinct trajectories or speaker assigns defined. The composer could then use the mixing score or manually controlled input levels to cross-fade between the different spatial treatments. Alternatively, the entire matrix could be considered to be an effects send and return system for studio work with, for instance, two 'dry' channels and six channels of processing being mixed together.
Besides the Banff performance, the DM-8 performed without problems at recent Vancouver New Music electroacoustic concerts, and is currently available for use in the Sonic Research Studio at SFU. Although an extended (16 x 16) commercial version has been designed for Richmond Sound Design in Vancouver, the existing prototype hardware and software configuration is extremely useful for electroacoustic diffusion. Those interested in the availability of the new 16-channel unit, the Audiobox, should visit Harmonic Functions' website: http://www.hfi.com.
This project was supported by a Computer-Integrated Media Research Grant from the Canada Council.