Scalable Connectivity Processor for Computer Music Performance Systems

Standard laptop computers are now capable of sizeable quantities of sound synthesis and sound processing, but low-latency, high quality, multichannel audio I/O has not been possible without a cumbersome external card cage. CNMAT has developed a solution using the ubiquitous 100BaseT Ethernet that supports up to 10 channels of 24-bit audio, 64 channels of sample-synchronous control-rate gesture data, and 4 precisely time-stamped MIDI I/O streams. Latency measurements show that we can get signals into and back out of Max/MSP in under 7 milliseconds. The central component in the device is a field programmable gate array (FPGA). In addition to providing a variety of computer interface capabilities, the device can function as a cross-coder for a variety of protocols including GMICS. This paper outlines the motivation, design, and implementation of the connectivity processor. 1. Context and Prior Work Hardware development for computer music performance systems has followed the standard pattern of technology evolution passing through the first two phases of design focus, function and price, to the final phase: usability. We have identified size and connectivity as primary usability issues for computer music performance systems. Laptop computers have recently become available with fast signal processing capabilities at a moderate cost and although their size makes them very attractive for musical performance, their constrained expansion capabilities limit connectivity. Currently there are no commercially available, low-latency, high-reliability, compact, multi-channel audio solutions for laptop computers. Furthermore the architecture of current laptops and most computers makes it impossible to synchronize acquired gestural data and sound I/O to satisfy the low latency/jitter needed for satisfactory reactive performance systems: 10±1ms (Freed, et al., 1997). The 10 ms latency criterion is not difficult to meet, but a maximum latency variation of ±1ms is difficult to achieve, especially when the stimulus gesture is represented as a MIDI event or a low rate signal from a nonsample-synchronous input source like a data acquisition card rather than a sample-synchronized audio input signal. The only computers with the requisite unified clock management and operating systems support for such tight synchronization are from Silicon Graphics. Unfortunately, even the smallest configurations of their machines, the O2 and Octane are too large and expensive for most performing musicians. One of our key design goals was to eliminate virtually all latency variation in low sample rate inputs like those from gestural input devices.