The Convolvotron

The Convolvotron, which was developed for NASA and is manufactured by Crystal River Engineering, provides a conceptually simple way to use HRTF's for spatial audio. Stripped to its essentials, it consists of two "convolution engines," each of which can convolve the same audio input stream with a finite segment of a head-related impulse response (HRIR) retrieved from a table of measured values. The outputs of the convolvers go through amplifiers to headphones worn by the listener. If the HRIR's for the listener are sufficiently close to the HRIR's used by the convolvers, the sound delivered to the listener's ears will contain all the proper spatial cues, and the sound image will be properly localized. The location will be determined by the particular azimuth, elevation, and (in principle) range used to index the stored tables.

This basic idea can be elaborated in several ways:

1. Multiple sources can be accomodated by replicating the hardware and summing the outputs to each ear.
2. Head motion can be accounted for by combining the absolute location of the source with the outputs of a head tracker to select the appropriate HRIR's
3. The tables can be indexed by azimuth and elevation only, with the distance from the source to each ear being used to introduce range/amplitude effects
4. The number of HRIR's stored in the tables can be reduced by using coarse spatial sampling and appropriately interpolating between nearby points
5. Echoes and room reverberation can be added by including a room simulation model
6. The system can be customized for a particular individual by measuring and using that person's HRIR's