Pure Stereo is a recent breakthrough in audio technology (licensed by Princeton University) that yields unprecedented spatial realism in loudspeakers-based audio playback allowing the listener to hear, through only two loudspeakers, a truly 3D reproduction of a recorded soundfield with uncanny accuracy and detail, and with a level of high tonal and spatial fidelity that is simply unapproachable by even the most expensive and advanced existing high-end audio systems.
BACCH Filters are optimized crosstalk cancellation filters that allow 3D audio reproduction over loudspeakers. They yield maximum crosstalk cancellation level without introducing any spectral coloration to the input signal. An introduction to BACCH filters can be found here.
This highly technical paper describes most of the basic aspects of BACCH filters (some aspects are not published for propriety reasons).