The human voice may have the most sonically complex timbre known to man, but the Bay Area's Sound Tribe Sector 9 is hell-bent on proving that man-made instruments can be just as compelling. Actually, while the voices of San Francisco's Audio Angel and Athens's Kozmos do show up on Artifact, Sound Tribe's latest release, they're usually eschewed by the group when on the road. Thankfully, even though the outfit also forgoes the aimless meanderings of most other improvisational rock acts, it has managed to keep the groove intact. In fact, groove is the centerpiece of Sound Tribe's sound. Koto and keyboard have never co-existed so harmoniously. Neither have sound and color, for that matter. The band's video projections often kick off with the starting key's relative chromatic frequency. Only a soul in its ninth lifetime -- or a schizophrenic -- would understand.