The future of pop music, we've been told, is techno: voiceless electronic stuff that embraces the blips and wing-dings of computer technology. We'll pin our hopes on PORTISHEAD, a trip-hop band with a healthy sense of the past. Their new album, due Sept. 30, balances drum tracks and droney rhythms with lush orchestrations and old-school turntable scratching; they bring the lounge music of the mid-'60s head to head with the high-tech millennium. Beth Gibbons has a chanteuse's gift for romantic despair: the darker her lyrics get, the more transparent and light her voice becomes. Nothing here has an overt pop feel, but the band's flair for textures makes each song a surprise.