CARTER: PIANO CONCERTO; VARIATIONS FOR ORCHESTRA. Ursula Oppens, piano; Cincinnati Symphony Orchestra conducted by Michael Gielen. New World NW-347 (digital/analog). Oppens fearlessly assumes the combative stances of the soloist in Carter's 1965 piano concerto, one of his more complex essays in pitting a leading instrumentalist against orchestral forces. The pianist elucidates the dense writing with poise and clarity. But conventional recording can scarcely capture the spatial requisites in Carter's scheme (a seven-member ensemble surrounds the pianist) and the results are somewhat uncommunicative. Gielen guides his orchestra with complete assurance through the more accessible 1955 variations, in which Carter's eclectic style (from impressionism to jazz) and interwoven thematic line skew the traditional format. The performances, deriving from air checks, lack distinguished sonics.