The artistic necessity for the jazz musician to communicate directly and intimately with his or her audience is attested to weekend after weekend at the One Step Down, a small club that, with a shoehorn, can squeeze in about 100 bodies.Internationally known interpreters of the idiom usually head the bill with a local rhythm section in support.

Trombonist Jimmy Knepper's credits would fill all the space allotted here. A representative list includes work with Charlie Barnet, Stan Kenton, Charlie Parker, Art Pepper and Charles Mingus. He is considered by many to be, for his instrument, the most important link between the swing period and the present.

Last night a free-wheeling, fastpaced "I Got Rhythm" with a Lester Young line was staccato and upper register while "Out of Nowhere" came out legato and mellow. A plumber's helper as a mute assisted him on a mean and lowdown "Things Ain't What They Used to Be."

Pianist Benny Aronov, cross-firing chords, and bassist Paul Langosch, coattailing a split second behind, worked out extraordinary overlapping patterns. Mike Smith's well-deployed drums were the carpet upon which the others danced.

The Jimmy Knepper Quartet plays again this evening.