While America was struggling through a staggering economic depression in the '30s, jazz musicians were busy joyously, renewing their artistic language.

The music was opening up, becoming more complex rhythmically, harmonically and melodically. The great names of the period - Armstrong, Ellington, Hawkins, Hodges, Tatum, Basie, Young - were extending their musical visions while building on the foundations laid by musicians of the previous generation.

They succeeded masterfully. One way to judge great art is by noting its timelessness long after its creation. The jazz of the '30s fits this formula. The recordings of the masters of the '30s still sound vital and refreshing almost half a century later. They are true classics.

Fats. Waller and Teddy Wilson, two pianists from that classic era, have just had reissues of some of their finest recordings.

Waller, a huge, hard-drinking man with an enormous appetite for food, is represented by "Fats Waller Piano Solos/1929-1941" (RCA AXM2-5518), a two-record set that traces his keyboard style from his first recordings to his last.

Waller's infectiously joyous vocals, which stamped him as the funman of jazz, caused most people to ignore his stature as a pianist. He also was a top flight songwriter, the composer of such standards as "Ain't Misbehavin,'" "Honeysuckle Rose," "Black and Blue" and "Squeeze Me."

Waller's key board approach was stride style, which featured a strong oompah bass line and glistening treble arpeggios. Pieces like "Handful of Keys" and "Valentine Stomp" were typical stride works, showing his debt to James P. Johnson, the father of stride piano.

But by 1934, Waller had begun to assert his own individuality in pieces like "African Ripples" and "Clothes Line Ballet." He played a smoother, more evenly spaced bass line and put increased emphasis on a treble line that broke more with original melodies.

"African Ripples" featured a tender lyricism and a magnificent rhythmic power and thrust. "Clothes Line Ballet" was characterized by delicate melodies alternating with vibrant, roving lines. This album also includes some charming renditions of his popular pieces such as "I've Got a Feeling I'm Falling," and "Keepin' Out of Mischief Now."

Despite Waller's pianistic grandeur and abundant musicality, he confined his artistic pulse by his loyalty to the stride approach while playing solo piano. While many other jazzmen were loosening up rhythmically, Waller continued to refine an established style.

Teddy Wilson, a well-known member of the Benny Goodman Trio and (later) Quartet, had the greatest impact in opening up the rhythmic limits of jazz piano in the '30s. The album, "Teddy Wilson: Statement and Improvisations, 1934-1942" (Smithsonian Collection R005), is a marvelous documentation of his evolution and influence.

Available only by mail order from the Smithsonian Collection, P.O. Box 1642, Washington, D.C. 20013, or in the Smithsonian's museum shops, the album finds Wilson as an accompanist for singers Billie Holiday and Mildred Bailey and in solo performances.

As pianist Dick Katz points out in his excellent liner notes, Wilson revised the stride piano approach by eliminating the oompah bass effect, leaving the left hand free to punctuate lightly as the right hand carried the melodic weight.

In the bass was a running line. In the treble were light, elegant melodies. His 1935 version of "Rosetta" is a fine example of this revised stride style in an early form.

By 1941, when he recorded "Rosetta" with bass and drum accompaniment, his approach - a light touch, sweeping, graceful melodies and cheery themes - had become part of the American musical picture. But Wilson pioneered the style, as is demonstrated by his work in 1934 with Red Norvo and his Swing Septet on "I Surrender Dear." His improvisations were melodically and rhythmically fluid. His whole style was marked by understatement.

Waller died in 1943, but Wilson at age 64 is still going strong - playing piano for any of us willing to listen.