TOTALLY unexpected but enormously impressive commemoration of the recent A Barto'k centenary is a five-disc survey of the Hungarian master's orchestral works conducted by A'rpa'd Joo' on the new Sefel label from Canada. The discs are actually pressed by KM in Los Angeles and the recordings were made with the Soundstream digital system from Salt Lake City, but the orchestras involved are the Budapest Philharmonic (which gave the premieres of several of the works involved) and the Budapest Symphony.

This is quite a stunning recording debut for a virtually unknown conductor. Joo' (prounced "Yoe") is 33, a native of Budapest, now a U.S. citizen. He came here in 1968, completed his studies at Juilliard and Indiana University, served on the Indiana faculty, then become conductor of the Knoxville Symphony, and since 1977 has been building up the Calgary Philharmonic.

In Calgary he attracted the attention of Joseph Sefel, a Hungarian-born technological entrepreneur who created the record company. The Barto'k project was Sefel's idea; recording it in Budapest was Joo''s. The discs, gorgeously recorded, handsomely pressed and packaged, and comprehensively documented, more than justify the expense and effort involved. Not quite all of Barto'k's orchestral works are covered; major omissions are the "Music for Strings, Percussion and Celesta," the Divertimento for String Orchestra and the music from the ballet "The Wooden Prince." But these are all available elsewhere, while the early Suite No. 2 has not been listed in Schwann for some time and the still earlier symphonic poem "Kossuth" has never been in domestic circulation before.

"Kossuth," which memorializes the 1848 revolution and its leader Lajos Kossuth, is very much in the manner of Liszt; it is not a great work, perhaps not an important one, but certainly an intriguing one for what it shows of the 22-year-old entering the orchestral arena under the influence of Liszt and Strauss. It is paired on SEFD-5005 with the Four Pieces for Orchestra, Op. 12, another little-known work but a much more substantial one, reflecting the strong influence (by 1912) of Debussy, but with a great deal of thoroughly original language as well. For delicacy of detail, for raw power where called for, and for all-round musical sense, it would be hard to imagine either work more convincingly performed.

The Suite No. 1, Op. 3, is packaged with the "Two Portraits," Op. 5, on SEFD-5006, and the Suite No. 2, Op. 4 (in the 1943 revision), comes with the "Two Pictures," Op. 10, on SEFD-5007. In the suites Joo' is possibly even more infectiously involved than Dorati, whose fine version of No. 2 is gone now but whose splendid recent one of No. 1 with the Detroit Symphony is available on London CS-7120. The "Portraits," with Andra's Kiss, solo violin, and the "Pictures" are marvelously evocative here.

The one disc among the five that struck me as less persuasive than the rest is SEFD-5008, which pairs the Dance Suite with the "Miraculous Mandarin" Suite. The latter is a bit rushed here and there, while the former is just a mite too expansive to hold together with the sort of spontaneity and easy flow found in the strongest competing versions. The great summing-up of Barto'k's orchestral activity, though, the Concerto for Orchestra, comes across with impactive brilliance to spare on SEFD-5009, even if the last two movements might have been a little tidier.

On all 10 sides, Joo' shows a superb feeling for the Barto'k idiom and, what is more, the ability to draw the highest level of performance from his two orchestras. His future recordings, with the London Symphony Orchestra in varied repertory, will be assured of serious attention.

Another digital recording of orchestral Barto'k--a remake of the Concerto for Orchestra and the Dance Suite by Sir Georg Solti, this time with the Chicago Symphony Orchestra--has been issued on London LDR-71036. The orchestra is unquestionably finer than either of Joo''s Budapest ensembles, the conducting is super-brilliant and the sound is, too, but these performances fail to come across as more than showcases for the assembled talent. Solti's earlier such pairing, with the LSO, now deleted, got closer to the heart of the music.

Solti contributes a personal note with the new disc, by the way, in which he reports his discovery that the tempo indication printed in the score for the Concerto's second movement is really slower than Barto'k intended, and that he has adjusted his pacing accordingly. But Fritz Reiner, who was close to Barto'k when the Concerto was composed, had already noted the discrepancy. In his classic recording of the work with the same orchestra (now on RCA AGL1-2909), he claimed no "discovery," but simply took the movement in question at the proper tempo, a bit faster than Solti's "new" one.