Jaap Schroeder's concerts incite either incredulous scorn or enthusiastic support from audiences and critics alike. Some people walk outmuttering "That's impossible" or "That's not what I intended to hear." Others give him standing ovations for helping them "really understand" at last the music of Beethoven or Mozart.

"It's kind of a civil war created in the audiences," says Schroeder.

Schroeder works as a violinist and conductor with musicians performing on period instruments and using period techniques, an approach as revolutionary to modern audiences as that of the most controversial contemporary composers. Despite its assault upon the "traditional" senses of many, this movement -- called "authentic performance," "original performance," "historical awareness," "music on period instruments" -- has found increasing adherents.

With numerous groups and performance series pursuing such musical authenticity, Washington has become something of a hotbed of the movement, and at the center of Washington's "authentic performances" are the internationally recognized ensembles of the Smithsonian's "Music on Original Instruments" series: the Smithson String Quartet, the Smithsonian Chamber Players and the Smithsonian Chamber Orchestra.

Tonight's performance by the Smithsonian Chamber Orchestra conducted by Schroeder -- inaugurating an annual series at the Kennedy Center Terrace Theater -- will represent another dimension of the music program of the National Museum of American History. Tonight's program will include Mozart's Bassoon Concerto in B-Flat Major, with bassoonist Dennis Godburn, Haydn's Symphony No. 45, "Farewell," and Beethoven's Symphony No. 3, "Eroica."

Founded in 1976, these ensembles have set the Smithsonian's Division of Musical Instruments apart from those of other museums. With a core group of eight musicians in residence, the program attracts other musicians from around the United States and Europe to perform and record here in Washington. These musicians have taken a museum program beyond facts and artifacts and into a movement every bit as avant-garde as minimalism.

"We at the Smithsonian have been in the forefront of the movement in this country," says cellist Kenneth Slowik, director of the Smithsonian's chamber music programs. "We have been able to get projects off the ground which have been impossible elsewhere in the country." Slowik, 32, a specialist in the viola da gamba literature, is the cellist of the Smithson String Quartet. He also teaches at the Oberlin Baroque Performance Institute.

Running counter to a modern esthetic that has led each international-caliber orchestra to sound more and more like the others, the Smithsonian groups and similar ensembles strive for a deeper awareness of the roots of classical music. "Against this kind of homogenization of music, we try to look at things on our own," says Slowik. "Putting it into an historical framework is a very exciting way of making the familiar unfamiliar."

To some, the phrase "authentic instrument" may summon visions of sackbuts, recorders and violas da gamba, but these medieval and Renaissance instruments merely mark the beginnings of early-instrument performance. Later works in performance document the piano's gradual evolution, from the harpsichord through various small intermediatestages to the virtuoso, 11-octave instrument of today. Period instruments are generally pitched lower than modern ones, sounding slightly out of tune to one accustomed to a modern concert pitch. With their gut strings, the violins are more mellow and less brilliant than their metal-stringed modern counterparts. Brass and woodwind instruments tend to stand out more in an orchestra, sometimes with a rawer edge, the valveless horns more difficult to play.

The authentic performance movement, however, is more than merely playing old music on period instruments. It is more like a reconquest of the music, not only to restore its original sound but also to emphasize the composer's virtuosity instead of the performer's.

What most people believe to be traditional Mozart or traditional Beethoven is in fact not the tradition of the composer's time but one comparatively recent. It developed, in part, from the evolution of instruments and techniques scaled to fill larger and larger concert halls with more reverberant sound. A Haydn or Mozart piano sonata played on a modern Steinway, where the timbres and tone are very different from those of an 18th-century piano, should be considered a translation of the original reading, and a rather free one at that.

Performers and composers have added further changes. While the beauty of baroque and classical compositions was recognized from the middle of 19th century, musicians believed they had to add something to earlier music. "Wagner and following people thought they could improve on performances by using other instruments and different technique to bring out the value," says Schroeder. Recognized as an authority on the violin literature and performance practices of the 17th, 18th and 19th centuries, Schroeder, 61, is one of the initiators of the authentic performance movement. A resident of Amsterdam, where he teaches at the Sweelnick Conservatory, Schroeder is also first violinist of the Smithson String Quartet and principal guest music director of the Smithsonian Chamber Orchestra. He regularly teaches at Yale University and the Juilliard School of Music as well.

"We have come to realize that Mozart and Beethoven knew pretty well what they were doing. The music was obviously written for those instruments. The whole idea of progress in instrumental construction and sound is absurd.

"You should give music ... the style and technique that intimately go together. What we try to do is get back to the origin and to feel, to write, to play the way the composer would have recognized. And I maintain, of course, that Bach and Beethoven would not have recognized the way a modern symphony orchestra is playing."

In resurrecting the original sound, the Smithsonian's extensive resources come into play. After obtaining a clean copy of the original score -- "one that has not been piled up with all sorts of suggestions of 19th- and 20th-century performers," says Slowik -- correct instruments must be matched to the music. With a collection of more than 2,000 instruments, including more than 250 keyboard instruments, the Smithsonian ensembles can match specific instruments to specific compositions. For example, this season the Chamber Players are performing a set of Beethoven trios, some from the late 1790s and some from the 1810s, a period in which the piano was undergoing rapid development. Consequently, they will use one piano from their collection for the earlier trios and a later piano for the later trios. "It's a matter of marrying the instruments and the music," says Slowik.

Period performances require string instruments with gut strings. Although gut strings have their disadvantages -- they don't stay in tune as well, they're not as loud, they can break in the middle of performance -- they have a rounder, mellower sound and color unknown to most contemporary audiences.

But it is the technique, mingling historical research with the artistic temperament, that sets this musical approach apart from modern performance. The instruments themselves demand adjustments from musicians. Steel strings, which replaced gut strings at the beginning of this century, forced musicians to play quite differently, using heavier bows with more tension and strength. With gut strings, the response is quite different in tone and color. "Many modern players find these old-style instruments somehow emasculated or not giving enough resistance," says Slowik. "All these things require a physical response, a change of attitude."

There are stylistic differences as well. Schroeder emphasizes the concept of "performance practice," which means considering not only the relationship of music to instruments, but also how the instruments were played during earlier periods. "Instead of stressing loudness and speed," says Schroeder, "we are now going back to the concept of resonance, of rhythmic flexibility, the whole approach to technique. Literally, flexible means that you are not as tense and that you have more color in your playing. You have to realize that to revert to the old material.

"More and more people have, of course, rallied to our side and recognized the instruments of that time do sound more exciting and more flexible and more colorful ..." Furthermore, performance practice requires coming to grips with specific stylistic variations from one composer to the next. "As we go into these other periods you say, 'Oh yes, for Haydn I should really try and do this differently than I do Bach,' " says Slowik. "You bring to it one's own experience based on treatises, work with the instruments, on things as far-flung as paintings which might give a clue. You then make interpretive choices which would be in keeping with what we know of certain stylistic norms of those earlier periods."

Of course, one can take the historical context too far. The first recordings made with this approach were considered rather formal and stale. "If you do it just to follow the rules, or just to be historical, that's really not all that much," says Slowik. "It doesn't mean that we wear period costumes and not bathe for three weeks ..."

When the authentic performance movement began in the early '60s with the Quadro Amsterdam -- composed of Schroeder, harpsichordist Gustav Leonhardt, flutist Frans Brueggen and cellist Anner Bylsma -- the repertoire was limited to baroque. By the time the group disbanded five years later, it had influenced many like-minded musicians to specialize in earlier music, and early-music ensembles sprang up in Amsterdam, Vienna, London and elsewhere.

By the early '70s, however, these musicians had raised their sights. "Because we come to it with our 20th-century biases," explains Slowik, "we have to ask ourselves after a while, 'What's next? If I were alive 200 years ago, wouldn't I also have experienced this stylistic shift {from baroque to classical}?' "

In moving on to classical and then early romantic works, these musicians rediscovered the novelty of each composer.Says Schroeder: "After years and years of working on Mozart and Haydn and Boccherini, moving up to Schubert makes you discover what the composer knew when he was writing ... It is a very exciting way of discovering things."

As the movement has progressed, it has begun to specialize and institutionalize. The first generation of players formed a broad class of "early musicians," people who often turned to earlier music because they were not up to the technical challenges involved in performing Paganini or Stravinsky, according to Schroeder. Now there are baroque players and classical players who specialize in the repertoire and performance practice of specific periods. As this approach expands further into 19th-century repertoire, there will no doubt emerge musicians specializing in the romantic period.

Perhaps more important, performance practice is now being taught to receptive students as an alternative approach in graduate programs like those at Juilliard and Yale. Whereas traditionally a musician will learn a block of music for a specific instrument in order to master the instrument, students may now adopt this more inquisitive approach and develop techniques specific to a period.

Still, some have their reservations about how far authentic performance can go. When they took on Mozart and then Beethoven, Schroeder and company met with raised eyebrows. Now people are telling him "Oh well, Beethoven, I can see that. But you can't do that to Schumann." And he responds, "Well, as a matter of fact, we intend to go on also with Schubert and Mendelssohn and Brahms. I would like to play Brahms in the right sound in the right technique with the right piano. I mean, there is no reason to stop anywhere."