If the phonograph had been invented in Bach's time, we'd know exactly howhe intended his Brandenburg concertos to sound.

Given that there's no such definitive recording, however, musicians rely on the next best thing: the notes Bach wrote, the instruments he used, and the accounts of his contemporaries regarding how they were to be played.

All of these sources inform the lyrical music-making of Pro Musica Rara, Baltimore's resident early- music ensemble, which will present a program of baroque masters today at 3:30 p.m. in its second concert of the season at the Baltimore Museum of Art.

"We can never know for certain exactly how this music sounded," says Shirley Mathews, Pro Musica's artistic director. "But just having the right equipment is half the battle, because then the music begins to mean something."

Interpreting early music - generally defined as works composed before 1750, or during the renaissance and baroque periods - has become classical music's biggest growth industry in the last 30 years.

Over that time, musicians have taught themselves not only how to play the forerunners of today's modern flutes, violins and cellos, but also how to adorn their melodies with the characteristic accents and ornaments employed by artists of that period.

As a result, contemporary audiences are so accustomed to hearing Bach and Scarlatti played on the harpsichord instead of the piano, for example, that many would be surprised to learn that just a generation ago, such performances were considered daring experiments.

Pro Musica Rara, a Latin name that translates roughly as "for rare (as in rarely performed) music," was founded in Baltimore in 1974 by a small group of Baltimore Symphony Orchestra players who wanted to create fresh readings of the baroque classics.

The first members were trumpeter Rob Roy McGregor, oboist Joe Turner, bassoonist Phil Coker, flutist Tim Day and violinist Bruce Wade. All were interested in learning to play baroque versions of their modern instruments.

"You couldn't just go out and buy a baroque oboe or flute in those days," Mathews said. "You had to have it specially made. The same went for violins and cellos."

The players ordered copies of the old instruments and began rehearsing in 1974. Their first concert was in 1975. It was, to say the least, a challenge.

"The biggest problem players had back then was intonation," said Mathews. "They hadn't yet figured out how to play in tune."

That wasn't all they had to contend with. Nobody really knew, for example, how to make reeds for the older oboes and bassoons. And unlike their modern counterparts, baroque woodwinds lacked the elaborate array of mechanical keys - a late 19th-century innovation called the Boehm system - most performers grew up with.

When it came to string instruments, the problems were daunting. While many famous violins, like the Stradivarius, were built in the baroque era, virtually none have survived in their original condition.

"All the Strads were remodeled in the 18th century to give them longer necks, metal strings, different angles to the bridges and bass bars," Mathews said.

These changes allowed performers to play louder and gave the instruments greater range. Moreover, the shape of the violin bow also changed, from the short, curved baroque bow to today's longer, nearly flat bow.

It took awhile for musicians trained on modern instruments to learn the techniques that would allow them play the older instruments well.

"Today we have wonderful younger virtuosos on period instruments who can play anything," Mathews said. "A lot of them are students of the members of this group."

Ensembles like Pro Musica Rara inherit a tradition that began in the mid-19th century, when the composer Felix Mendelssohn revived regular public performances of the music of J.S. Bach.

Bach had been virtually forgotten during the 100 years after his death. Mendelssohn's revival inspired a movement to rediscover the great works of the past.

At the turn of the century, the English instrument-maker and musicologist Arnold Dolmetsch launched a one-man crusade to revive long-neglected instruments like the harpsichord, the lute and the recorder.

Even more important, Dolmetsch urged performers to reacquaint themselves with the writings of Bach and Handel's contemporaries on how their music should be played.

His then-radical ideas got a popular boost in the 1920s and 1930s from such virtuoso players as organist Albert Schweitzer and harpsichordist Wanda Landowska.

Landowska, in particular, waged a virtual 40-year-long campaign win acceptance of the harpsichord. Schweitzer was a scholar-musician who, in addition to performing, wrote a monumental two-volume study of Bach's music.

Neither Schweitzer nor Landowska actually played on period instruments, and their ideas about what constituted "authentic" performance practices today appear naive at best.

Yet their insistence that the earlier practices could be rediscovered and that they could form the basis for modern performances had a tremendous impact. They played superbly, and their passionate proselytizing encouraged an entire generation of musicians to rethink their approach to the classics.

By the 1960s, a revival of renaissance and baroque music was well under way, starting with pioneering ensembles in New York and Boston.

Boston, in particular, became a major center of period instrument-making, led by such craftsmen-scholars as harpsichord builders William Dowd and Carl Fudge.

Today, the period instrument approach has been so widely accepted that traditional orchestras have largely ceded the field to ensembles that specialize in early music.

At the same time, the movement's original goal of "authentic performance practice" has come to be seen as more an ideal than a fixed set of rules.

As the writer Jan Swafford put it, "Most early music specialists have let go of the illusion that we can really find out everything about how Machaut, or Bach or even Haydn played their music ... [and] have admitted that our way of doing any music cannot help but reflect our own times."

Still, the early-music movement has brought an astonishing vitality and excitement to music long mired in stale tradition. And it has allowed modern audiences to hear the classics with fresh ears.

"Every generation has a new layer to add," Mathews said. "We used to claim absolute authenticity, but we don't do that anymore. The main thing is to get the right instruments, play in tune and do the best we can."

What: "The Glory of the Baroque," music by Bach, Locatelli and others

When: 3:30 p.m. today

Where: The Baltimore Museum of Art, Art Museum Drive at North Charles Street

Tickets: $15, $12 for seniors and BMA members, $6 for students, free for children under 16

Call: 410-433-0041

Pub Date: 11/22/98