Early music pioneer Jordi Savall talks about his upcoming Glasgow dates

Of all the great movers and shakers associated with the 1970s early-music revival Savall has always followed his own musical and ideological path. He's an extraordinary combination of fastidious scholar and ardent free spirit, as passionate about embracing new musical cultures and improvisation as he is about investigating forgotten medieval scores. He's also tremendously heartfelt: "If the face is the mirror of the soul," comes a typical quote from the sleeve notes to a recent album, "a people's music is the reflection of the spirit of its identity."

The Catalan musician "discovered" the viola da gamba in 1965 at the age of 23. As a boy he had sung in his local choir and he went on to study the cello in Barcelona, but when he was introduced to what was then still a dinosaur rarity of an instrument he was captivated by its expressive, intimate voice and never looked back. Just under a decade later he and his wife, the Catalan soprano Montserrat Figueras, founded one of Europe's most important early music ensembles, Hespèrion XX (it became Hespèrion XXI at the turn of the millennium). They toured the world playing forgotten scores from the 16th and 17th centuries, but from the start also integrated traditional folk music of the Spanish Sephardic Jews.

Savall's work has spanned the realms of world, folk and classical music ever since, and has achieved commercial success across all three in a way that few other musicians have managed without ditching chunks of integrity along the way. Next week he performs a pair of concerts in Glasgow: the second, a programme called Music of the Emperors, explores the music of Spain during the reigns of Emperors Charles V and Philip II. "Basically it's dance music," he explains from his home near Barcelona, "the kind of ostinato dances where a repeating pattern is established and then we improvise around it. Almost the whole concert is improvised. Styles such as La Folia come from early Spain, and we mix in later music from Mexico that is a prolongation of the same tradition."

The other Glasgow concert turns to music from our own shores. A few years ago Savall and the Irish harp player Andrew Lawrence-King recorded The Celtic Viol, a collection encompassing Scottish and Irish music from the 17th and 18th centuries. The project had been a long time coming: it was as far back as the late 1970s that Savall, on tour in Kilkenny, found himself captivated by spirited local fiddlers and flautists busking in the streets. "What incredible vitality!" he writes in his introduction to the album. "And it was magical to see so many musicians living their music with that degree of intensity and emotion!"

It struck Savall that he should play this music on the viola da gamba. "In the 17th century viols were family instruments," he says. "Viol consorts" – ensembles of variously sized viola da gambas – "were a very popular way for people to make music together in their own homes. After the violin arrived in about 1680 the popularity died down. But compare a photograph of my treble viol to a painting of [18th century Scottish fiddler] Niel Gow's instrument. They're not so different, eh? The only real difference is that mine has six strings and his four, and that mine has frets." It's tricky to make real glissandos with frets, whereas Gow would have likely used slides for all sorts of colouring – the way fiddlers and flautists do today.

Other encouraging clues came from the Manchester Gamba Book, the largest existing manuscript of solo viol music that dates from around the 1660s. Among its some 250 pieces are almost 30 ways to tune the instrument, which for Savall proves just what a flexible beast the gamba was (and still is). Some of those tunings are exactly the same as the tunings of a bagpipe, "and that means," he says, "that the viol must have had a very real connection with the traditional Celtic music of its day".

Savall went about choosing his material for The Celtic Viol like the early music scholar that he is. He delved into collections of 17th century Scottish and Irish tunes, most of them printed in the 19th and early 20th century, and listened to as many and as wide a range of recordings as he could, old and new. The resulting line-up includes a few well-known tunes – Niel Gow's Lament for the Death of his Second Wife gets pride of place – alongside many rarities.

The sound of the albums is shrouded in echo; the recording was made in the Catalan Benedictine hillside monastery of Sant Pere de Rodes and sounds accordingly misty and mystical. "I love the space this music allows for its slow airs," he says. "Sometimes the reaction I get from Celtic musicians is that my rendition is too sad, too serious, not lively enough. I know I'm an outsider to the tradition, but I hope that what I bring is a new angle – that of a baroque and renaissance specialist."

Savall describes The Celtic Viol as "a fervent tribute to the art of transmission". Notated music might have allowed "a formidable development of musical forms and instruments, but at the same time has contributed to the neglect and relegation to a second-class category of all those forms of living music which traditionally accompanied the daily lives of the vast majority: in other words, popular music."

At heart he's an idealist, and The Celtic Viol, like so much of his work, is about ploughing a level playing field. "Similar to the way I've approached Sephardic music over the years, and all my repertoire, in fact, my approach to Celtic music has been to go back as far as possible to the original material. By stripping away the centuries of interpretation I'm trying to let the music speak for itself."

Jordi Savall plays at the University of Glasgow Memorial Chapel on May 1 and at Kelvingrove Museum on May 2.