The bridge between classical and popular music is getting pretty congested, what with all the crossover traffic these days. There is scarcely an opera or art-song star who doesn't now regularly trot out show tunes, to say nothing of entire Broadway roles. Nor does it take much encouragement to get Nigel Kennedy, Yo-Yo Ma or André Previn to start improvising. 

But what about the other direction? Here the traffic is lighter, the steps taken much more tentative. Jazz vocalist Bobby McFerrin has successfully led Beethoven's Seventh Symphony on a couple of occasions. Jazz pianist Keith Jarrett plays Bach, Bartók, Shostakovich and Peggy Glanville Hicks, but he has become something of a recluse in the jazz world. And nine years ago in San Francisco an uncharacteristically stiff Frank Zappa tried his hand at conducting Varèse . 

Thus seeming practically a class by himself is Wynton Marsalis, the New Orleans-bred, Juilliard-trained trumpeter, who is one of the most popular and outspoken jazz musicians of our days, but who can also peal off Bachian and Handelian trumpet flourishes with seeming improvisatory ease, as he does on a new Sony recording of Baroque music for soprano and trumpet. 

At the age of 30, Marsalis is something of an institution. Before he had even reached the legal drinking age of 21, he was already making a name for himself, playing with Art Blakey. By 1983 he had released best-selling, Grammy-winning Jazz and classical albums. And the difficulty of catching up with him in New York late last year indicated just how chaotic living in those many musical worlds can be. The apartment at Lincoln Center he has temporarily taken over is strewn everywhere with pages of a ballet score he is madly finishing for Garth Fagan. Jazz is playing on a portable CD player while an assistant is busy scoring. Another assistant is taking care of serving a take-away Chinese lunch. Marsalis is in town for the ballet, an appearance in a Carnegie Hall Christmas programme with Battle, Frederica von Stade and André Previn (for video release this Christmas), and to present a programme of Louis Armstrong films at the Film Society of Lincoln Center. He is just back from South America and soon will be off touring again.

Marsalis makes a great show of enjoying, of thriving on, his musically and geographically peripatetic existence. But he admits it is not as easy as it looks, especially the moving between the jazz and classical worlds. 'It is hard,' he says. 'It takes a lot of time to practise both styles. Each style requires a different technique and a different sound. On the trumpet you have to play with a different attack.' 

More important is the whole difference in musical philosophy, however, which better explains why so little crossover gets beyond superficial hand-shaking, despite the fact that many jazz players are trained in classical techniques and all but the most head-in-the-sand young classical players grow up participating in a world dominated by pop culture. 

'In jazz you have to spend hours practising to learn harmony and other basics. You should know them in classical music, too, but you can play a trumpet part in a symphony without knowing what the harmonic structure of the piece is. In jazz you can't do that. 

'In jazz you have to construct a whole philosophy around what the music is about and then learn to play it. In classical music you just have to learn to play it, but you have to achieve a very high level of technical expertise. A lot of the comparisons between jazz musicians and classical music compare a composer to an instrumentalist. Well, you can't compare Richard Wagner to Louis Armstrong, because Richard Wagner's achievement is of a composer whose vision is then executed by 100 musicians. The achievement of Louis Armstrong is democratic. It's not just that he's great, but how do other people sound with him? You know everybody has to play together.'

And when Marsalis talks about playing with Kathleen Battle, he thinks a lot about how he sounds with her. 'I have learned a lot working with her' Marsalis says, noting that the first time he performed classical music in concert was a performance of Bach's Cantata No 51 eight years ago – the work reappears on the new Sony release with John Nelson leading the Chamber Orchestra of St Luke's. 

'Just listening to that glorious sound during a week of recording,' Marsalis explains, 'I learned a lot about expression, because in the earliest jazz, the instrumentalist imitated the vocalist, the blues singers. That's how they got all those effects on their horns. And then, later, the vocalists came to imitate the musicians. 

'But what I love most in all of music is the kind of pure expression that I hear in a voice like Kathy's. I don't like a lot of artifice. The only time I like artifice is when it is artifice. I don't like the illusion of expression, like a lot of vibrato or trills. I like that stuff when it's actually being used as ornamentation. 

'Pure emotion is very hard to identify, because it's in the middle. It doesn't go one way or the other. It's not like an angry sound or a happy sound. It'sjust a sound with a certain type of beauty, and real beauty has two components: it's real ugly but it's real beautiful. What I'm saying might be crazy sounding, but that's what Kathleen Battle's sound has in it. It has a lot of qualities. It has qualities that are real sweet and pure, but it has a lot of bite in it too. It has everything in it that I like.'

It is tempting to think of Marsalis, with his reverence for purity, his profound knowledge of the musical traditions of classical and jazz, his often paying homage to the jazz greats of history, as almost the ultimate classicist. But call him that, and he bristles angrily. 'You 're wrong, I'm truly a modernist,' he counters. 'I don't believe in a style. That's an old-fashioned conception. What being modern means is that every day in your life you interact with stuff that's ancient to modern. If you want to go to Central Park and get in one of those horses with your old lady, you want to do that. You don't say, "Oh, man, I can't do this, because people used to do it. I want to get in an airplane only." Modern life is everything that came before it and modern life.' 

'But, you know, it's easy to get wrapped up in style. I play bebop; I play modern; I play jazz. This is classical music, this is pop music. You need the terminology, but as an artist you can't get caught up in it. If you address the fundamental expression of music, whatever the idiom, you will find something innovative. Because you're modern. There's no way in the world you can feel about living in the world what Louis Armstrong felt. You can't even if you wanted to.' 

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