Never heard of Philip Glass? The guy's a classically-trained musician, often playing with a small ensemble of avant-garde performers and occasionally turning his hand to grand opera and string quartets. But David Bowie has heard of him, David Byrne has worked with him, and both Paul Simon and Laurie Anderson have recorded with him.

And the music of The Philip Glass Ensemble isn't all that inaccessible — it dominates films such as "Koyaanisqatsi" and Paul Shrader's "Mishima", it provides the CBS Masterworks label with their biggest-selling albums, and once in a while you can catch breakdancers bopping about to it from Covent Garden to New York's Times Square.

What else do you want to know? Well, Glass studied at the Juillard School in New York, became interested in Indian music and studied with Ravi Shankar, drove taxis and worked as a carpenter to make ends meet, and formed a small instrumental ensemble and his own record label, Chatham Square, to release his music. His early pieces such as "Music In Similar Motion" and "Music With Changing Parts" didn't make much of an impression, often emptying concert halls due to their severely minimal nature.

But Glass signed to Virgin Records in the wake of the success of Mike Oldfield and Tangerine Dream, and put out "Music In Twelve Parts" and the film music album "North Star".

From that point there was no stopping the man. "Dance", one of his first modern ballet pieces, the heavily synthetic theatre piece "The Photographer", and most of all the opera "Einstein On The Beach", confirmed him as one of the world's top-selling classical artists — but one whose appeal crossed over into the world of amplified rock music. The Philip Glass Ensemble plays loud, with mixing engineer Kurt Muncaksi unashamedly centre-stage and the sounds of synthesisers and Emulators mixed with flutes, brass and often voices.

Glass was most recently in London to supervise the premiere of "Akhnaten", the third of his "portrait-operas" after "Einstein On The Beach" and "Satyagraha" (which dealt with Gandhi's early life and was transmitted on Channel 4 TV last Christmas).

Even though "Akhnaten" is a largely orchestral piece, there's a Yamaha DX synthesiser hidden away in the orchestra somewhere, and for a very good reason, as Glass explained. "Because 'Akhnaten' doesn't have a large orchestra, the synthesiser helps to smooth out some places where the wind and brass parts don't get much time to breathe. I wrote the piece in 1983 and I've found different ways to do that now, so I don't use it any more — but in the studio we usually have a synthesiser doubling the brass parts, or some Emulator strings underneath the real ones.

"I work a lot with synths — I've got nine in my band and I didn't realise how they'd crept up. We didn't use them until about 1978 when they were really polyphonic — and they aren't really now, because you still get notes stolen after you play five or six keys. But now I've got an Emulator and an Emulator 2, two DX9s, an Oberheim, a Prophet, a Roland JX3-P and so on, and I'd say at this point that the synth has a generic sound of its own; when I write parts for the synths I used to write 'woodwind' and 'brass' to indicate the kind of sound, but now I tend to just write 'bass synth' or 'wind synth' because they have sounds of their own."

The way Glass uses synths seems to mystify and even alienate the rest of the classical world, so what exactly do they offer him in the studio?

"Well, I've done a lot of recording with synthesisers — even when you think you're hearing an acoustic instrument there's a synth doubling it an octave below, which gives you a much bigger bottom end. The soundtrack of 'Mishima' (launched at this year's Cannes festival) is for strings and harp, but the harp is a DX9 and plays parts no real harp could play. Then when I write for the real instrument I have to make that adjustment again.

"I doubled all the 30 live strings on the recording of 'Satyagraha' with synths and scarcely anybody can tell where one ends and the other begins. In fact there's some very difficult wind playing in 'Satyagraha' and I used synths to smooth out the rough places and give a very deep sound.

"But all this is just part of recording technology, and has been for 15 years. I'm surprised that more people in the classical business haven't done what Michael Riesman, Kurt Muncaksi and myself have done — our kind of experiments in acoustic/synth combinations aren't generally done in classical music, and in pop they don't use this instrumentation, so it's a real breakthrough area."

Even so, it's surprising how technological the recording of an album which ends up sounding "classical" can be. "The Photographer" was recorded to a click track, but that's a fairly synthetic piece — more surprisingly, the opera "Satyagraha" (which Glass has just recorded for release on CBS later this year) used many of the same techniques.

"We had the conductor take the rehearsal pianist through the piano score, then we set a click to it with a Dr Click or one of those damn stupid things — we've got them all, anyway. You can just adjust the tempo to include all the accellerandi, so we had a complete click track. Then we took off the piano, put on the high strings, the winds, the chorus and so on.

"Basically we record each piece horizontally and then vertically, so it may seem longer our way — but in fact it's quicker, because when we get done there's virtually no editing. All we do is use a razor blade to put the header tape on the front."

Now that he's composing for advanced instruments like the Emulator 2 and Oberheim synths, doesn't Glass feel himself becoming a slave to the recording studio?

"Not at all. My studio at home is just a Baldwin upright piano and a big desk, and I really know how a piece should sound as I'm writing it down. If it sounded any different when we got to the rehearsal stage I'd be worried! But now we've jointly built a 24-track studio with two other guys, and so I don't have that terrible bugaboo of 'studio time'. Apart from the album pieces, I do a lot of preparation for live performances — the designers and directors as well as dance companies work from rough tapes, so I make a synth tape of a whole piece in real time and even put scratch track voices on."

Such a prolific composer must surely have a heavy day-to-day schedule. Does Glass have a strict regime, or can he only compose when the Muse is with him? And what equipment does he use in the studio?

"We're busy all the time; usually I get up around seven, work on a new composition until noon, then go into the studio for a few hours. At the moment we have in there an MCI 24-track machine, Soundcraft 16- and 24-channel boards and a whole shitload of noise reduction and digital delays. There's a 15 by 30 foot control room for the synths and a 10 by 12 foot room large enough for about a dozen string players.

"I always record in sections, and now we're not worried about hours we can break down into the smallest sections we want, so space for 12 string players is enough. We've made our own samples for the Emulators, but string and choir sounds are very difficult; it's OK for dummy tapes though, and we play in tandem with the Emulator 2, DX9s and Oberheim to arrive at a sound. But you can't really get away with it for a real record apart from percussion and gong samples — I can even do a string quartet on the Emulator 2, but I'd still do it a voice at a time."

On the subject of string quartets, back to 'Mishima' and some thoughts on the world of film music. Glass's soundtrack for the wordless "Koyaanisqatsi" was a massive critical success — did he work in the same way on "Mishima"?

"Paul Shrader partly shot the film to a working tape of my score — I composed 60 minutes and thought he'd use about 30, but in fact he asked me for five more. I went down to see a lot of the shooting, and he used my working tape of the music, re-cut it and looped it to make it fit, and then gave me the working tape back all cut to pieces. Then I had to re-write the score to fit, as on 'Koyaanisqatsi'; but at least this way the film-maker has some aural reality to work to. What most people don't realise is that film editors just can't cut without music, and if they don't have your music to hand they'll use Wagner or whatever they can. And, of course, it's very disturbing having to write to match someone else's temporary music. So I prefer my way of working, even if it does mean writing the score twice."

The album of "Mishima" will probably be released on Elektra/Asylum, since CBS have enough Glass Product with "Satyagraha" and an album of songs with Paul Simon, Laurie Anderson, David Byrne and others coming up. Other recent works include Act 1 of a new opera, "The Making Of The Representative For Planet Eight", in conjunction with Doris Lessing who wrote the book of the same name. The Netherlands Opera are due to perform this in March '86. Then there's "The Juniper Tree", a theatre piece based on The Brothers Grimm, due to premiere in Boston in December, and a ballet piece for Twyla Tharp which she hasn't rehearsed yet. And in Spring 1987 the Stuttgart Opera are putting on all three "portrait operas" — "Einstein", "Satyagraha" and "Akhnaten".

Back to the studio though, and time to check out rumours that Glass had finally begun to use sequencers rather than manual playing in his music.

Not true, as it turned out. "We have a computer sequencer which Michael Riesman feeds percussion parts into in digits, but I'm not so keen on that method. It's an experiment, and we're mostly using the computer as a word processor for the scores. You see, Michael is such an extraordinary keyboard player that he's better than most machines, he doesn't break down or forget his programs. It seems to me quicker to work that way.

"In any case, sequencers aren't much good to me, because the music changes so much. It's the same for two or three measures then it changes, and that may go on for two or three hundred measures. It's not simply a pattern that repeats; people thought that's what it was, but the problem is not that it doesn't change, but that it changes all the time. If it didn't change it would be unlistenable; it's the accretion of small changes that gives the music its life, and that represents a terrific problem in sequencer programming.

"The people playing 'Akhnaten' at the English National Opera have to keep watching for the changes all the time — a lot of the dramatic tension of the music evolves out of the rate of change, so you need live players who have both attention and stamina."

So what parts does Glass take when his Ensemble plays live? "I take the easy parts! I don't practice any more because I simply don't have the two hours or so per day you need to keep your fingers in shape. I used to be the first keyboard, and in 1975 I became second to Michael Riesman. Then we got a third guy to play my parts because I couldn't even play the second parts any more. If you saw my part of the score now it would say 'Organ 3'! I'm much better off writing than practising, so Michael has become Music Director and he rehearses the ensemble. I just go to the first and the last few rehearsals.

"I'm a performer as well as a composer — I do 50 concerts a year, and that's a lot of work. We have a six-week tour in the Fall, three weeks in Spring and more in Summer, around 12 weeks a year altogether. I can't rehearse the ensemble with that schedule, but my booking agent says he can't book dates without me playing, and the fact is that I love to play. I've been playing in front of an audience for years and I don't want to give it up, so the ideal situation is for me to play and not to practise!"

Glass is being modest, of course. The "Four American Composers" series by Peter Greenaway that was shown on Channel 4 amply demonstrated the speed and stamina necessary for his keyboard technique, and it's a mistake to think that his pieces are simply repetitive and so are easy to play. In fact, some of them provide the greatest challenges facing musicians in the classical avant-garde field today.

Glass will undoubtedly continue to go from strength to strength, and the unique crossover position his music occupies will probably educate classical music lovers about the rock world and rockists about the classics for years to come.

The linking factor is his application of modern technology, of which he says, "I've always found it very friendly. I love it, and it's been very helpful to us." What a refreshing attitude — makes you wonder what Scarlatti would have done if all those years ago he had been given a 24-track studio and an Emulator 2.