Luciano Berio speaks with passion and conviction. Whether the questions deal with matters technical or philosophical, the Italian composer responds easily and articulately, getting to the point right away and then spinning a virtuoso variation on some thematic thought. His casual appearance - hunched over a table, a chewed cigar stub between the fingers of his right hand - belie the intensity of his words. The man is a thinker. As concerned as he is with his own artistic products, he seems to be concerned even more with the function of all artistic products in contemporary society.

Berio's visit to St. Louis early this month was his first in 19 years. The occasion in 1966 was the performance by the St. Louis Symphony of Berio's ''Allelujah II.'' This time he was in town to supervise rehearsals of his new six-minute ''Call (St. Louis Fanfare),'' for brass quintet, which will be performed Aug.and 30 in conjunction with the opening of the renovated St. Louis Union Station. He said he was impressed with the growth of the city, with its ''new energies,'' with its ''desire for renewal.''

In fact, he hinted that if it were not for St. Louis'interest in renewal, his brass quintet might not have been written.

The commission came from the St. Louis New Music Circle in celebration of its 25th anniversary.

Berio said he was approached last fall by Elizabeth Gentry Sayad, the organization's chairman, and asked to produce a ''short, festive'' piece for the events at the train station. He was as busy then as he is now, with large-scale works for the Amsterdam Concertgebouw, the Orchestre de Paris and the Chicago Symphony still on the drawing board. Berio told Sayad he'd have to think about it.

''And I did, for several weeks,'' he said. ''Two things helped me decide that, yes, I should do this piece. In the first place, I've been interested in the techniques of brass instruments for some time now. The latest piece in my 'Sequenza' series, 'Sequenza X,' is for solo trumpet, and the premiere took place in Los Angeles just last November. So I saw this as an opportunity to continue exploring certain ideas I had.

''Secondly, the occasion seemed to be very worthwhile. This old building that is being brought back to life, that's something worth celebrating. They wanted festive music for a festive occasion. Festivity is an important aspect of our lives. At least it should be. So I decided that I could make a contribution to this.''

Berio said he composed his fanfare in four days in late July. He added, though, that he knew how the piece would ''work'' as soon as he agreed to write it.

''Always, you start with the architecture of the piece,'' he said. ''In this case, I wanted to deal with four 'privileged' musical intervals, and I wanted to move from a structure that was simple to one that was complex. I had ideas, too, for an evolution of temporal elements, something that would grow from simple periodic rhythm, like a metronome, to passages played as fast as possible, with a continuum of speeds in between. This was the plan, the 'global view' of the piece. It came to me instantly. When I sat down to write the piece in July it was just a matter of filling in the details.''

Some of the details contained in the piece are fragments of melody borrowed from W.C. Handy's famous ''St. Louis Blues.'' Berio said the references are so subtle that they probably won't be noticed by listeners. ''It was just something to get me started,'' he said, ''just a bit of material with which I could begin working. It's actually a very timid reference - the song implies an idiomatic way of singing or playing, and the jazz style does not play a role at all in this piece.''

For Berio, the inclusion of a reference - however hidden - to older music is a typical gesture.

Born in 1925, Berio emerged from the World War II years as a dedicated young ''modernist.'' His teachers at the conservatory in Milan had steered him in the direction of an eclectic sort of neo-Classicism spiced with jarring dissonances, but an encounter with the German composer Karlheinz Stockhausen at Darmstadt in 1954 convinced him to jump on the bandwagon of 12-tone serial music. Berio's approach to 12-tone writing was never as rigorous as Stockhausen's, however, and in many of his works from the late 1950s the ''rules'' of serialism are discarded in favor of lyrical gestures. From 1955 to 1961 Berio served as director of the new electronic music studio operated by Italy's national radio station. Even his electronic pieces from this period reflect his need to create music that was ultimately more expressive than intellectual.

Freed from the procedural restrictions of serialism, by themid-1960s Berio had developed an eclectic musical language in which any materials could be used if they helped further communication between composer and listener. He produced a number of works (the 1960 ''Circles,'' the 1966 ''Sequenza V'' for solo trombone) that call for overt theatrical action to be incorporated into the musical performance. The third movement of his 1968 ''Sinfonia,'' for chorus and orchestra, features a collage of quotations from the 20th-century repertory overlaid on the scherzo of Mahler's ''Resurrection'' Symphony; the tape part of his 1969 ''Questo vuol dire che'' contains a mix of musics from non-Western countries. Berio's only full-length opera, a work from 1970 titled ''Opera,'' is in effect an olio show of assorted vernacular and ''high art'' theatrical styles fitted around a destruction-of-mankind allegory.

In a set of interviews with the Italian musicologist Rossana Dalmonte published in book form earlier this year (''Luciano Berio: Two Interviews,'' Marion Boyars, Inc., $19.95), Berio says: ''Music must be capable of educating people to discover and create relations between different elements, . . . and in doing that it speaks of the history of man and of his musical resources in all their acoustic and expressive aspects.''

He elaborated on the thought during his St. Louis visit.

''I believe strongly in hierarchies, in values,'' he said. ''The fascination of music is that it is meaningful on many levels. It's wrong to listen for only one element, because there are so many things going on. There's more than just melody in Bach, and there's more than just pitch relationships in Schoenberg. The listener's depth of understanding has to do with the number of relationships he can find. The ideal listener is the one who can catch all the implications; the ideal composer is the one who can control them.

''It was easier to do this in the past, I think, because there was only one musical language, only one grammar. Today each composer has to invent his own language - it's much more difficult.

''My language, my style, has to do with bringing together different things. There is a psychological need for this, especially in our modern society. I think that deep down we all feel a need to create a harmony out of the multitude of things we know - not just the things of the present, but also the things of the past.

''These references to 'St. Louis Blues' in my 'Call' - they are just symbols, and musically they are not at all important. I know where they are, but I'm sure the listeners will not hear them. They don't need to know about them in order to take in the piece. If they do know about them, well, maybe it will help them find one more 'relationship' in the music.''