It sounds at first like an electronic whine, a build-up of noise in the amplifier. Then there's Lou Reed's voice, young but hardened: "When the smack begins to flow, and it shoots up the dropper's neck, and I'm rushing on my run, then I feel just like Jesus's son." Behind it all, there's that strange keening, humming note. Listen to the Velvet Underground's Heroin on headphones and you realise it's not feedback after all, not a synthesised warble, but the rich timbre of a violin playing a single note, held for a disturbingly long time. It's the darkest thing in the darkest of songs. If Reed sounds as if he's made a pact with the devil, then the musician who plays that buzzing fiddle - John Cale - must be the devil himself.
The Venice Biennale
Starts 7 June
Until 22 November
When I was 15, I used to come home every night from school to Cale, listening to my German compilation of Velvet Underground classics on headphones, escaping from north Wales to downtown Manhattan. I remember buying their second album, White Light/White Heat, and finding a track called The Gift especially arresting. A spoken-word recording, it tells the story of how a young man tried to mail himself to his girlfriend, who unfortunately decided to open the package with a saw. But this wasn't Reed's accent - it was a Welsh voice. Not my part of Wales, but south Wales. Apparently, one of these unimaginably sophisticated Manhattan art-punks was Welsh, like me.
John Cale is Welsh all right - so Welsh that he is representing his nation at this year's Venice Biennale. Next month, he will make his debut as a visual artist, with an ambitious audio-visual installation in the Wales Pavilion at the world's most famous art festival. And here he is, sitting in a corner, a cup of coffee in his hand. I have to admit I find myself thinking more about the Velvets than Venice: sure, it's great he's made an art film - but what about the night Andy Warhol got the Velvet Underground to play a convention of psychiatrists at Delominco's steak house? The psychiatrists were appalled. "That was revenge - Lou's revenge," Cale says, "and I was all for it." As a teenager, Reed had been given electric shock treatment to "cure" him of homosexuality. "Lou and I were going to put out a record with his psychiatrist's letter on one side and my arrest record on the other," he says. "I was arrested for being in posession of chemical substances." This wasn't Warhol's fault: Cale was arrested earlier in the 1960s, while playing viola in an avant-garde classical ensemble, La Monte Young's Theatre of Eternal Music.
Cale was born in Garnant in the valleys, and played viola in the Welsh Youth Orchestra. He went to America on a prestigious Tanglewood music scholarship, having survived an interview with the composer Aaron Copland, and first visited New York to hear a summer concert at the Lincoln Centre. "I remember being apalled by the smell of the city," he tells me. "It was hot and humid - it stank. I was terrified, but I had to just put my head down and charge. All the happenings were going on - people were just putting events on at the drop of a hat."
What I have wanted to know since I was 15 is not just how Cale got to New York, but why. Why did a nice Welsh boy who played the viola want to live among the freaks of Warhol's Factory, and make freakish music? Cale's Venice installation, Dyddiau Du/Dark Days - a confrontation with his heritage, a return to his home town - offers some kind of answer. What has it been like, filming in Wales? He can't get over the landscape, he says. He's been in Snowdonia, where he walked up the unstable slate heaps of Dinorwic quarry - "I'd hear clink, clink, clink all the way up the mountain." It's been a moving, sometime upsetting experience, and he's exhausted by it. Wales in the early 1960s, he remembers, was somewhere you had to leave: "The place scared me; it was going nowhere." He went to the US, seduced by "the eternal optimism of American society". Today, whenever he has a gig in Wales, there is a Cale clan gathering. It's a typical family reunion, he says, everyone bickering, tensions rising.
Cale's first language is Welsh, even though his father, a miner, was exclusively English-speaking. The family home was dominated by his grandmother, who enforced a Welsh-only rule. He learned English at school and wasn't able to speak to his father until he was seven. He grew up among the sounds of a musical nation, and on the film's soundtrack he includes a rugby crowd roaring the national anthem: this is played over film of him being tortured by waterboarding, a scene that suggests some ambivalence towards his homeland.
The image of a house ruled by the grandmother - the "nain" - is troubling, but typical of a Wales where the older generation have a particular kind of authority. In the 1980s, my own escape from this was to listen to Heroin; Cale's was a lot less vicarious.
His musical brilliance got him to New York and there, in 1963, aged 21, he performed in an eight-hour piano marathon of Erik Satie's Vexations, organised by avant-garde composer John Cage. He then joined La Monte Young's band, which was experimenting with "drone" - the sustaining of the same note for a very long time. The point of drone, for Cale, is sensory deprivation - "but what I was really hoping for was something that could reach into the subconscious. I though we could do that with the Velvet Underground." The difference between John Cale playing one note and other, later punks playing one note is that he could play all the others, too - if he wanted. The reason he didn't was because, at the time, he evidently didn't want to be the good Welsh boy, deep as his patriotism is.
It is not surprising that Cale has now turned to art, for he owes so much to an artist: Andy Warhol produced the Velvets' first album and designed its banana peel cover. No figure in modern culture is more misunderstood than the Velvets' manager, and nobody speaks up for Warhol more eloquently than Cale. He won't hear a word against Andy. The Factory, he insists, was a true underground - "it was outrageously creative and vital" - and Warhol cared about, and properly curated, the Velvets. A rare bit of footage Warhol shot in the Factory shows Cale fiddling with the amplifier, while Reed strums and drummer Maureen Tucker knocks out her steady, dry beat. Warhol listened carefully, and remembered it all. "He was the one who'd remind us of an idea we'd forgotten."
Cale is still smarting from what he sees - amazingly - as the tragic waste of the band. Warhol took them to the west coast, he tells me. While they were away, Bob Dylan's manager took out a lease on their Manhattan venue. This was part of Dylan's feud with Warhol, whose world is caricatured in lyrics on the 1966 album Highway 61 Revisited. Then Reed sacked Warhol and Cale. A new manager, says Cale, "appealed to Lou's desire for glory". In the years since, Reed and Cale have occasionally got back together - but from the furious way he talks, I'd say any further reunion was unlikely.
All through the solo career that followed, Cale has returned again and again to his Welshness. He has recorded Dylan Thomas poems, and in the early 1970s composed a nostalgic Thomas-inspired song, A Child's Christmas in Wales. And apart from the Thomas obsession, there is a lyricism to his music, one that struggles with his severity and evokes all those years in the Welsh Youth Orchestra.
Weeks after we meet, I am watching Cale's work for Venice on curator Bruce Haines's laptop. (Haines secured the commission after writing to Cale's record company and inviting him to make an artwork - simple as that.) Seeing a five-screen video projection on a computer screen is not ideal, and sounds that in Venice will echo through a brewery reach me on headphones. I am told by Haines to skim through the bits that drag.
I don't touch the cursor once. I am transfixed. This is fantastic. It was one thing hearing Cale talk about his return home, another watching this raw material transformed. The camera focuses on architectural details in an old house - Cale's old house. As it scrutinises peeling paint on a door frame, a passionate, wistful song fills the soundtrack. Cale sings of having to "Hollywood" this place - as if the very act of filming it is a betrayal. In a wonderful spoken-word recording, like a cross between Dylan Thomas and William S Burroughs played on an old radio, Cale says his childhood home was "the first place he became aware of symmetry". On film, he is walking up a mountain, his feet clinking on loose black slate.
The images in Dyddiau Du/Dark Days are lucid and exact, but it is the audio that makes this film extraordinary. In effect, Cale has created a filmed concept album and called it an artwork. It is utterly compelling, deeply felt. His renewed relationship with Wales continues to grow: he has just finished filming a documentary about drug-taking in the valleys for BBC Wales.
You can leave Wales, but Wales doesn't leave you, it seems. Cale's Welshness is in everything he does - in the melancholy of his voice, the anger of his music. It is there in his string-saturated version of Leonard Cohen's Hallelujah: Cale gives the song all the passion of a Welsh choir. And in his Venice work, you see him go up to a piano in an old Welsh chapel. He sits there, opens it; sits there some more; gets up. He never plays it.
The segment is a homage to John Cage, and at the same time an eloquent expression of this exile's return. He wants to play that piano again, but he can't. It resembles the instruments and the halls where he first knew music. He wants to be in the place he needed to get away from, but he can't. Cale has created a magnificent allegory of migration and loss, a poem of memory and distance.