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Paul McCartney says the world is finally ready to hear Carnival of Light, the psychedelic track that was too weird for 1967.
For Beatles fans around the world, it has gained near-mythical status. The 14-minute improvised track was recorded in 1967 and played once in public.
It was never released on record because three of the Fab Four thought it too adventurous.
The track, a jumble of shrieks and psychedelic effects, is said to be as far from the melodic ballads that made Sir Paul McCartney famous as it is possible to imagine.
But now McCartney has said the public will have the chance to judge for themselves.
"It does exist," McCartney says on a BBC Radio 4 arts programme to be broadcast this week.
Talking to John Wilson, the presenter of Front Row, the former Beatle says he still has a master tape. He says he suspects "the time has come for it to get its moment. I like it because it's the Beatles free, going off piste".
In the 40 years since Carnival of Light was recorded by McCartney, Ringo Starr, George Harrison and John Lennon in the Abbey Rd studios in London, its collection of disparate rhythms has become a kind of holy grail for Beatles obsessives.
The track was put together on January 5, 1967, in between the group working on the vocals for the song Penny Lane.
Once released, it should prove that the Beatles, and McCartney in particular, were even more avant-garde in their tastes than many gave them credit for.
People who heard the track on the one occasion the recording was played publicly, at a London music festival in 1967, say it features the sound of water being gargled and strangled shouts from Lennon which vie with church organs and distorted guitar sounds.
"We were set up in the studio and would just go in every day and record," McCartney tells Wilson.
"I said to the guys, this is a bit indulgent but would you mind giving me 10 minutes, I've been asked to do this thing. All I want you to do is just wander round all of the stuff and bang it, shout, play it. It doesn't need to make any sense. Hit a drum, wander to the piano, hit a few notes ... and then we put a bit of echo on it. It's very free."
McCartney had been commissioned to create a piece for an electronic music festival at the Roundhouse Theatre in north London by his friend Barry Miles. The event, the Million Volt Light and Sound Rave, was organised by International Times, an underground newspaper.
Many in the audience had no idea they were listening to a Beatles track.
Other performers included Delia Derbyshire, whose work at the BBC Radiophonic Workshop included working on the theme for Doctor Who.
McCartney, who this month releases his third experimental album of new work under the alias the Fireman, regards Carnival of Light as evidence of how musically adventurous he has always been. For the three other Beatles the track was just an oddity. George Harrison dismissed it as too weird.
The piece was inspired, McCartney says, by the works of composers John Cage and Karlheinz Stockhausen. In his book Complete Beatles Recording Sessions, Mark Lewisohn describes "distorted, hypnotic drum and organ sounds, a distorted lead guitar, the sound of a church organ, various effects and, perhaps most intimidating of all, Lennon and McCartney screaming and bawling random phrases including 'Are you all right?' and 'Barcelona!"'
Carnival Of Light was considered for inclusion in the exhaustive 1996 Anthology compilation. "We were listening to everything we'd ever recorded," McCartney says. "I said it would be great to put this on because it would show we were working with really avant-garde stuff ... But it was vetoed. The guys didn't like the idea, like 'this is rubbish'."
McCartney revealed that George Harrison disparaged sonic experimentation as "avant-garde a clue".
Sir George Martin, the Beatles producer who oversaw the track, has described it as "one of those weird things. It was a kind of uncomposed, free-for-all melange of sound. It was not considered worthy of issuing as a normal piece of Beatles music at the time."
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