During the long hot day of July 2, several singers, including Bono of U2, went briefly off-key while the performance of others, such as Pete Doherty of Babyshambles, was dismissed by critics as an embarrassment.

Blushes will be spared thanks to a computer program called Auto-Tune, described by some as “Botox for the vocally challenged”.

“By cutting up and reassembling notes it can erase the wrinkles and bum notes and make a karaoke drunk sound like Frank Sinatra,” said one technician, who declined to be named.

Auto-Tune has aided singers such as the Spice Girls and Cher, but the four-DVD collection will set its technology against the memory of people who saw the concert.

Unlike well-known pop tricks such as lip-synching, Auto-Tune has remained largely a trade secret. Purists loathe it, saying it makes every singer sound similar to Milli Vanilli, the pop act infamously caught miming.

Crispin Murray, Live 8’s audio producer, confirmed that the DVD due on sale on November 28 had made use of the Auto-Tune technique: “We are just setting out to make the artists sound as good as they can.” Some of the artists who performed during the biggest live concert in the world have been to a west London studio to assist Murray in “tweaking” their sound in the mixing room, but this did not involve re-recording vocals.

Last week Doherty, who played with the Libertines before his drug problems led him to set up his own group, was recording in the studio next door to Live 8’s engineers, but it is not known whether he popped in for a damage report.

His erratic performance of Marc Bolan’s Children of the Revolution with Sir Elton John was roundly dismissed as one of the low points of the global spectacular.

Pete Waterman, the veteran pop producer, said that it would need a lot more than tweaking to make the song enjoyable. “If they can tune that up, I want that tuner please. That would be a miracle machine,” he said. “Mind you, every record producer in the world could do with one of them.”

All the bands who performed at Live 8 surrendered their rights to the performances so nobody can refuse to be on the DVD, as Led Zeppelin did with the original Live Aid show.

It was not just musical differences that prevented some bands joining Live 8 — politics was a big reason for the Rolling Stones turning down the invitation.

Keith Richards, the Stones guitarist, said this weekend that he had not wanted to take part as he found Live 8 “a bit nebulous”.

“I couldn’t believe the pressure, even from 10 Downing Street,” he added. “I heartily applaud what they were trying to do, except that it was tied in with government policy and I always try and separate politics and music.”

Richards said he had been “hit on by Sir Bob [Geldof] and Sir Mick [Jagger]”.

“But I said to Mick, ‘We ain’t doing it, pal. You can do it, but I ain’t’.”

He added: “Bob’s a nice bloke and all that, but ultimately he’s the one who comes off best, isn’t he?”

Whether it was politics or sound quality that concerned the artists, Alex Needham, deputy editor of NME, the music magazine, said that using Auto-Tune would be cheating the Live 8 audience: “It is unforgivable to corrode the authenticity and integrity of the day. Live music is not supposed to be flawless, it’s all about the emotion.”

Jill Sinclair, executive in charge of producing the DVD, said: “It will be different to the live broadcast and it will be better but in terms of rewriting history, we aren’t doing that. We are cleaning history up.”

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