Image copyright Getty Images Image caption This photo shows Turing (right) and colleagues working on the Ferranti Mark I Computer in 1951

The earliest known recording of music produced by a computer - a machine operated by Alan Turing, no less - has finally been made to sound exactly as it did 65 years ago.

It's hardly chart-topping material. The performance is halting and the tone reedy.

It starts with a few bars of the national anthem, then a burst of Baa Baa Black Sheep, followed by a truncated rendition of Glenn Miller's swing hit In The Mood. ("The machine's obviously not in the mood," an engineer can be heard remarking when it stops mid-way.)

But the rudimentary audio track is a landmark - the first time that music played on a computer is known to have been recorded.

It was captured by the BBC in the Autumn of 1951 during a visit to the University of Manchester, where the Ferranti Mark 1 - the world's first commercially available general purpose computer - was based.

Image copyright British Library Image caption Jack Copeland (l) and Jason Long (r) have restored the recording to how it would have sounded in 1951

The recording was captured on a 12-inch (30.5cm) acetate disc. But when Professor Jack Copeland of University of Canterbury in Christchurch and composer Jason Long examined the disc, they found the audio had been distorted.

It "gave at best only a rough impression of how the computer sounded", Copeland and Long wrote in a blog for the British Library. But now they say they have restored it to how it actually would have sounded in 1951.

The Ferranti Mark 1 may not have been the first computer to have played music - that distinction, it's been widely claimed, went to an Australian machine called CSIRAC that played The Colonel Bogey March some months before. But no recording has ever surfaced.

Media playback is unsupported on your device Media caption The world's first computer music

The music program was written by a maths master at Harrow called Christopher Strachey, a friend of computing pioneer Alan Turing, who had written the Ferranti Mark 1's instruction manual in his role as deputy director of Manchester University's Computing Machine Laboratory.

The Ferranti had the capacity to produce an instruction called a "hoot", which produced short burst of sound lasting a fraction of a second. Turing realised this could be used to produce musical notes. He intended that this would be used to issue alerts when a job was finished and so on, but Strachey saw the potential to perform proper melodies.

Image copyright Bodleian Library and Camphill Village Trust Image caption Christopher Strachey said working at the huge computer felt like being at the controls of a battleship

As well as being fascinated by computer programming - he would go on to become one of the UK's foremost computer scientists - Strachey was a skilled pianist.

Turing trusted Strachey enough to leave him alone with the computer for a night. "I sat in front of this enormous machine," Strachey later recalled, "with four or five rows of 20 switches and things, in a room that felt like the control room of a battleship."

There's some dispute about what he did next. Chris Burton of the Computer Conservation Society (CCS) says Strachey wrote a program for playing draughts on the machine, and when the program terminated it played God Save the King. Others say Strachey's program was purely for playing music.

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Prof Jack Copeland spoke to the Today programme on BBC Radio 4 on Saturday 1 October shortly after 08:50 BST

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The result was crude at best. The machine could only approximate the pitch of many notes.

"It was fairly imprecise," says Burton.

But word spread that a computer was capable of performing music and a BBC outside-broadcast team arrived later in the year to record a segment for Children's Hour.

Image copyright Chris Burton Image caption The original acetate disc was saved by an engineer at Manchester University

It's not clear who programmed the three pieces of music they recorded. A number of technicians had begun programming melodies into the machine and even Strachey's version of God Save The King may have been amended.

After the recording, a university engineer called Frank Cooper asked the BBC team for a copy. They cut him a version of the original, and this was eventually passed to the CCS and the unrestored version was made public in 2008.

By analysing the recording, Copeland and Long realised it was playing at the wrong speed, possibly as a result of the recorder's turntable running too quickly as the acetate was cut.

As they knew the notes the computer was actually capable of playing, the pair were able to calculate exactly by how much the recording needed to be speeded up in order to exactly match the sound made by the Ferranti Mark 1. They also removed extraneous noise from the recording - though not the engineer's voice.

"It was a beautiful moment when we first heard the true sound of Turing's computer," Copeland and Long wrote. Now anyone can hear it in all its somewhat ramshackle glory.

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