In 1951, a group of BBC broadcasters arrived at the Computing Machine Laboratory in Manchester, England, for a music recital. There, they made the first recording of computer-generated music, produced by the Mark II computer invented by Alan Turing, widely considered the father of computing.

The recording, stored on a 12-inch acetate disc, holds snippets of the British national anthem, the nursery rhyme, “Baa Baa Black Sheep,” and a performance of the Glenn Miller’s “In the Mood.” The recital is famous for being the earliest instance of computer-generated music, but it was not until recently that researchers from the University of Canterbury in New Zealand noticed that the tunes were distorted.

Now these researchers have remastered the recording, which had been preserved in digital form in the British Library’s Sound Archive. Using specialized software and traditional editing, they restored the computer’s original sound, which resembles a primitive synthesizer.

“The computer itself was scrapped long ago, so the archived recording is our only window on that historic soundscape,” wrote the researchers – computer scientist Jack Copeland and composer Jason Long – in a blog post about the restoration.

Listen to the recording here.

Playing musical notes is one of the lesser known features of Turing’s computer, which filled most of the laboratory’s ground floor. The gigantic contraption was connected to a loudspeaker – Turing called it the “hooter” – which emitted a short burst of sound when it received a special instruction. When these bursts were repeated fast enough, the computer created what sounded like a note. By adjusting the pattern of the bursts, engineers could play different notes.

Turing, however, was not interested in creating music. He used them instead to troubleshoot the computer, playing different sounds when a program finished or an error occurred. It was not until 1951, when a budding computer scientist named Christopher Strachey wrote the first music programs, that the Mark II became an instrument.

The computer-generated music was imperfect: the computer could only produce an approximate version of a note. But when Copeland and Long analyzed the recording nearly 65 years later, they found there was a significant amount of distortion.

The researchers had previously catalogued all of the sounds that the computer could emit, using Turing’s original programming manual as a guide. They discovered that the recording contained many notes that were impossible for the machine to produce.

“The frequencies in the recording were not accurate,” the researchers said.  “The recording gave at best only a rough impression of how the computer sounded.” By figuring out how badly the recording deviated from computer’s capabilities, they concluded that the recording was playing too slowly.

“This was most likely the result of the mobile recorder's turntable running too fast while the acetate disc was being cut,” Copeland and Long said.

The researchers repaired the recording by playing it faster and filtering out extraneous noise. They also used pitch-correction software to remove a “wobble” in the speed of the recording.

“It was a beautiful moment when we first heard the true sound of Turing's computer,” Copeland and Long said.

The music’s restoration was an interesting experiment, but the researchers said that it also helped to highlight an overlooked part of Turing’s legacy. Turing is known for breaking the Nazis’ Enigma code during World War II, but he also played a big role in transforming the computer into a musical instrument.