Few recordings are as well known as "A Day In The Life", the closing track of the Beatles' 1967 album Sgt. Pepper's Lonely Hearts Club Band. It was made on four-track tape. Now a Toronto engineer says he has pulled it apart into 38 separate tracks.

John "Wheels" Hurlbut describes the experiment in a first-person article for the audio trade site Production Expert. The site says he started with Steinberg's SpectraLayers and then discovered MVSEP.com, "a little known product" whose name is short for Music & Voice Separation, and which he "found to be very impressive in his tests".

Hurlbut, who is 61, has been working in studios since at least the early 1990s. "There is one thing, though, that I never thought I'd see in my lifetime," he writes, "and that is the ability to remove different instruments and sounds from a mono or stereo file."

Why four tracks matters

The Beatles began recording the song on 19 January 1967 at EMI Studios, Abbey Road, according to The Beatles Bible. On those first takes, John Lennon's acoustic guitar, Paul McCartney's piano, George Harrison's maracas and Ringo Starr's congas were all taped together onto a single track.

That is the problem in a sentence. Once several instruments share one track, there is no fader that will pull them apart again.

The same source records that 40 orchestral musicians were hired for the session on 10 February. EMI technical engineer Ken Townsend devised a way to run two four-track machines in sync so the orchestra could be recorded several times over.

How "unmixing" works

Stem separation, or unmixing, is software that listens to a finished recording and sorts it back into its component parts.

Hurlbut traces the technology to Spleeter, which he says the research division of the French streaming service Deezer released in November 2019. Deezer Research describes Spleeter as a source separation library with pretrained models that can split a song into vocals, drums, bass, piano and "other". Training such models, it says, requires a dataset of isolated sources.

In plain terms, the software has studied instruments on their own, and so learns to recognise a voice or a drum kit inside a crowded mix.

Hurlbut writes that Peter Jackson's team adapted the approach for the documentary Get Back, training it to identify the Beatles' voices, their vintage instruments and the room acoustics. He adds that Giles Martin's team later used it to lift Lennon's vocal from a demo for "Now and Then", released in 2023.

MVSEP offers a long menu of these models. Its website lists separate ones for vocals, drums, bass, piano, guitar, wind and strings, plus one that divides a drum track into kick, snare, toms and cymbals.

That hints at how the count can climb far beyond the number of original tracks: a stem can be split, and then split again. Hurlbut's own step-by-step account is in his Production Expert article.

Restoration, not remixing

The Beatles test sits alongside a much larger project. Hurlbut writes that he is mixing albums from the EMI Music Canada catalogue in Dolby Atmos, working from digitised multitrack tapes held in an archive hosted at the University of Calgary.

He is careful about what he calls the results. "I don't have any creative license to 'remix' these songs, so I call them 'restoration' mixes, replicated as closely as I can to the original mixes from back in the day," he writes.

The AI tools fill the gaps. Hurlbut describes using SpectraLayers to remove tape hiss from analogue multitracks, and to extract a gang sound effect from the stereo mix of Glass Tiger's "The Thin Red Line" because it was missing from the multitrack. On a Grapes of Wrath song, he "surgically painted out" loud guitar string squeaks.

On Tom Cochrane's 1985 song "Boy Inside The Man", he says the kick and snare samples used in the original mix were never put back on the multitrack, so he rebuilt them with the help of a separated drum stem. "Without AI, it would have been very hard to finish the mix while still honouring the original, at least in my view," he writes.

He is frank about the limits, too. Of a 2023 mastering job he writes: "There were a lot of artefacts on all the tracks back then (2023!), but since it was combined back into a stereo mix, you can't hear it at all."

For anyone who looks after old recordings, that is the real story. Parts that were fused together on tape decades ago, or lost altogether, need not stay that way.

Of the wider project, Hurlbut says: "It is my way of honouring their work and hopefully giving it more longevity."