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Anything that’s rock n’ roll
"She had a guitar and she taught him some chords, the sky was the limit..."
In 1991 Johnny Depp starred in the video for a song called Into the Great Wide Open, by Tom Petty. He played Eddie Rebel, the main character of the song, who had a guitar but not a clue. Then again, you don't need much more than a guitar and some chords to play most rock songs.
People all over the world want to know the chords to their favourite songs, so they can easily play them on their guitar or with their band. Unfortunately, songwriters like Tom Petty don’t usually publish the chords they base their songs on. What happens instead is that people try to figure out which chords he played by listening to the song. Professional transcribers sell their chords in sheet music books, while others just put up a page on the Internet for everyone to use. For rare songs, finding either of these can be a problem, so hobby musicians often end up having to transcribe the chords themselves. Unless...they can get a computer to do it for them!
This is what Matthias Mauch and Simon Dixon from the Centre for Digital Music at Queen Mary have been doing. It’s a three-step process. First you have the computer calculate the pattern of what goes on at every beat of a song. Since that gives you a very messy view of the music, the second step orders the beats into bars, eliminates short ‘phantom’ chord changes in the middle of bars, assigns the key to the song and so forth. To do that all simultaneously Matthias uses a computational model developed by statisticians called a ‘dynamic Bayesian network’ and feeds it what we know about chords, bars and keys in general. That means that when the model runs, it comes up with results that make musical sense. The third step is the easiest: write the results of the process in a way that musicians can use it, known as a lead sheet.
So, one day soon you may no longer need to search the web looking for chords. Your chord-savvy computer may just listen to the music, hand you the lead sheet, and off you go. Of course that doesn’t guarantee you will sound as good as Tom Petty, so don’t blame the computer if your audience ends up telling you: "Don’t come around here no more".