I built an audio player to easily compare multiple interpretations of the same piece. Here's an interactive demo, and a video to give you a sense of how it works:

What does it mean to interpret classical music?

At first glance, sheet music is prescriptive: the composer has provided all of the notes, the dynamics (forte, piano), tempo (lento, presto) and changes in tempo (de/accelerando).

In practice, however, the interpreter has a lot of leeway. In some extreme cases, such as the Cadenza in solo concertos, the performer gets to improvize a melody based on a chord progression. Some pieces include ornamentation (eg. trills, etc) which are largely left up to the performer to interpret.

That said, cadenzas and ornaments are somewhat rare. In general, every piece is under-specified by the composer. This gives the performer a lot of leeway to express themselves through the performance, selecting tempo, phrasing, articulation and tone.

Example: Bach's Goldberg Variations

The Goldberg Variations were composed by Johann Sebastian Bach in 1741, and then popularized by Glenn Gould in his debut album in 1955, transforming a work once considered esoteric into one of the most iconic piano recordings.

In 1981, a year before his death, Gould recorded the pieces again. After a long period of reclusion, he was able to revisit the variations and produce a completely different take. In an interview, he said:

...since I stopped playing concerts, about 20 years, having not played it in all that time, maybe I wasn't savaged by any over-exposure to it...

Compare Gould's 1955 and 1981 recordings

Both the 1955 and 1981 recordings are available on YouTube, of course. I found that listening to two distinct performances is not the same as having one integrated player. So I built one: a player specifically for comparing multiple interpretations of the same piece.

Here is a demo that lets you compare the first variation from the Goldberg Variations. Try it out here. You can use your keyboard to skip between interpretations (↑, ↓) just as easily as you can seek within a track (←, →). The mouse works as well. Note that I haven't tested at all on mobile. Sorry, it's just a prototype and I'm on paternity leave 😇

I also tried it on Mozart's Requiem

I am a huge fan of Mozart's Requiem, and once came across an online thread debating which conductor's performance was the best. I soon found myself listening to a dozen or so different versions of the same piece. When I was a younger music appreciator, I would often wonder what the point of a conductor really was. I no longer have this question.

Just to give you a taste for how different the interpretations are, here's an example of three conductors performing the Introitus, the first movement in the Requiem. Check it out here, but be patient as it may take a minute to load and decode the audio. Böhm's brooding tempo and lumbering chorus (ugh) contrasts especially well with Levin's crisp and minimalist take.

Technical details

For this prototype, I focused on creating a reasonable UI to play back and interact with multiple time-aligned performances of the same piece. An index file specifies metadata for each track, most importantly the URL to the label file and the URL to the audio file. Each label file is a text file with lines in the format START_TIME END_TIME BAR_NUMBER.

To create the label files, I manually annotated the waveform. Even with Audacity's extremely useful label track feature, it was a lot of manual work to go through the score, and find each bar's time range in each recording. At the end of the day, I had start and end times for each bar. For times that don't fall exactly on bar lines, I linearly interpolate between the bar boundaries, which works reasonably well, but is sometimes a bit off. More granular timing references would address this better, but that currently means doing more manual labor. No thanks!

Science, help me automate this, please

An obvious question is how to automate the labor of synchronizing a recording to a score. In general, I think this is an unsolved problem, especially for complex tracks containing hundreds of instruments and varying levels of background noise.

An promising approach that could work for solo piano music might be to use something like Onsets and Frames to extract piano rolls and then apply something like a Dynamic Time Warp (DTW) in piano roll space. A more general approach might be to synthesize each bar into raw audio (from MIDI), and then align recordings to synthesized audio using something like DTW based on a Constant-Q transform (CQT).

My brief and ill-guided attempts to do something like this on real-world examples didn't yield good enough results. Any ML/DSP experts want to take this on?