The magic of Pandora derives from a simple principle: a song listeners enjoy should lead to other songs they'll enjoy. Pandora is an Internet music service with an unusual twist: you merely select a song or artist you like and the system builds a playlist of additional songs based on those musical characteristics.

After Pandora builds the playlist (which it calls a station), the music starts streaming to your Web browser--for free. You can then refine the playlist by clicking a thumbs-up or thumbs-down button as subsequent songs play. Other buttons call up more information on the song or the artist or whisk you to Amazon or the iTunes Music Store to buy the CD or file. (Pandora makes money by collecting referral fees for those clicks and by displaying ads in the Web interface.)

Using Pandora is simple, and the results can be surprisingly good. But as you'd imagine, there's a lot of innovative technology behind the scenes to make it work. I met with chief technical officer Tom Conrad to find out more. Before reading further, why not visit Pandora yourself and set up a little background music?

Pandora Thumbs Up

Pandora's main window shows the stations you've created at left, with the current station highlighted. (Stations with a curving arrow in the name were recommended by another Pandora listener.) The main area shows the currently playing song and the two previous songs, which scroll to the left as new songs enter the queue or as you skip ahead. Here we have rated two songs thumbs-up and one song thumbs-down.

Decoding the Music Genome

Pandora (which is also the name of the company) grew out of the Music Genome Project, which company founder Tim Westergren began six years ago. Westergren had studied music at Stanford and toured the country in a band for many years, eventually spending time discussing music with film directors. He became fascinated with the way directors described the music they were looking for, which led to his wondering what made people enjoy certain types of music. He asked himself, "If people haven't found any music that they love since college, and artists are struggling to find an audience, is there a role for technology to help bridge the gap?"

Westergren started the Genome Project from the idea of creating a platform for connecting people with music that they'll love based on music they already enjoy. The project uses experts called "music analysts" to deconstruct music into its fundamental parts and capture the results into a database. Pandora has 40 professional musicians who come to the office every day and listen to one song at a time, analyzing each in anywhere from 200 to 400 dimensions. (The dimensions are somewhat different for each genre of music.)

Pandora chose the dimensions because they are quantitative. For instance, how breathy are the vocals? Is the music diatonic or chromatic? The music analysts are trained to be able to score songs consistently. In fact, one of the test cases is, "Could a group of 10 musicologists listen to a song and agree on one score for a particular element?"

Pandora Analysts
Pandora analysts plow through CDs in search of musical match-ups.

Once a song is captured, the system loads the analysis into an n-dimensional vector space. To find matches, Pandora uses "nearest-neighbor" computation. For example, let's say you like "Hips Don't Lie" by Shakira. Pandora will take that song and find its nearest neighbors in 200-vector space (the number of data points used for this genre). The resulting songs form a playlist of songs quantitatively similar to "Hips Don't Lie."

As a lover of jazz, I found this system worked remarkably well. The vast majority of the time, my stations stay true to the original profile. But I've heard from others that, although very good, stations built on a particular pop tune are not as accurate. Some listeners have complained that Pandora's database over-represents American music.

Tom Conrad, Pandora CTO

Tom Conrad, Pandora CTO

There is a bit of magic here. For instance, Pandora doesn't weight all attributes equally. The vocal-performance attributes might be more important than how the rhythm guitar is played.

"One of the things we can do for playlist generation is play with that notion of weighting," Conrad says. "We do that to help give form to the listening experience--beyond just a random collection of similar songs. As you listen to Pandora, you'll find there are musical themes that are shared across songs and weave in and out of the listening experience. So, you might hear a sequence of songs that feature similar percussion, and then a sequence of songs where the emphasis is on vocal similarities, and then a sequence of songs that have similar brass sections--which is unique to us because we understand, on a song-by-song basis, what the music is.

"What is exciting about the Music Genome Project, with respect to Pandora the radio-listening experience, is that by understanding the music on a song-by-song basis we can put together a playlist that has a much more natural ebb and flow than you might be able to do with collaborative filtering data," Conrad says.

I asked him to compare the Music Genome Project with Gracenote's new machine-listening approach that takes a song and decomposes it into elements using computers. "When we started the Music Genome Project, the state of the art in machine listening could decompose about six to eight elements," he replied. "Something about the tempo, the key signature, the song form, etc. Over the last 5 years there has been some dramatic progress, but the state of the art today is about 20 elements--a tenth of what we collect for a pop song.

"Since we use a human analyst to analyze song by song, we've experimented with using a smaller number of elements," he continued. "We've determined that you can't create interesting playlists with only 20 attributes. But we do keep an eye on machine listening as it might provide a way to augment the manual analysis."

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