Tim Westergren is one of those entrepreneurs who built an online company to fill a void he saw in the physical world.

His Oakland-based Pandora is the leading radio service on the Internet, which has to be satisfying for a musician who says he spent a "long time trying to make a living" in music beforehand.

In fact, Pandora, which recently filed for a $100 million IPO, now hosts a database of some 800,000 songs from more than 80,000 artists. This massive playlist is what you have to choose from in order to populate your own private music channel.

Around 80 million registered users have chosen to do just that, probably half of them via Pandora's iPhone app.

"For a decade now, we have been assembling a musicological taxonomy called the Music Genome Project," says Westergren. "We have around 20 music analysts who identify the essential elements of a song," -- things like key, rhythm, tempo, instruments used, etc.

"These musical fingerprints are essentially the DNA of a song," Westergren explains. "So when you launch your own station, we use this DNA along with other data to identify musical relationships between the songs you choose and other music in our collection."

It works like this:

After Pandora's human experts thave determined which of the 480 intrinsic elements in a particular song you like are dominant, it begins building a profile of your individual musical taste.

"Then you essentially train the system to get better for you," says Westergren. While you are building your own channel by choosing which songs you like,  Pandora's algorithm scours its databse to suggest new songs musically similar to the ones you've chosen.

Then, by indicating either a thumbs-up or a thumbs-down, you indicate to the algorithm how to better recognize your preferences and refine its suggestions to you in the future.

"We also aggregate feedback from millions of others with similar channels to yours to bring you 'the wisdom of the audience' to help you find other songs you'll like," Westergren adds.

One factor Pandora does not take into account is a song's popularity over the conventional airwaves. "We don't care about that, we want to help create a rising tide of artists, I call it a 'musician middle class,' to give many more performers visibility than they otherwise would have had."

So it's about the "long tail" of music, not the top hits you keep hearing over and over ad naseum on your favorite FM radio station. And yes, one of the relatively unknown performers whose music you can find on Pandora is Tim Westergren himself.

Just search for his former band, Yellow Road Junction.