I'm obviously showing my ignorance, here, but how exactly does mythtv manage Closed Captioning? What part of the system works as the VBI decoder?

The reason I ask is because I'm wondering how hard it would be to write an app that read in the VBI data from the tuner as it was going on, so it could then be stored off into some text file, for example. Anyone have any ideas if this is possible? Or already done?

Thanks,
-Brian