The way decent ones work has very little to do with filtering. It is to do with stereo. In most "ordinarily" produced (pop/rock) tracks, you will have the vocals in the centre along with the bass and kick drum. All the other instruments tend to be panned, if only slightly, to the left or right. Then, you just use a width expander to seperate these instruments totally, and a low cut on your remaining vocal track to remove the bass instruments. You are left with either a solo vocal, or an instrumental track if the vocal is inverted and mixed back with the original. It can be highly effective, it depends how the recording was produced really, some tracks from the 60s/70s with things hard panned all over the place clearly won't work.