The game Rocksmith and some polyphonic guitar tuners (like the PolyTune) are able to recognize, at least to some extent, multiple fundamental frequencies in single-source audio.
I know there are some methods for multiple fundamental frequency detection, like MUSIC and ESPRIT, but is it likely that these products implement them or leverage several assumptions (such as: audio is from a guitar, audio is from a single source) about the application domain to make a simpler method work?
In the second case, which simpler methods are likely to be used? Simple peak-picking on a spectrum, with domain-specific heuristics? Or something more complex?