audio filter with comparator?

**broken link removed**

Here is the schematic of a what seems to be a fairly nice digital guitar tuner. I have been experimenting on a filtering amplifier for a guitar project. Could I use this circuit up to only a certain node before the comparator to achieve my fundamental frequency filtering? Or is the comparator there to capture the first fundamental of the polyphony?

In short what does a comparator really do? Why would it be needed in this circuit?

Thanks in advance,

Your link is faulty.!

edited link to functionality

fairly nice guitar tuner