ISP DecimatorNoise gate circuit. Lots of more information in the thread.
Just remember that D12 is not an 1N4001, it's an 1N5819
LinksThis page previously linked to a reverse engineering thread on DIYSB. That thread has now been removed so the links stopped working. Here are a few other links.
Verified vero layout
PCB layout on Google Docs
Another PCB layout and other useful information