Data Slicers
From The RadioReference Wiki
Many applications featured on these pages require either the 2-Level FSK interface or the 4-Level FSK interface, otherwise known as a data slicer. You must build this interface, or buy it from some of sources listed at the bottom of the page. NOTE that these interfaces DO NOT provide computer control for your scanner, but rather they interface the output from your scanner so that the computer and application can decode the digital signals.
These interfaces connect to the discriminator output on you scanner (and ONLY the discriminator output) This is usually a connection inside of your scanner that must be made with a soldering iron.