Almost all radio receivers, scanners included, are designed for voice communications. Voice is pretty narrow-band, consisting of frequencies between 300 and 3000 Hz. This is not the full range of voice, but it's good enough that the Bell System determined that using just this band of frequencies to pass voice communications would be sufficient for 99.999% of all users. Most FM radio receivers use a low-pass filter, located just after a function called the discriminator, to remove higher frequencies that the ear would interpret as noise. It's this filter that is a problem for data communications like ATCSMon.
The "discriminator tap" is a hack on a radio receiver that allows us to get at the audio information before the low-pass filter, and therefore get more frequency data that would normally be necessary for standard voice communications. The late Bill Cheek built up a huge compendium of these hacks for a variety of radio receivers, and that compendium lives on at a variety of websites. Just do a search for "Bill Cheek discriminator" and you should be able to find it.
The discriminator tap is a way to bypass the low-pass filter and get the wide-bandwidth signal for our signal processing purposes. It means that you'll have to open the case of a radio receiver, solder in a couple of components, and obviously void the warranty on anything that you just bought. If you're lucky enough to have a radio receiver that already has a discriminator output (like an ICOM PCR1000, labeled the "packet - 9600 baud" output, then it's already done for you.
Once you've got a receiver with a discriminator tap, the ATCSMon software-based data receiver can do the rest.
-- JonAdams - 30 Oct 2006