Readers - These are units that usually have their own display, in the form of a LCD readout. These kinds of units have been on the market for many years. Sadly, though, for many people just starting out in the digital hobby, these units become rather out of date for their time. They don't have any upgrade path, so new modes cannot be added. There still are some being sold on the market, and the potential users should be aware of this rather big pitfall.

TERMINAL UNITS - These units require a display of some kind (PC, video terminal) to function. Here, we must consider 2 different categories;

• Hobby Level (including ham units) - Many of these units are still used by hams and hobbyists due to their reliability and superior filtering. However, like readers, the modes they receive tend to become out of date very quickly and are rarely, if ever, updated.
• Professional Level - In this category, modes are updated from time to time, allowing the user to adapt and keep up with the evolution of the digital scene. Sometimes they also have tuning aids (a big help when trying to tune a signal in with less-than-optimum conditions) and other analysis tools.

SUPPORT PROGRAMS - There are several programs available today to interface with the terminal units, sometimes adding more functionality than they originally had. While there are a good number of Windows programs, there are still some DOS programs available; this makes buying a PC a great deal easier, as 386/486 laptops and old desktops can still be found at hamfests and flea markets.