As Tom, W6EIJ wrote, the 599ZX outboard audio DSP unit is a good adjunct for many radios. Another outboard audio DSP unit that I can highly recommend is the JPS NIR-12. However, the KDSP2 is not an outboard DSP unit. It is internal to the Elecraft K2 and is contolled by the K2's front panel buttons. 73, de Earl, K6SE