ADC DD-4AX is a digital delay processor for adding subtle reverb and echo effects to home stereo systems to create natural sounding spacious listening environments. This is a vintage signal processor that was a precursor to surround sound. The unit will add delay to the primary speakers and also has two speaker outputs for adding even more dramatic delay, making the room seem much larger. User manual included. This unit is in good working condition. I am the original owner and I have the original user manual. This delay unit was a lot of fun to use and made my system sound incredible.