AC synthesis is the creation of alternating current (AC) using direct current (DC) electronics and special software.  In RC4 Series 3 dimmers, this is done using two 2 DC dimmers in tandem.  AC power is needed to make a telephone ring, or make EL wire light up.  It is the kind of power that comes from a regular power outlet in a home or office.  DC power is what comes from a battery.