a three - pole double - throw switch is provided to selectively interrupt the video signal from an input connector to an output connector for display of white characters generated by a microprocessor on a dark screen of a crt monitor , or vice versa , and route the video signal through an inverter for display of dark characters on white screen . one pole of the switch is used to provide the vertical sync signal to the power input terminal of the inverter , thus powering the select switch box solely from the transmitted vertical sync signal . a capacitor filters the power supply thus provided . the select switch box requires no external power and is virtually free of rf energy radiation while providing noise free data display with equal bandwidth response in both modes .