Pulse width modulation conversion circuit for controlling a color display monitor

Claim 1

A conversion circuit for a video display terminal having twenty-seven selectable colors comprising: an eight-color CDM of the type having only three RGB video input lines, a cathode ray tube controller of the type having six output color control signal lines capable of defining sixty-four logic states, each signal line having two logic states being defined by full width pulses or the absence of full width pulses, said six output color control signal lines providing three RGB color pairs of output color control lines, three pulse width modulation conversion circuits each having one of said three RGB color pairs of output control lines coupled to its input, each said pulse width modulation conversion circuit having its output coupled to one of said three RGB video input lines of said eight-color CDM, each output control line of said pulse width modulation conversion circuit having at least three possible pulse width modulation conditions in response to the two logic states on said control signal lines, said logic states being full width pulses and the absence of full width pulses and said pulse width modulation conditions being partial width pulses, full width pulses and the absence of pulses, and a dot clock generator coupled to said three pulse width modulation conversion circuits and to said cathode ray tube controller for synchronizing said pulses, whereby said eight-color CDM is converted to display as many colors as a twenty-seven color CDM.