A line doubler is an electronic device that converts analog or digital television (TV) video signals into a format suitable for display on a computer monitor. Some line doublers allow for the display of more than one TV signal on a single computer monitor.
The heart of a line doubler for analog (conventional) TV viewing is an analog-to-digital converter (ADC). A scan converter is also necessary, both for analog and digital TV, because the refresh rate of a computer display is generally not the same as that of a TV video signal.
The term line doubler is occasionally confused with the term splitter. A splitter is a device that allows two signal lines to merge into one line, or one line to be split into two lines, while maintaining a proper impedance match for all the input and output lines.