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https://en.wikipedia.org/wiki/Video_scalerIt primarily changes a video resolution to another one (up/down scaling) or adapts to a specific/custom screen resolution.
It can also convert analog video to digital (Composite -> 1K HDMI) or digital video to analog (HDMI -> S-Video), or serve as an aggregator/switch to accommodate various entries into a single output.
Some will also use algorithms/filters to improve signals/perceived quality (such as Darbee).
In the context of LaserDisc, the goal is mainly to upscale/filter/improve the Composite/S-Video signals to be displayed on modern screens and projectors that might lack analog inputs, as well as correct for aspect ratio, adjust color/contrast/etc. Ideally take both video and audio signal (analog or coax digital audio or AC3RF after demodulation) from a LaserDisc player and turn it into 1K/4K HDMI audio+video to be passed to an A/V system.
Julien