To produce a key signal from a video signal, a clip insert control on the front panel is used to set a threshold level to which the video signal is compared. In luminance keying, any video (brightness) level above the clip level will insert the key; any level below the clip level will turn the key off. The clip level is adjusted to produce an optimum key free of noise and tearing. In the Key Invert mode, this clip relationship is reversed, allowing video below the clip level to be keyed in. This is used for keying from dark graphics on a light background.