a) The process that establishes a fixed level for the picture signal at the beginning of each scanning line. b) The process whereby a video signal is referenced or “clamped” to a DC level to prevent pumping or bouncing under different picture levels. Without clamping, a dark picture would bounce if a white object appeared. Changes in APL would cause annoying pulsations in the video. Clamping is usually done at zero DC level on the breezeway of the back porch of horizontal sync. This is the most stable portion of a TV picture.