a) Refers to the 0 IRE level which exists before and after horizontal sync and during the vertical interval. This voltage level allows the electron beam to be turned off while it is being repositioned (retracing) across the face of the CRT into the position needed to start tracing the next visible line. b) The level of the front and back porches of the composite video signal. c) The level of a composite picture signal which separates the range containing picture information from the range containing synchronizing information. Note: This term should be used for controls performing this function (IEEE 100).