An STC filter (not to be confused with Sensitivity/Time Control) is also sometimes known as Fast Time Constant, or FTC, a name more descriptive of its function: It is a circuit in the video amplifier whose purpose is to suppress echoes on the display in inverse proportion to the rate of rise of the input signal, or in other words to the steepness or sharpness of the leading edge of the envelope. The effect if used in a terrain mapping radar mode would be to eliminate or reduce the intensity of echoes with slower rise and decay times which return from gently varying terrain, whilst preserving sharper echoes from features like buildings, riverbanks or coastlines.