Video is applied via a gain controlled IF amplifier (11) to a video detector (13) and amplifier (15). Sync and equalising pulses are fed from the amplifier to an active filter (21-26) and then to a comparator (30), which produces an output variation for changing an AGC voltage produced by generator (40) and AGC capacitor (41) if the pulse peaks exceed a threshold. The filter (21-26) delays and attenuated noise impulses, but can also cause noise impulses to remain above the threshold for a period of time defined by pulse stretching in the filter. A transistor (32) is responsive to a potential difference of predetermined sense and magnitude between the input and output of the filter to curtail the output variation of the comparator (30). Such a difference exists when a delayed leading edge of a noise pulse is present at the output of the filter and the trailing edge is at the input.