The deviation of an FM signal modulated by a video signal is detected by measuring the average period of the FM signal at a measurement rate proportional to the average frequency of the FM signal. The value of each average period measurement is compared with a predetermined number to produce an indicator signal each time the measured value is less than or equal to the predetermined number. The effect of overshoot of the video signal due to preemphasis is minimized by detecting the successive occurrence of at least two indicator signals to provide a further indicator signal which may be used for visually monitoring video peak levels and/or for protecting a utilization device to which the FM signal is supplied.