the reversing switch and hence a controlled steepness of the edge in the interruption of the lf signal an interruption with suitably controlled edge steepness in turn shortens the time required for the lf signal to drop or rise again at a minimal interference spectrum as a result a control function of predetermined duration can be performed within a shorter interruption of the lf signal which markedly improves the avoidance of perceptibility of the interruption for the duration of the blanking pulse the potential at the blanking pulse input 8 is dropped from approximately 5 0 v to 0 v the voltage values given in the description below are based on the use of npn transistors of type bc 848 c as t 1 and t 2 and pnp transistors of type bc 858 c as t 3 t 4 and t 5 fig3 on the left shows the voltage course between the base of the transistor t 3 and the base of the transistor t 4 which acts as a reference during the descending edges or in other words at the onset of a blanking pulse and on the right during the reascending edge that is at the end of a blanking pulse during the state of repose of the blanking line phase i the transistor t 3 is blocked and the current from the constant current source t 5 flows entirely via the transistor t 4 whose base at this time is at approximately 5 5 v with the onset of the descending edge of the blanking pulse in switching phase ii the voltage between the control electrodes of the two transistors t 3.