105 and possibly 104 may be separated from the charging circuits 102 and 106 and operate for example with a special sawtooth capacitor it is alternatively possible to feed back the sawtooth voltage v c occurring at capacitor c in accordance with the broken line 108 shown in fig1 to the synchronizing signal frequency detector 105 and possibly the detector 104 and to evaluate this sawtooth voltage in for example an integrated circuit the connection of an external capacitor c then requires only one terminal 1 to this end the circuit may be of such a construction that when the device is switched on and synchronizing pulses are still absent it produces a sawtooth voltage having the lower frequency of 50 hz at capacitor c when then 50 hz synchronizing pulses s occur at the input terminal 103 the charging circuit 102 is switched by the first synchronizing pulse detector 104 in the described manner in such a way that as shown by the curve 111 in fig2 the period of the natural frequency is extended until the voltage v 3 has been reached and the discharge 112 is initiated at an earlier instant by the synchronizing pulse s by means of a comparison circuit the sawtooth produced is compared with a voltage v 4 which is approximately halfway the voltage v 1 which is reached at 50 hz synchronizing pulses and the voltage v 6 at which the sawtooth edge 111 is prematurely ended by the discharge 113 when synchronizing pulses of the other frequency of 60 hz are received when this threshold voltage has.