analog converter 21 and from the output of 21 the signal um 34 is supplied through a capacitor 22 and a diode 23 to the output terminal 25 which is connected to a voltmeter 26 a resistor 24 is connected between ground and a junction point between capacitor 22 and the diode 23 and the resistor 24 and diode 23 comprise a rectifier circuit if the repetition frequency of the timing pulses 19 is selected to be sufficiently high the output signal um 34 at the output of the digital to analog converter 21 will have an amplitude which periodically fluctuates the 34 period 34 is defined as being the length of time required by the interrogation circuit 18 to interrogate all the inputs e1 through en one time the capacitor 22 cuts off the mean dc value from the voltage um 34 and the rectifier circuit comprising the diode 23 and resistor 24 is supplied with an ac voltage whose amplitude corresponds to the amplitude fluctuations of um 34 therefore at the output terminal 25 there will occur a dc voltage ug which represents a signal for measuring the amplitude fluctuations of um 34 and um 39 and thus of the distortion caused by the attenuation existing between the circuit points 7 and 8 in the frequency band determined by the individual frequencies f1 through fn if the signal ug is displayed with the use of a voltmeter 26 each of the adjusting attenuation control elements 2a of the attenuation corrector 2 and the adjusting elements 4a of the attenuating corrector 4 which influence the attenuation characteristic curve of the correctors 2 and 4 can be adjusted during the display in a simple manner so as to assure that ug.