h if it is assumed that the color difference signal r y becomes negative diode 54 39 is blocked further and diode 56 39 becomes conductive when reaching its threshold voltage a current flows through diode 56 39 and a resistor 59 39 to ground potential and leads to a negative voltage at the non inverse input of sum and difference amplifier 55 39 thereby the output level of sum and difference amplifier 55 39 is switched from h to l this state is maintained until the input voltage drops below the threshold voltage of diode 56 39 the amplifying device has therefore the function of a so called window discriminator when a specific color difference voltage is exceeded in positive as well as in negative direction the output level of sum and difference amplifier 55 39 assumes the l state by means of a resistor 60 positioned between the non inverse input and the output of sum and difference amplifier 55 39 a certain hysteresis conduct of the sum and difference amplifier is produced color difference signals with superimposed noise portions because of this measure do not result in a continuous back and forth switching of the amplifier device a signal with l level can only be obtained at the output of nand gate 51 when the two output levels of sum and difference amplifiers 55 39 and 55 34 are in the h state as initially mentioned saturation threshold detectors 21 28 and 29 are.