after an adjustable period and this entails the lowering of the measured voltage us 1 by a step u03b4u if the control circuit 24 records a change in us 2 from 1 to 0 for this resetting the test matching factor f 3 t is incremented again at the instant t 5 this oscillating motion of the test matching factor f 3 t is now periodically repeated further up to the instants t 6 etc the described oscillating motion takes place however only when no visible picture is traced the matching factor f 3 a for the display mode always remains unchanged in this case because the signal us 2 alternates between the values of 0 and 1 this has the advantage that after an optimum setting has been reached the television picture does not jitter on the picture screen because the correction current does not change in the display mode the correction of the matching factor f 3 a for the display mode is found from experience to occur relatively infrequently because the entire circuit principally serves to correct long term fluctuations which are caused by temperature effects in particular after the receiver is switched on however experience has shown that such a rare change in the matching factor f 3 a or the correction.