A level comparator is used to check the incoming message header pulse amplitude and if it is of a level higher than an established level a signal is transmitted to update a digital counter whose digital output is converted by a digital to analog converter to establish an increased analog reference level. If the incoming message header is lower the counter is decremented to establish a lower reference level. Then the levels of any signals present at fixed intervals after the header or sync pulse are measured and recorded. These measured levels constitute the value of correction required for subsequent pulses and are added to or subtracted from following data pulses as required.