in a magnetic recording system utilizing a non - return - to - zero format the reproduced recording is fed to positive and negative threshold detectors having no need for electrical hysteresis as well as , via differentiator means , to a zero - crossing detector . the signal from the positive threshold detector and one polarity of the signal from the zero - crossing detector are used to trigger a latch means to a first state to emit a signal having a first level , and the signal from the negative threshold detector and the other polarity of the signal from the zero - crossing detector are used to trigger the latch means to a second state whereby the signal emitted by the latch means is switched to a second level .