Abstract: Clocking pulses are provided by providing a number of equally spaced potential mark locations serially disposed in the intended direction of relative movement of the marks past a detection location wherein there are provided marks contrasting with the background at every nth one of the potential mark locations and where marks are selectively provided or not at the other potential mark locations to provide information. Means are provided for detecting such marks or the absence thereof when the locations are moved relatively to said detecting means, in a relative direction to move serially therepast. At least n detectors are provided to disposed in said relative movement disposed so that all sense the status of a location simultaneously and at least one is sensing a mark at one of the `nth` locations.