Prior analog to digital conversion techniques have used various methods. FIG. 1A represents a typical periodic sampling technique where an analog signal 10 is sampled every Tsample, so that the sampled values 12 may then be converted to a digital representation. FIG. 1B represents an alternative level-crossing technique where an input signal 14 is compared to a multiplicity of reference values 16, and the time periods, Dti, between reference level crossings of input signal 14 are recorded along with the specific level crossings.