Abstract: In an analogue to digital converter, the analogue signal applied thereto is sampled at successive time intervals and at each sample a digital signal is produced and the time interval between successive samples is not necessarily constant but is dependent upon the rate of change of amplitude of the analogue signal during said time interval. The digital signals may be applied to a counter to produce data representative of the time interval between successive samples and also to indicate whether the analogue signal has increased or decreased between successive samples. This data may then be recorded in a manner to provide substantially constant recording density.