A sorting decoder captures the rank-order of a set of input analogue signals in the digital domain using simple logic components such as self-timed first state elements, without requiring conventional analogue-to-digital signal converters. The analogue signals are each compared against a monotonic dynamic reference and the resulting comparisons are snapshot by a self-timed first state element for each input signal, or the last member of a sorted collection of input signals, at the time when it reaches the reference signal, so that a different snapshot representing the signal value ranking relative to the other signal values is produced for each input signal. The resulting rank-order estimation snapshots are binary signals that can then be further processed by a simple sorting logic circuit based on elementary logic components.