the output signal from the sensor(s) in suitable fashion, for example determining one or more signal parameters by counting the number of peaks in the signal, or determining their relative positions and/or amplitudes. Throughout this specification, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.