Abstract: The present invention provides a method of overcoming the contamination of physiological signals with noise caused by characteristics of the electrical supply to measuring devices. The method exploits the periodic and spectrally stationary nature of noise. The method can be implemented in software for easy calculation and display of calculated results for interpretation and use of the resulting relatively uncontaminated signals. The method can be applied where measurements are made of physiological parameters of humans or any other animal. The invention includes apparatus for acquiring and processing physiological signals from a subject included at least one sensor for acquiring at least one signal and at least one microprocessor means for processing the at least one signal, the microprocessor means including means for storing a whole number multiple of an artefact waveform for calculating the line-noise component of data derived from the at least one sensor.