Estimation of Signals in Noise

Abstract

During the recording of any signal, it is inevitable that some undesirable signals loosely termed noise is also picked up. This noise may be inherent in the measuring apparatus or it may be generated by other systems in the vicinity of the recording. In physiological measurement it is very common to find other physiological signals that provide undesirable noise to the measurement. For example, during the recording of the EMG from muscles in the trunk, the ECG is often unavoidably picked up presenting an undesirable interference. If the signals are well separated in their frequency composition, it may be possible to use frequency filters that permit the passage of the desired signal while attenuating the other interfering signals. Such filtering is included in the analogue signal recording apparatus. It is, of course, also possible to perform such filtering digitally after the signal is digitized. Such digital filtering may be done either on-line while the data is being collected or off-line after the data is recorded and stored. The advantage of using off-line processing is that more sophisticated filters can be used; for instance during off-line processing the restriction of causality is relaxed since for the calculation of the output at any point in time, input values both before and after that point in time are available.