Thesis - Instituto de Telecomunicações

90 CHAPTER 4. SIGNAL PROCESSING AND FEATURE EXTRACTIONFigure 4.14: Mean wave of an ECG signal. The continuous line represents the mean waveand the dashed lines are the mean wave ± the standard deviation of the waves.4.3.2 Feature ExtractionAfter the signal processing we have access to a mean wave from which we compute featuresrelated to the waves P, Q, S, T. The R wave is used for time alignment, setting the initialinstant of the beat (t R = 0). As this time instant is always zero, it will not be used in thefeature vector. The amplitude of the R wave has been normalized to one, so this amplitudeis also not used as a feature.To locate the time position of each of the P, Q, S, T waves we use the following rules(all time values are positive, measuring the absolute distance to the central wave R):1. t P - The first maximum before R wave (reverse time);2. a P - The amplitude of the P wave;3. t Q - The first minimum before R wave (reverse time)4. a Q - The amplitude of the Q wave;5. t S - The first minimum after R wave6. a S - The amplitude of the S wave;7. t T - The first maximum after R Wave