Tham khảo tài liệu 'advanced biomedical engineering part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Biomedical Time Series Processing and Analysis Methods The Case of Empirical Mode Decomposition 71 effect on the whole process of the EMD algorithm especially at the level of the number of iterations required. Optimum threshold values are still under investigation in the research field concerning the method as well as in the effect on the set of IMFs and the relation of certain IMFs with the underlying physical process 19 . Each component extracted IMF is defined as a function with equal number of extrema and zero crossings or at most differed by one with its envelopes defined by all the local maxima and minima being symmetric with respect to zero. The application of the EMD method results in the production of N IMFs and a residue signal. The first IMFs extracted are the lower order IMFs which captures the fast oscillation modes while the last IMFs produced are the higher order IMFs which represent the slow oscillation modes. The residue reveals the general trend of the time series. Fig. 2. Experimental respiratory signal processed with Empirical Mode Decomposition. At the upper plot is depicted the original signal Axis Y of a dual axis accelerometer which is sampled in both axes by a mote of a Wireless Sensor Network 18 . 5. Statistical significance of IMFs Intuitively a subset of IMF set produced after the application of EMD on biomedical ECG time series is related to the signal originating from the physical process. Although high correlation values between the noise corrupted time series with specific IMFs may occur there is a difficulty in defining a physical meaning and identifying those IMFs that carry information related to the underlying process. The lack of EMD mathematical formulation and theoretical basis complicates the process of selecting the IMFs that may confidently be separated from the ones that are mainly attributed to noise. Flandrin et al 21 studied fractional Gaussian noise and suggested that 72 Advanced Biomedical Engineering EMD acts as a .
