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As the carriers of electromagnetic information about the cardiac system, both magnetocardiography (MCG) and electrocardiography (ECG) are non-stationary signals (NSS). The essential and crucial features of NSS are the local time-frequency characteristics (LTFCH). Under the constraint of the uncertainty principle in the time-frequency domain (TFD), the LTFCH of those non-stationary signals can be determined by evolutionary spectrum analysis (ESA). In this paper, by analyzing the QRS complexes of both MCG and ECG data from healthy volunteers, we have derived some characteristic parameters (CHPs) in the TFD, such as the quality factor (Q) and center frequency (f z ). According to the experiment results, we believe that there are threshold ranges of Q and f z values in the dynamic behavior of the cardiac system for healthy projects. If the CHPs are in the proper ranges, the cardiac system is in a normal condition and homeostasis can be self-correcting. In contrast, if the values of the CHPs are less than the proper ranges, the homeostasis of the cardiac system is lost and some cardiac diseases may result. The assumption of the threshold ranges is verified by sample data from different cardiac diseases, and the results support the assumption. The CHPs are not only a kind of new reference criterion for ECG in clinical diagnosis, but also afford a technological route for the application of MCGs in cardiology. The heart is the dynamic organ of the blood circulation system. The transmembrane potential changes during myocardial cells' vital processes are a biological electromagnetic signal source within living matter. Electrocardiography (ECG) and magnetocardiography (MCG), which are the carriers of electromagnetic information about the human cardiac system, can be acquired by non-invasive detection devices [1][2][3][4]. ECG and MCG signals, which are both NSS [5], have similar shapes. They both consist of a P-wave (atrial depolarization), QRS complex (ventricular depolarization) and T-wave (ventricular repolarization): the S-T segment reflects the repolarization status of myocardial cells. Transient information contained in the QRS complex *Corresponding author (email: xinyuanliu@pku.edu.cn)is extremely important for identifying changes of electromagnetic status in the cardiac system, and it can also provide clues about some cardiovascular diseases. Theoretically, in the TFD the frequency spectrum of a NSS is time-dependent, which means that traditional Fourier transform and the common wavelet transform methods are not suitable for analyzing LTFCH of those signals [6]. The evolutionary spectrum and the corresponding methods are used for detecting time-frequency domain features of NSS [7,8]. We have analyzed the features of the evolutionary spectrum from the QRS complex of healthy subjects' MCG and ECG signals, and then obtained the time-frequency domain characteristic parameters, that is the quality factor Q and the center frequency f z . The results show that the MCG and ECG are both information carriers of cardiac elec...

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The recovered message signal in the synchronized Chua’s system, which is proposed for secure communications, is analyzed by means of both numerical simulations and analytical methods. The results show that the characteristic of “frequency response” of the system is independent of the amplitude of the input message signal, and the amplitude of the recovered signal is suppressed by the system in the lower frequency range and amplified in the higher frequency range. Specifically, we find that, as the frequency of the input signal increases continuously, there is a band of frequency over which a resonance type of phenomenon occurs in the system whereby the input message’s amplitude is amplified, but the location of this band lies beyond the range of the main spectrum of the chaotic driving signal. The phase difference between input signal and output signal is also obtained. Furthermore based on numerical simulations, the characteristic frequency response of a synchronized Lorenz’s system is given, which also shows a band of resonance in the lower range of frequencies.

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