Time-domain Sequence Research Articles

Reconstruction of non‐minimum phase function from only amplitude data

AbstractA procedure for a non‐minimum‐phase reconstruction from magnitude data alone is presented. A non‐minimum‐phase component is recovered, based on the fact that the auto‐correlation of the time domain sequence is equal to a discrete cosine transformation of the square of the magnitude response. An all‐pass filter is also utilized in order to reduce the optimization variables. Since a non‐minimum‐phase reconstruction does not have a unique solution, a causal time‐domain response is selected along with the smallest poles in magnitude. This theory is applied to antenna pattern synthesis and scattering parameters of microwave filters to illustrate the applicability of the proposed approach. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 212–216, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10560

On the rank of the discrete Gabor transform matrix

By using the discrete Gabor transform or expansion, the time domain sequences are mapped into joint time-frequency domain matrices or vice versa. In many applications, it is more effective to process signals, i.e., two-dimensional matrices, in the joint time-frequency domain than in the time or frequency domain alone. From the mathematical point of view, the processing of the discrete Gabor coefficients is no more than matrix computation. So it is beneficial to understand the properties of the Gabor coefficient matrix. In this letter, we shall investigate the rank of the Gabor coefficient matrix of a one-dimensional time domain signal, which is one of the most important matrix properties.

Early Detection Of Diminished Baroreflex Sensitivity In Diabetic Patients Without Evidence Of Cardiovascular Autonomic Neuropathy

Diabetic cardiovascular autonomic neuropathy (CAN) carries an increased risk of mortality. Decreased baroreflex sensitivity (BRS) has been identified as a predictor of increased mortality following myocardial infarction. We evaluated spontaneous BRS in 39 healthy control subjects (C: age (mean ± SEM): 41.5 ± 1.9 years) and 116 diabetic patients (64% Type 1, 36% Type 2; age: 45.8 ± 1.4 years; diabetes duration: 16.9 ± 1.0 years; HbA1c: 9.2 ± 0.2%) using cross‐spectral analysis between systolic blood pressure and heart rate in the low‐frequency (LF) and high‐frequency (HF) bands as well as time domain (sequence) analysis in the supine and standing positions over 10 min. According to previously suggested definitions based on autonomic function tests (AFTs), 36 patients had definite CAN (CAN+: 3 of 7 indices abnormal), 13 had borderline CAN (CAN[+]: 2 of 7 indices abnormal), and 64 had no evidence of CAN (CAN−: 1 of 7 indices abnormal). Maximum gain in cross‐spectral LF band (standing) was significantly reduced in CAN− as compared with C (5.2 ± 0.4 vs. 7.2 ± 0.8 ms/mmHg, p < 0.05). Moreover, maximum gain in cross‐spectral HF band was significantly lower in CAN− than in C (supine: 12.0 ± 1.2 vs. 17.9 ± 2.5 ms/mmHg, p < 0.05; standing: 4.9 ± 0.5 vs. 8.7 ± 1.0 ms/mmHg, p < 0.05). The slope of the regression line between defined increases or reductions in systolic blood pressure and R‐R intervals was significantly reduced in CAN− compared to C (supine: 10.6 ± 0.7 vs. 14.2 ± 1.6 ms/mmHg, p < 0.05; standing: 5.6 ± 0.4 vs. 8.1 ± 0.7 ms/mmHg, p < 0.05). Similar differences were obtained when comparing the CAN− and CAN[+] groups, the latter showing significantly reduced BRS by both techniques (p < 0.05). In contrast, no such differences were noted when comparing the CAN[+] and CAN+ groups. In conclusion, reduced spontaneous baroreflex sensitivity is an early marker of autonomic dysfunction at a stage when autonomic function tests do not yet indicate the presence of CAN, while cases with borderline CAN show a degree of BRS abnormality that is comparable to the level seen in definite CAN. Prospective studies are needed to evaluate whether reduced BRS is a predictor of mortality in diabetic patients.

Synthesis of fractional gaussian noise using linear approximation for generating self-similar network traffic

The present paper focuses on self-similar network traffic generation. Network traffic modeling studies the generation of synthetic sequences. The generated sequences must have similar features to the measured traffic. Exact methods for generating self-similar sequences are not appropriate for long traces. Our main objective in the present paper is to improve the efficiency of Paxson's method for synthesizing self-similar network traffic. Paxson's method uses a fast, approximate synthesis for the power spectrum of the FGN and uses the inverse Fourier transform to obtain the time-domain sequences. We demonstrate that a linear approximation can be used to determine the power spectrum of the FGN. This linear approximation reduces the complexity of the computation without compromising the accuracy in synthesizing the power spectrum of the FGN. Our results show that long traces can be generated in much less time. To compare our method with existing ones, we will measure the running time in generating long and short sample paths from the FGN. We will also conduct experiments to show that our method can generate self-similar traffic for specified Hurst parameters with high accuracy.

Assessment of baroreceptor-cardiac reflex sensitivity using time domain analysis in patients with IDDM and the relation to left ventricular mass index.

Autonomic dysfunction in insulin-dependent diabetic (IDDM) patients has been associated with abnormalities of left ventricular function and an increased risk of sudden death. A group of 30 patients with IDDM and 30 age, sex and blood pressure matched control subjects underwent traditional tests of autonomic function. In addition, baroreceptor-cardiac reflex sensitivity (BRS) was assessed using time domain (sequence) analysis of systolic blood pressure and pulse interval data recorded non-invasively using the Finapres beat-to-beat blood pressure recording system. 'Up BRS' sequences-increases in systolic blood pressure associated with lengthening of R-R interval, and 'down BRS' sequences-decreases in systolic blood pressure associated with shortening of R-R interval were identified and BRS calculated from the regression of systolic blood pressure on R-R interval for all sequences. We also assessed heart rate variability using power spectral analysis and, after expressing components of the spectrum in normalised units, assessed sympathovagal balance from the ratio of low to high frequency powers. IDDM subjects underwent 2-D echocardiography to assess left ventricular mass index. Standard tests of autonomic function revealed no differences between IDDM patients and control subjects, but dramatic reductions in baroreceptor-cardiac reflex sensitivity were detected in IDDM patients. 'Up BRS' when supine was 11.2 +/- 1.5 ms/mmHg (mean +/- SEM) compared with 20.4 +/- 1.95 in control subjects (p < 0.003) and when standing was 4.1 +/- 1.9 vs 7.6 +/- 2.7 ms/mmHg (p < 0.001). Down BRS when supine was 11.5 +/- 1.2 vs 22 +/- 2.6 (p < 0.001) and standing was 4.4 +/- 1.9 vs 7.3 +/- 2.5 ms/mmHg (p < 0.003). There were significant relations between impairment of the baroreflex and duration of diabetes (p < 0.001) and poor glycaemic control (p < 0.001). From a fast Fourier transformation of supine heart rate data and using a band width of 0.05-0.15 Hz as low-frequency and 0.2-0.35 Hz as high frequency total spectral power of R-R interval variability was significantly reduced in the IDDM group for both low-frequency (473 +/- 62.8 vs 746.6 +/- 77.6 ms2 p = 0.002) and high frequency bands 125.2 +/- 12.9 vs 459.3 +/- 89.8 ms2 p < 0.0001. When the absolute powers were expressed in normalised units the ratio of low frequency to high frequency power (a measure of sympathovagal balance) was significantly increased in the IDDM group (2.9 +/- 0.53 vs 4.6 +/- 0.55, p < 0.002 supine: 3.8 +/- 0.49 vs 6.6 +/- 0.55, p < 0.001 standing). Thus, time domain analysis of baroreceptor-cardiac reflex sensitivity detects autonomic dysfunction more frequently in IDDM patients than conventional tests. Impaired BRS is associated with an increased left ventricular mass index and this abnormality may have a role in the increased incidence of sudden death seen in young IDDM patients.

Efficient dynamic synchronous machine simulation with harmonics

Within this paper, an efficient procedure for the dynamic simulation of faulted synchronous machines operating in the presence of harmonics is presented. The algorithm employs steady-state frequency domain techniques for simulation of the transmission system sequence networks and time domain methods for synchronous machines represented in the odq frame of reference. The procedure affords greater accuracy than conventional fault analysis techniques based strictly on steady-state methods, yet requires far less computational time than full system dynamic simulations such as those using the EMTP.

Simplified inverse z‐transform

A simplified method for the determination of inverse z‐transforms is presented. This technique provides an alternative to partial‐fraction expansion. With knowledge of the signal poles and the time domain sequence, the inverse z‐transform can be accomplished by solving a set of simultaneous equations. The time domain sequence is easily found by long division of the z‐transform.

Time series versus fourier transform methods for estimation of respiratory impedance spectra

Most current techniques to estimate respiratory system mechanical input impedance spectra (Z rs) use digitally created (Fourier transform (FFT) based) random noise. Recent Z rs data reported from 0.1–4 Hz and above 32 Hz display sharper, more distinct spectral features. When expressing Z rs as a power spectral density (PSD), such features suggest the application of a time series spectral estimation approach. Here, a simulation study was performed to compare the quality of impedance (PSD and Z rs) estimates from the time series technique to those from the FFT approach in the presence of measurement noise. Random noise pressure and flow time domain sequences were simulated for two different networks, one which exhibits impedance features reported from 0.1–4 Hz and one which exhibits impedance features reported above 32 Hz. In the time series method, autoregressive (AR), moving average (MA), and autoregressive-moving average (ARMA) models were fit to the pressure and flow sequences separately. The estimated PSD and complex Z rs spectra were compared to the true spectra calculated from the models. Results show that the time series PSD estimates were reasonable even in the presence of additive measurement noise. Conversely, with additive noise, the time series estimates of the complex Z rs showed a negative real part which is physiologically inappropriate. This occurs because of the loss of phase information inherent to the time series approach. Regardless of measurement noise, the FFT estimates of impedance were always close to the true impedance and always superior to the time series estimates. We conclude that an accurate estimate of the PSD or complex Z rs spectra from digitally created FFT-based random noise is best obtained using the traditional FFT method.

A new procedure for interfacing the transmission line matrix (TLM) method with frequency-domain solutions

The authors present a new procedure that interfaces the transmission-line matrix (TLM) method with frequency-domain solutions of electromagnetic fields. Frequency-domain solutions are transformed into appropriate time-domain sequences using the discrete Fourier transform (DFT). Hence, the corresponding boundary Johns matrix can be determined with minimum computational effort. The subsequent treatment consists in convolving the streams of TLM impulses incident on the boundary with a Johns matrix generated with the new approach. The method is used to obtain the time-domain reflection sequence of wideband absorbing terminations in a rectangular waveguide in the dominant mode operation. In addition, the time-domain analysis of pulse penetration through a sheet with high, but finite, conductivity is presented. Good results demonstrate the efficiency of the proposed procedure. >

Tissue contrast enhancement: Image reconstruction algorithm and selection of TI in inversion recovery MRI

It is clearly demonstrated that the proper application of the inversion recovery imaging pulse sequence is dependent on the method of image reconstruction and the selection of TI for optimum tissue contrast. There are two methods of 2DFT image reconstruction of IR sequence time-domain raw data. The first is a modulus-image reconstruction algorithm (contrast-obliterating option), and the second is a phase-correction routine for reconstructing “phase-sensitive” true IR-images. The second option generates proper “in-phase” images, retains proper scale of contrast, but can invert the algebraic sign of image-values under certain conditions. A series of “phase-sensitive” and “modulus” reconstructed brain images, obtained with conventional and optimized new IR pulse sequences, are shown to demonstrate these effects. They illustrate the considerable advantages gained, in practical clinical situations, if one generates “phase-sensitive” true IR-images from IR-sequence raw data at optimum TI for tissue contrast enhancement.

Effects of time window on the estimation of coherence function for the system with delay.

The effects of the shape of time window on the estimation of the coherence function for the system having group delay are investigated. It is well known that the time-windowing determines the spectral resolution and the sidelobes, and that the suitable time window should be selected from that point of view. Time-windowing has another effect on the estimation when the system has group delay. Since the cross power spectrum obtained from DFT's corresponds to the circular convolution of the input and output sequences in time domain, the windowed input and output include the uncorrelated components in the initial and final parts of the windows due to group delay. Consequently, the estimation of the coherence function is biased toward zero. The estimation is improved by using the Hanning window as far as the delay is sufficiently shorter than the window duration, but the rectangular window becomes superior to the Hanning window when the delay is longer than about 0.18 times of the window duration. Theoretical and experimental results indicate the effects of time window on the estimation of the coherence function.