• 음성과학
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• 제8권4호
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• pp.45-58
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• 2001

Modulated components of a speech signal are frequently used for speech coding, speech recognition, and speech synthesis. Time-frequency representation (TFR) reveals some information about instantaneous frequency, instantaneous bandwidth and boundary of each component of the considering speech signal. In many cases, the extraction of AM-FM components corresponding to instantaneous frequencies is difficult since the Fourier spectra of the components with time-varying instantaneous frequency are overlapped each other in Fourier frequency domain. In this paper, an efficient method decomposing speech signal into AM-FM components is proposed. A variable bandwidth filter is developed for the decomposition of speech signals with time-varying instantaneous frequencies. The variable bandwidth filter can extract AM-FM components of a speech signal whose TFRs are not overlapped in timefrequency domain. Also, amplitude and instantaneous frequency of the decomposed components are estimated by using Hilbert transform.

• 한국방사선학회논문지
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• 제12권4호
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• pp.491-496
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• 2018

1.5Tesla와 3.0Tesla의 MRI 검사의 DWI (diffusion-weighted imaging) 펄스시퀀스에서 노이즈 스펙트럼을 분석하여 MRI검사의 기초자료를 제공하여 임상에서 적용하는데 목적이 있다. MRI 검사에서 ACR (American College of Radiology) 팬텀과 노이즈 스펙트럼은 Wavepad sound editor version 8.13 (NCH software, Green wood Village, CO, USA)로 FFT (fast Fourier transform), TFFT (time based fast Fourier transform)를 분석하였다. MR 1.5Tesla와 3.0Tesla의 DWI 펄스 시퀀스에서 검사실에 따른 노이즈 스펙트럼 및 FFT와 TFFT를 분석하였다. 1.5Tesla에 비해 3.0Tesla에서 FFT 및 TFFT에서 주파수 진폭의 노이즈 임계값은 1.5Tesla에서 -6 dB 사이였고, 3.0Tesla에서는 0 dB 사이로 분석되어 환자의 소음감소를 위한 DWI 펄스시퀀스를 환자에게 적절하게 임상에서 적용할 필요가 있다.

• Geomechanics and Engineering
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• 제30권1호
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• pp.1-10
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• 2022

Seismic ground response evaluation is one of the main issues in geotechnical earthquake engineering. These analyses are subsequently divided into one-, two- and three-dimensional methods, and each of which can perform in time or frequency domain. In this study, a novel approach is proposed to assess the seismic site response using two-dimensional transfer functions in frequency domain analysis. Using the proposed formulation, a program is written in MATLAB environment and then promoted utilizing the equivalent linear approach. The accuracy of the written program is evaluated by comparing the obtained results with those of actual recorded data in the Gilroy region during Loma Prieta (1989) and Coyote Lake (1979) earthquakes. In order to precise comparison, acceleration time histories, Fourier amplitude spectra and acceleration response spectra diagrams of calculated and recorded data are presented. The proposed 2D transfer function diagrams are also obtained using mentioned earthquakes which show the amount of amplification or attenuation of the input motion at different frequencies while passing through the soil layer. The results of the proposed method confirm its accuracy and efficiency to evaluate ground motion during earthquakes using two-dimensional model. Then, studies on irregular topographies are carried out, and diagrams of amplification factors are shown.

• 한국전자파학회논문지
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• 제29권3호
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• pp.192-199
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• 2018

본 논문은 기존 DFT-s-SSB(Discrete Fourier Transform spread Single Sideband) OFDM(Orthogonal Frequency Division Multiplexing) 시스템이 가지는 문제점을 해결하는 개선된 DFT-s-SSB OFDM 시스템을 제안한다. 기존 DFT-s-SSB 시스템은 SSB 변조를 적용하기 위하여 PAM(Pulse Amplitude Modulation) 변조를 사용한다. 때문에 변조레벨이 높아질수록 BER(Bit Error Rate) 성능열화가 심각해진다. 그리고 오직 LSB(Lower Sideband) 스펙트럼을 통해서만 송신이 가능하며 LSB와 USB(Upper Sideband) 스펙트럼을 동시에 사용하여 송신하는 경우, 간섭이 발생하고 스펙트럼 복구가 올바르게 이루어지지 않는다. 제안하는 시스템은 이러한 문제를 해결하기 위해 2/3 convolution coding을 적용하여 BER 성능을 개선하고 DFT 크기를 조절하고 USB 스펙트럼을 선택하는 방법을 제시하여 남은 스펙트럼 자원을 이용할 수 있도록 한다. 또한 다중사용자를 지원하는 환경 혹은 제한적인 대역폭을 갖는 환경에서 이 시스템을 이용하는 경우 스펙트럼을 절반만 사용하기 때문에 남은 스펙트럼 자원을 이용할 수 있고 스펙트럼 효율을 향상시킬 수 있다.

• 지구물리와물리탐사
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• 제7권1호
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• pp.45-50
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• 2004

This paper describes a method of detecting formation boundaries, and permeable fractures, from frequency-domain Stoneley wave logs. Field data sets were collected between the depths of 330 and 360 m in well EE-4 in the Higashi-Hachimantai geothermal field, using a monopole acoustic logging tool with a source central frequency of 15 kHz. Stoneley wave amplitude spectra were calculated by performing a fast Fourier transform on the waveforms, and the spectra were then collected into a frequency-depth distribution of Stoneley wave amplitudes. The frequency-domain Stoneley wave log shows four main characteristic peaks at frequencies 6.5, 8.8, 12, and 13.3 kHz. The magnitudes of the Stoneley wave at these four frequencies are affected by formation properties. The Stoneley wave at higher frequencies (12 and 13.3 kHz) has higher amplitudes in hard formations than in soft formations, while the wave at lower frequencies (6.5 and 8.8 kHz) has higher amplitudes in soft formations than in hard formations. The correlation of the frequency-domain Stoneley wave log with the logs of lithology, degree of welding, and P-wave velocity is excellent, with all of them showing similar discontinuities at the depths of formation boundaries. It is obvious from these facts that the frequency-domain Stoneley wave log provides useful clues for detecting formation boundaries. The frequency-domain Stoneley wave logs are also applicable to the detection of a single permeable fracture. The procedure uses the Stoneley wave spectral amplitude logs at the four frequencies, and weighting functions. The optimally weighted sum of the four Stoneley wave spectral amplitudes becomes almost constant at all depths, except at the depth of a permeable fracture. The assumptions that underlie this procedure are that the energy of the Stoneley wave is conserved in continuous media, but that attenuation of the Stoneley wave may occur at a permeable fracture. This attenuation may take place at anyone of the four characteristic Stoneley wave frequencies. We think our multispectral approach is the only reliable method for the detection of permeable fractures.

• 전자공학회논문지D
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• 제34D권9호
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• pp.36-42
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• 1997

Optical filters with graded index profiles are designed by applying the fourier transform to a riccati equation which governs the reflection and transmission characteristics of inhomogeneous refractive index distributions. The inhomogeneous refractive index profile of an optical filter with specified target spectrum is obtained through iterations. The spectra response of the inhomogeneous refractive index layers are analyzed by using runge-dutta numerical method to solve the differential euations of the amplitude and the phase of reflection coefficient derived from the riccati equation and the results are in good agreement with the resutls obtained by using matrix method.

• 감성과학
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• 제1권1호
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• pp.105-111
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• 1998

중추신경계돠 자율신경계에 나타난 후각에 의한 감성변화를 측정하기 위해 후각자극은 0.6% 오렌지향과 2.5% valeric acid를 수증기로 포화시키는 후각 자극기를 통해 일정 flow와 농도로 시행하였다. 대조자극은 방안 공기로 향자극 전후에 시행하였으며 뇌파자극은 international 10-20 systim에서 4채널을 사용하였다. 이외에 ECG, EOG, heart rate, skin conductance와 respiration를 기록하였고 뇌파분석은 fast Fourier tuansform analysis의 power spectra로 하였다. 그 frequency banes는 delta(0-4.5Hz), theta(4.5-7Hz)은 쾌하고 친숙하게 valeric acid는 불쾌하고 성가시게 평가되었다. 뇌파분석에서 쾌와 불쾌 자극간의 차이는 PG2-A2 channel 에서 alphal의 자극전후의 차이를 나타내었으며 불유쾌한 자극에서는 모든 channel alphal, alpha2와 beta파 증가를 보였다. 또한 heart rate, galvaric skin resistance는 쾌자극에서 감소양상을 나타내었으며 불쾌자극에서는 반대경향을 보였다. 호흡에서는 쾌자극에서 호흡수 감소경향과 input/output amplitude dutation와 duration의 증가경향을 보였으며 불쾌자극에서는 반대양상을 보였다. 결론적으로 쾌와 불쾌 후각자극으로 감성변화를 뇌파와 자율신경계에서 볼 수 있었다.

• 전산구조공학
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• 제10권3호
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• pp.175-184
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• 1997

본 논문의 목적은 부등침하를 받는 회전 쉘의 최대 응답과 표준편차와의 관계를 규명하는 것이다. 이를 위해 표준편차에 대한 최대응답의 비(.eta.)및 그 개략값(.eta./sub apr/)를 통계적 수법으로 조사하였다. 또한 탁월차수(predoninant harmonic number)의 함수로 표현되는 .eta./sub apr/을 구하기 위한 식을 제안하였다. 한편 침하는 Fourier급수로 표시하였다. 이때 각 항은 2개의 계수를 포함하게 되는데 하나는 진폭 스펙트럼이고 다른 하나는 위상각이다. 여기서는 위상각은 확률변수로, 진폭은 확정값으로 가정하였다. 해석에서는 .eta., .eta./sub apr/의 특성을 조사하기 위해 2가지 type의 진폭 스펙트럼을 실측데이터를 근거로 설정한 후, 각 type에 대해 100 sample씩의 위상각을 가정하여 사용하였다. .eta.,.eta./sub apr/은 응답의 종류, 쉘의 위치 그리고 진폭 스텍트럼의 type에 상관없이 거의 일정한 값으로 분포하며 그 값은 2.0에서 2.5사이의 값이 된다. 또한 .eta./sub apr/의 값은 .eta.의 값과 거의 비슷하지만 약간 큰 값으로 나타났다. 결론으로 부등침하를 받는 회전 쉘의 최대 응답은 표준편차의 .eta./sub apr/(약 2.5)배 임을 밝히고 있다.

• Ocean Systems Engineering
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• 제2권3호
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• pp.189-203
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• 2012

New experiments focusing on the evolution characteristics of nonlinear wave trains were conducted in a large wave flume. A series of wave trains with added sidebands, varying initial steepness, perturbed amplitudes and frequencies, were physically generated in a long wave flume. The experimental results show that the increasing wave steepness, increases the speed of sidebands growth. To study the frequency and phase modulation, the Morlet wavelet transform is adopted to extract the instantaneous frequency of wave trains and the phase functions of each wave component. From the instantaneous frequency, there are local frequency downshifts, even an effective frequency downshift was not observed. The frequency modulation increases with an increase in amplitude modulation, and abrupt changes of instantaneous frequencies occur at the peak modulation. The wrapped phase functions show that in the early stage of the modulation, the phase of the upper sideband first diverges from that of the carrier waves. However, at the later stage, the discrepancy phase from the carrier wave transformed to the lower sideband. The phase deviations appear in the front of the envelope's peaks. Furthermore, the evolution of the instantaneous frequency exhibits an approximate recurrence-type for the experiment with large imposed sidebands, even when the corresponding recurrence is not observed in the Fourier spectrum.

• Wind and Structures
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• 제16권5호
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• pp.433-455
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• 2013

A phase delay spectrum model towards the representation of spatial coherence of stochastic wind fields is proposed. Different from the classical coherence functions used in the spectral representation methods, the model is derived from the comprehensive description of coherence of fluctuating wind speeds and from the thorough analysis of physical accounts of random factors affecting phase delay, building up a consistent mapping between the simulated fluctuating wind speeds and the basic random variables. It thus includes complete probabilistic information of spatial stochastic wind fields. This treatment prompts a ready and succinct scheme for the simulation of fluctuating wind speeds, and provides a new perspective to the accurate assessment of dynamic reliability of wind-induced structures. Numerical investigations and comparative studies indicate that the developed model is of rationality and of applicability which matches well with the measured data at spatial points of wind fields, whereby the phase spectra at defined datum mark and objective point are feasibly obtained using the numerical scheme associated with the starting-time of phase evolution. In conjunction with the stochastic Fourier amplitude spectrum that we developed previously, the time history of fluctuating wind speeds at any spatial points of wind fields can be readily simulated.