• 한국음향학회지
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• 제14권4호
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• pp.38-45
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• 1995

본 논문에서는 CELP (Code Excited Linear Prediction) 부호화기의 장구간 예측단에 MBE(Multi-Band Excitation) 방법을 도입하여 4.8kbps 이하의 낮은 전송률에서 성능을 향상시키는 방법을 제안한다. 제안한 방법에서는 기존의 CELP 방법으로 장구간 예측을 한 후에도 여전히 남아 있는 주기적 성분들을 다시 한번 다중대역(multiband)으로 장구간 예측을 한다. 이때 전 스펙트럼을 기본 주파수의 하모닉 간격으로 대역 분할하고, 주기적 다중대역 여기 신호는 각 대역 내의 하모닉들에서 여기 신호 스펙트럼과 근사한 크기를 갖는 정현파 (sine wave)의 합으로 표현함으로써 실제 여기 신호의 특성을 잘 반영하도록 한다. 제안된 방법의 성능 평가를 위해서 4.8 kbps의 전송률에서 컴퓨터 모의 실험을 하였다. 음질 평가의 비교 기준 대상으로 4.8 kbps DoD CELP와 4.4 kbps IMBE를 선정하여 주관적인 음질 평가를 실시한 결과 4.8 kbps DoD CELP보다 우수하였고, 4.4 kbps IMBE와는 비슷하였다.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2008년도 하계종합학술대회
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• pp.873-874
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• 2008

In this paper, we propose a packet loss concealment (PLC) algorithm for CELP speech coders, which is based on multiple adaptive codebooks by using comfort noise for the lost packet recovery. The multiple adaptive codebooks are composed of a conventional adaptive codebook to model periodic excitation of speech and another adaptive codebook to provide a better estimate of excitation when packets are lost in the speech onset region. The performance of the proposed PLC algorithm is evaluated by implementing it into the G.729 decoder and compared with that of the PLC algorithm employed in the G.729 decoder by means of perceptual evaluation of speech quality (PESQ). It is shown from the experiments under different burstiness of packet loss rates of 3% and 5% that the proposed PLC algorithm provides higher PESQ scores than the G.729 PLC algorithm.

• 한국소음진동공학회논문집
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• 제23권8호
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• pp.734-741
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• 2013

In contrast of external excitations, parametric excitations can produce a large response when the excitation frequency is away from the linear natural frequencies. The Mathieu equation is the simplest differential equation with periodic coefficients, which lead to the parametric excitation. The Mathieu equation may have the unbounded solutions. This work conducted the stability analysis for the Mathieu equation, using Floquet theory and numerical method. Using Lindstedt's perturbation method, harmonic solutions of the Mathieu equation and transition curves separating stable from unstable motions were obtained. Using Floquet theory with numerical method, stable and unstable regions were calculated. The numerical method had the same transition curves as the perturbation method. Increased stable regions due to the inclusion of damping were calculated.

• 대한조선학회논문집
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• 제28권1호
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• pp.69-82
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• 1991

축방향의 주기적인 하중으로 가진되는 긴 수직보의 동적안정성에 관하여 연구하였다. 해석방법으로서 Galerkin방법을 이용하여 무한원 연립 Mathieu형 미분 방정식을 얻었으며, 안정성영역을 나타내는 도표를 얻기 위하여, 섭동법과 수치적인 방법을 사용하였다. 또한 이두가지 방법으로 구한 결과를 서로 비교 검토하였다. 여러가지 경계조건에 대한 안정영역을 구했으며, 김쇠의 영향, 평균인장력 및 다중 주파수 파라메터 기진의 영향에 관해서 집중적으로 연구하였다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 1997년도 추계학술대회논문집; 한국과학기술회관; 6 Nov. 1997
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• pp.145-153
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• 1997

Dynamic characteristics are investigated when a nonlinear system showing periodic and chaotic responses under harmonic excitation is exposed to random perturbation. About two well potential problem, probability of homoclinic bifurcation is estimated using stochastic generalized Meinikov process and quantitive characteristics are investigated by calculation of Lyapunov exponent. Critical excitaion is calculated by various assumptions about Gaussian Melnikov process. To verify the phenomenon graphically Fokker-Planck equation is solved numerically and the original nonlinear equation is numerically simulated. Numerical solution of Fokker-Planck equation is calculated on Poincare section and noise induced chaos is studied by solving the eigenvalue problem of discretized probability density function.

• Bulletin of the Korean Chemical Society
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• 제17권5호
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• pp.461-463
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• 1996

The laser-induced fluorescence excitation spectrum of 7-amino-4-(trifluoromethyl)coumarin in a supersonic jet has been recorded in the 340-352 nm region. The electronic band origin was observed at 28622.8 cm-1. Vibrational assignments for the three fundamental low-frequency modes and eight combination bands have been made for the S1 electronic excited state. The out-of-plane vibrations of this molecule have been characterized from the low-frequency assignments of the spectrum. The periodic potential energy function for the CF3 torsion, which satisfactorily fits the observed data, were also determined to be V(Φ)=95X(1-cos3Φ)-32X(1-cos6Φ) where Φ is the torsional angle. The relatively low torsional barrier of 99 cm-1 in S1 state could be explained by the small steric interactions between the functional groups attached to a bicyclic ring.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 1998년도 춘계학술대회논문집; 용평리조트 타워콘도, 21-22 May 1998
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• pp.88-93
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• 1998

Since rattling noise, which occur in mechanical linkage with free play or glove boxes in passenger cars, play an important role in the generation of industrial noise and vibration, it is interest to study these dynamics. A difference equations are derived which described the motions of a mass constrained by pre-compressed spring and forced by a high frequency base excitation. Two types of saddle are founded from these difference equations and the stable and unstable manifolds are constructed in these saddle point. For a certain region in a parameter space of exciting displacement and coefficient of restitution, transversal intersections of stable and unstable manifolds exist. Therefore it is founded that there are large families of periodic and irregular non-periodic motions in rattling system i.e. chaos motion is observed.

• 한국공간정보시스템학회:학술대회논문집
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• 한국공간정보시스템학회 2004년도 춘계 학술발표회 논문집 제1권1호(통권1호)
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• pp.161-168
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• 2004

The dynamic instability of snapping phenomena has been studied by many researchers. Few papers deal with dynamic buckling under loads with periodic characteristics, and the behavior under periodic excitations is expected to be different from behavior under STEP excitations. We investigate the fundamental mechanisms of the dynamic instability when the sinusoidally shaped arch structures are subjected to sinusoidally distributed excitations with pin-ends. The mechanisms of dynamic indirect snapping of shallow arches are especially investigated under not only STEP function excitations but also under sinusoidal harmonic excitations, applied in the up-and-down direction. The dynamic nonlinear responses are obtained by the numerical integration of the geometrically nonlinear equation of motion, and examined by Fourier spectral analysis in order to get the frequency-dependent characteristics of the dynamic instability for various load levels.

• 동력기계공학회지
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• 제3권3호
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• pp.14-21
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• 1999

The wave nature of heat conduction has been developed in situations involving extreme thermal gradients, very short times, or temperatures near absolute zero. Under the excitation of a periodic surface heating in a finite medium, the hyperbolic and parabolic heat conduction equations and the damped wave equations in heat flux are presented for comparative analysis by using the Green's function with the integral transform technique. The Kummer transformation is also utilized to accelerate the rate of convergence of these solutions. On the other hand, the temperature distributions are obtained through integration of the energy conservation law with respect to time. For hyperbolic heat conduction, the heat flux distribution does not exist throughout all the region in a finite medium within the range of very short times(${\xi}<{\eta}_l$). It is shown that due to the thermal relaxation time, the hyperbolic heat conduction equation has thermal wave characteristics as the damped wave equation has wave nature.

• 한국산업정보학회논문지
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• 제4권1호
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• pp.58-62
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• 1999

본 논문은 엔진마운트에서 주기적인 미지의 외란에서 나타나는 시스템의 입출력으로부터의 시스템 동특성을 인식할 수 있는 응용가능성을 보여 주고자 한다. 일반적으로 외란 진동과 파장형태는 확실히 알려져 있지 않고 임의적으로 발생되고 있다. 단지 제어량인 입력과 외란이 섞여 있는 출력측정량만이 시스템인식을 위하여 사용될 뿐이다. 보다 일반화되어 개발된 시스템인식기술 알고리즘은 승용차 능동엔진마운트의 진동제어를 위하여 능동제어에 앞선 미지의 주기적외란을 포함한 동특성시스템에서의 수학적 모의시험을 통하여 시스템인식기술의 완벽성을 나타내고 있다.