• Journal of the Korean Wood Science and Technology
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• v.33 no.4 s.132
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• pp.9-14
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• 2005

Changes of sound absorption capability and anatomical features of wood by delignification treatment was estimated. Sound absorption coefficients of wood and delignificated wood had been measured by the two microphone method and anatomical changes of delignificated wood examined by SEM observation. The sound absorption coefficients of delignificated wood generally seemed to be higher than those of normal specimen. Especially, in the frequency range of 2 to 4 KHz, they was about 50% higher than those of normal specimen. Abundant small cracks generated on the cross sectional surface of delignificated wood and the weight of delignificated wood decreased about 8% than that of normal wood. It was considered that the small cracks formed by delignification treatment behaved as a sound absorbing pore.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2008.10a
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• pp.227-230
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• 2008

기존의 대표적인 디지털 통신방식으로 주파수 편이 변조(FSK: frequency shift keying), 위상 편이 변조(PSK: phase shift keying), 진폭 편이 변조(ASK: amplitude shift keying) 방식들이 있다. 본 논문에서는 디지털 통신에서 비트 전송률을 높이기 위하여 스케일링 함수(scaling function)와 웨이브릿(wavelet)을 이용한 2비트 스케일을 갖는 웨이브릿 편이 변조(WSK: wavelet shift keying)신호의 복원 알고리즘을 제안한다. 변조시 스케일링 함수(scaling function)와 웨이브릿(wavelet) 그리고 이 두 신호를 반전하여 2비트 스케일 즉, 00, 01, 10, 그리고 11로 변조하였다. 복조시에는 변조된 신호를 2개의 2진 정합필터에 통과시켰다. 그리고 정합필터를 거친 파형을 조합하여 4가지 신호를 복원하였다. 모의실험 결과 제안한 알고리즘이 기존의 웨이브릿 편이 변조 시스템에 비해 전송효율이 향상되었음을 알 수 있었다.

• Journal of the Korean Society for Nondestructive Testing
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• v.25 no.2
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• pp.127-132
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• 2005

The optical glass lens is required high dimensional precision such as the lack of defect. In this paper, we examined the detectable defect by using the resonant ultrasound spectroscopy(RUS). The RUS is the measurement system which is to excite the specimen and to inspect the differences of resonant frequency pattern between acceptable specimen and specimen which has some defects. In this paper, for nondestructive evaluation by using RUS, we measured the resonant frequency of each specimen which is spherical and aspherical glass lens. With the results, we knew the polishing processing degree of spherical glass lens by the measured resonant frequency and could evaluate the characteristic of aspherical glass lens about some flaws.

• Composites Research
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• v.16 no.2
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• pp.9-17
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• 2003

We have implemented a system of falling weight impact tester in order to evaluate the impact energy absorbing characteristics and impact strength of CFRP laminate plates. The absorbed energy of T-300 orthotropic composites is higher than that of quasi-isotropic specimen over impact energy 7J, but in case of using T700 fiber, much difference does not show. Also, absorbed energy of T-300 orthotropic composites, which are composed of the same stacking number and orientation became more than that of T700 fiber specimen however there was no big difference in case of quasi-isotropic specimens. Delamination area of impacted specimens was measured with ultrasonic C-scanner to find correlation between impact energy and delamination area. Delamination area and frequency responses were evaluated between impacted and unimpacted specimens. There is a strong correlation between frequency responses and impact-induced delamination. The presence and scale of damages have been investigated based on the variations of frequency responses.

• Journal of Sensor Science and Technology
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• v.3 no.2
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• pp.57-64
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• 1994

This paper describes the measurement of tissue blood flow by the the self-mixing effect of laser diode. A Laser doppler signals due to the moving object and the tissue blood flow were detected by the self-mixing effect of laser diode. The Doppler shifted frequency was changed linearly with the driving frequency of moving object and was increase after the exercise. The results of in-vivo experiment were consistent with the result of exercise physiology.

• The Journal of the Acoustical Society of Korea
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• v.41 no.2
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• pp.218-226
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• 2022

The main characteristics of the underwater acoustic channel have varying multipath and fast fading. Especially, Frequency Shift Keying (FSK) signals with multiple frequency bands caused loss of information bits allocated on the specific frequencies due to selective fading phenomenon. Therefore, this paper proposes frequency gain control algorithm based on preamble error rates. The proposed algorithm estimates optimal gain value in the range of preamble error rate with less than 10 % for specific faded frequency. By employing turbo equalized FSK signals with rate of 1/3, the experiment was conducted on a lake in Munkyeong city with distance of 300 m to 500 m. The result confirms that packets are decoded successfully by applying proposed algorithm as increasing number of iterations.

• 정보화사회
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• s.107
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• pp.75-77
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• 1996

위성통신에서는 위성에 실린 고가의 중계기를 가능하면 많은 지구국이 공용하는 편이 효율적이 된다. 그 때문에 중계기에 할당된 주파수 대역을 나누어 각 지구국에 할당해야 한다. 또 자동차와 휴대전화 등의 이동통신에서도 기지국과 이동기를 연결하는 무선회선에서는 다수의 가입자가 제한된 무선을 이용할 수 있는 것과 같은 방법을 사용할 필요가 있다.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2000.10a
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• pp.172-175
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• 2000

본 논문에서는 무선마이크용 특정소출력무선기기의 송수신단을 설계하고 제작하여 특성을 측정하였다. 무선마이크용 특정소출력무선기기의 주파수 대역 740.000~752.000MHz를 사용하였고, 점유주락수대폭의 허용치 200kHz, 최대주파수편이 $\pm$75kHz를 만족하도록 설계하였다. PLL Synthesized, 주파수 변조, Dual Conversion 방식을 사용하였으며 Compander Type은 2:1이다. 설계된 무선마이크 시스템의 출력은 10mW이하이고, 50-20,000Hz의 주파수 응답을 가지며 Hi-fi Audio를 전송할 수 있다. 송수신단의 Total Harmonic Distortion 은 1.5% 이하이다.

• Journal of IKEEE
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• v.22 no.3
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• pp.800-804
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• 2018

This paper presents a novel HF band differential quadrature phase-shift keying (DQPSK) orthogonal frequency-division multiplexing (OFDM) communication system. The system can deliver 3.6 kbps with a bandwidth of about 3 kHz. In a digital modem, OFDM with 32-point fast Fourier transform is used. In the system, each subcarrier uses DQPSK modulation. Hence, a demodulator does not require carrier phase recovery and symbol timing recovery. And, each subcarrier employs CRC error check code individually. By using CRC code for each subcarrier, bit error caused by multipath fading can be recovered simply.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.10C
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• pp.999-1006
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• 2005

The difference threshold test (DTT) declares erasures whenever the difference between the largest and second largest energy detector outputs does not exceed a given threshold. we show that the DTT outperforms the ratio threshold test (RTT) for finite modulation size and diversity order in the detection of M-FSK signals. The asymptotic performance for infinite modulation size and diversity order is then investigated. It is shown that the minimum signal-to-noise ratio required to achieve error-free communication for the DTT is the same as that for the RTT.