• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.58 no.1
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• pp.49-58
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• 2022

In September and October 2020, combined acoustic and trawl surveys were conducted in the northwestern sea of Jeju Island. In the survey area, a region, so called Jeju region, was designated to esimate the biomass of chub mackerel and jack mackerel using a trawl survey method and frequency difference method. In the September survey, the weight ratios of jack mackerel and chub mackerel to the total catch were 24.6% and 2.8%, respectively, and in the October survey, those ratios were 24.9% and 20.7%, which were used to calculate their biomass (trawl survey). Using the frequency difference range (-8 to -3dB) corresponding to two species in 120 and 200 kHz, their biomass was estimated (frequency difference method). As a result, the biomass of two species from the trawl method was 3252.3 tons in September and 5777.0 tons in October. The estimated biomass by the frequency difference method was 4926.6 tons in September and 7521.5 tons in October. It was the first trial to estimate the biomass of two species using the trawl and frequency differencing methods in South Korea although there were some differences between two methods. In addition, horizontal distributions of acoustic backscattering strength over the entire survey area were mapped.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.51 no.4
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• pp.435-443
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• 2018

The Antarctic krill Euphausia superba is important commercially and ecologically as a basic component of the Antarctic Ocean ecosystem. To manage this resource, it is important to determine the distribution and standing of krill in the water layer. Acoustic methods can capture information about the entire water layer quickly. Acoustic surveys were conducted from March 3 to March 14, 2017, using the commercial fishing boat Sejong (7,765 tons). Acoustic systems with a frequency of 38 kHz and a 200 kHz commercial echo sounder (ES70, Simrad, Norway) were used and the acquired data were processed using post processing software. The density and standing of Antarctic krill were determined using the two-frequency difference method, using the characteristics of two frequencies. To compare the frequency difference of krill, the method using the frequency difference according to the krill length, recommended by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) and the values extracted according to the krill length at survey stations where only krill were collected during the study period, were compared. The frequency difference ranges were 3.96-5.91 dB and -3.0~13.8 dB, respectively.

• Journal of the Korean Institute of Telematics and Electronics S
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• v.35S no.4
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• pp.85-95
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• 1998

In this paper, a new watermarking method for a copy protectionof images is proposed. The proposed method adaptively embeds a watermark in the frequency domain of images using human visual system model. For this purpose, the Just Noticeable Differences(JNDs) of each frequency coeffeicient value of a luminance plane is first found using Watson and Solomon's visual system model. An invisible maximum watermark value with is different in every position according to the characteristics of images is determined usig JND and Minkowski metric. A low frequency domain is divided into two sets based on a PN-sequence to protect thewatermark from the attack. The watermarks are added to one set of coefficients and detecting a watermark, the difference between the mean values of absolute coefficient values of both sets is calculated. The embedded watermark is tested using statistical hypothesis based on test static dertermined by the ean difference. To demonstrate the perfromance of the proposed method, the new watermarking method is applied to a high frequency image and low frequency images. Experimenatal results show the watermark is invisible and robust to JPEGlossy compression and noise.

• Proceedings of the Optical Society of Korea Conference
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• 2000.02a
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• pp.14-15
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• 2000

We study the microwave characteristics of traveling-wave photodetector using the finite-difference time-domain method. We present two parameters for design, the width of PIN region and the thickness of i-layer, and analyze TWPD's property in frequency domain.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.36 no.5
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• pp.541-548
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• 2003

Species identification in hydroacoustic survey is one of the key requirements to estimate biomass of organism and to understand the structure of zooplankton community. Feasibility of species identification using two frequencies (38 and 120 kHz) was investigated on the basis of mean volume backscattering strength difference (MVBS). Virtual echogram technique was applied to two frequencies data sets that obtained from surveys in the Antarctic Ocean and Yellow Sea. Virtual echogram method using MVBS revealed the possibility of species identification, which species identification relying on visual scrutiny of single frequency acoustic data resulted in significant errors in biomass estimation. Through noise cancellation using MVBS, much of the acoustic noise caused by acoustic instruments could be removed in new virtual echogram, and the biomass estimation and data quality was improved.

• Journal of electromagnetic engineering and science
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• v.4 no.3
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• pp.113-118
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• 2004

As an application of ring filters, a new and simple method to determine an inherent-ring-resonance frequency is introduced. The ring filter consists of a ring and two short stubs. They are connected at 90$^{\circ}$ and 270$^{\circ}$ points of the ring and the ring filter may be seen in such way that two filters are connected in parallel. Therefore, if the two powers of the two filters are out-of-phase at the output, the power excited at the input can not be delivered. That can be done by making difference in length of the two short stubs, and when a certain condition is satisfied, a frequency exists where all the excited power is reflected. That is the very inherent-ring-resonance frequency. In the lossless case, the return loss with the condition reaches 0 dB at the inherent-ring-resonance frequency but does not with conductor, dielectric losses and so on. Therefore, the effective loss tangent at a frequency of interest may be obtained correctly. To verify the method, two ring filters have been fabricated in microstrip lines and the measured results show good agreement with the predicted ones.

• Journal of Broadcast Engineering
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• v.15 no.2
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• pp.217-223
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• 2010

In this paper, an interchannel interference (ICI) cancellation method is proposed for multiple frequency offsets in orthogonal frequency division multiplexing (OFDM) systems. When several same signals are received from different transmitters simultaneously, multiple frequency offsets may occur at the receiver because of the frequency difference of the oscillators of the two transmitters and the receiver. This causes degradation of system performance because OFDM systems are very sensitive to the frequency offsets. In this paper, we propose a method to eliminate the effect of the multiple frequency offsets for OFDM systems. The method is accomplished in two steps: compensation of the frequency offset in the time domain and subsequent cancellation of the ICI in the frequency domain. Through computer simulations, we verify the effectiveness of the proposed ICI cancellation method.

• Structural Engineering and Mechanics
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• v.22 no.6
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• pp.719-739
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• 2006

An analytical method for the free vibration of two circular plates coupled with an inviscid and compressible fluid is developed by the Rayleigh-Ritz method. The fluid is bounded by a rigid cylindrical vessel and two circular plates with an unequal thickness and diameter. It was found that the theoretical results could predict well the fluid-coupled natural frequencies with an excellent accuracy when compared with the finite element analysis results. As the fluid thickness increases or the plate thickness difference increases, an abrupt curve veering in the natural frequency loci of the neighboring modes and drastic changes in the corresponding mode shapes are observed. The mode localization frequently appears in the higher modes and in the wide gap between the plates because of a decrease in the fluid coupling owing to the fluid dispersion effect.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.51 no.3
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• pp.305-312
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• 2018

This study was carried out to investigate euphausiid abundance in an acoustic survey from March 2010 to July 2010 to suggest reasonable usage and management of the euphausiid Euphausia pacifica in the coastal area of Guryongpo Pohang. The acoustic data were analyzed by the 2-frequency difference method and the distorted wave born approximation acoustical theoretical model, which is used for organisms of weak target strength with small scatter, such as euphausiid scatter, among other marine organisms. The distribution and monthly density of euphausiids were estimated in the survey area. The results show that the volume back scattering strength frequency difference for euphausiid was 13.91-7.6 dB, and their monthly averag density was $28.2g/m^2$.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.19 no.10
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• pp.1028-1033
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• 2009

To detect faults and monitor thinning on a pipe, many people use ultra sonic sensors that are operated in high frequency range. Because there are many modes in high frequency range, it is difficult to find faults and monitor pipe thinning on a structure. If we deal with signals in a low frequency range which include only A0 wave and S0 wave, the information of monitoring and diagnosis can be easily obtained. In this paper, the technique for exciting low frequency range using ultra sonic sensors is proposed. The main idea of the proposed method comes from the beat phenomenon. The beat frequency is equal to the absolute value of the difference in frequency of the two waves. If the beat frequency is tuned by two ultra waves, we can excite A0 mode and S0 mode of structures. To verify the proposed method, we have performed a steel plate and pipe experiments. Experimental results show that two ultra sonic sensors can well excite low frequency range.