A wavelet packet adaptive filtering algorithm for enhancing manatee vocalizations

Approximately a quarter of all West Indian manatee (Trichechus manatus latirostris) mortalities are attributed to collisions with watercraft. A boater warning system based on the passive acoustic detection of manatee vocalizations is one possible solution to reduce manatee-watercraft collisions. The success of such a warning system depends on effective enhancement of the vocalization signals in the presence of high levels of background noise, in particular, noise emitted from watercraft. Recent research has indicated that wavelet domain pre-processing of the noisy vocalizations is capable of significantly improving the detection ranges of passive acoustic vocalization detectors. In this paper, an adaptive denoising procedure, implemented on the wavelet packet transform coefficients obtained from the noisy vocalization signals, is investigated. The proposed denoising algorithm is shown to improve the manatee detection ranges by a factor ranging from two (minimum) to sixteen (maximum) compared to high-pass filtering alone, when evaluated using real manatee vocalization and background noise signals of varying signal-to-noise ratios (SNR). Furthermore, the proposed method is also shown to outperform a previously suggested feedback adaptive line enhancer (FALE) filter on average 3.4 dB in terms of noise suppression and 0.6 dB in terms of waveform preservation.     

Background noise cancellation for improved acoustic detection of manatee vocalizations

The West Indian manatee (Trichechus manatus latirostris) has become endangered partly because of an increase in the number of collisions with boats. A device to alert boaters of the presence of manatees, so that a collision can be avoided, is desired. A practical implementation of the technology is dependent on the hydrophone spacing and range of detection. These parameters are primarily dependent on the manatee vocalization strength, the decay of the signal's strength with distance, and the background noise levels. An efficient method to extend the detection range by using background noise cancellation is proposed in this paper. An adaptive line enhancer (ALE) that can detect and track narrow band signals buried in broadband noise is implemented to cancel the background noise. The results indicate that the ALE algorithm can efficiently extract the manatee calls from the background noise. The improved signal-to-noise ratio of the signal can be used to extend the range of detection of manatee vocalizations and reduce the false alarm and missing detection rate in their natural habitat.     

Theoretical detection ranges for acoustic based manatee avoidance technology

The West Indian manatee (Trichechus manatus latirostris) has become endangered partly because of watercraft collisions in Florida's coastal waterways. To reduce the number of collisions, warning systems based upon detecting manatee vocalizations have been proposed. One aspect of the feasibility of an acoustically based warning system relies upon the distance at which a manatee vocalization is detectable. Assuming a mixed spreading model, this paper presents a theoretical analysis of the system detection capabilities operating within various background and watercraft noise conditions. This study combines measured source levels of manatee vocalizations with the modeled acoustic properties of manatee habitats to develop a method for determining the detection range and hydrophone spacing requirements for acoustic based manatee avoidance technologies. In quiet environments (background noise approximately 70 dB) it was estimated that manatee vocalizations are detectable at approximately 250 m, with a 6 dB detection threshold, In louder environments (background noise approximately 100dB) the detection range drops to 2.5 m. In a habitat with 90 dB of background noise, a passing boat with a maximum noise floor of 120 dB would be the limiting factor when it is within approximately 100 m of a hydrophone. The detection range was also found to be strongly dependent on the manatee vocalization source level.     

Autocorrelation based denoising of manatee vocalizations using the undecimated discrete wavelet transform

Recent interest in the West Indian manatee (Trichechus manatus latirostris) vocalizations has been primarily induced by an effort to reduce manatee mortality rates due to watercraft collisions. A warning system based on passive acoustic detection of manatee vocalizations is desired. The success and feasibility of such a system depends on effective denoising of the vocalizations in the presence of high levels of background noise. In the last decade, simple and effective wavelet domain nonlinear denoising methods have emerged as an alternative to linear estimation methods. However, the denoising performances of these methods degrades considerably with decreasing signal-to-noise ratio (SNR) and therefore are not suited for denoising manatee vocalizations in which the typical SNR is below 0 dB. Manatee vocalizations possess a strong harmonic content and a slow decaying autocorrelation function. In this paper, an efficient denoising scheme that exploits both the autocorrelation function of manatee vocalizations and effectiveness of the nonlinear wavelet transform based denoising algorithms is introduced. The suggested wavelet-based denoising algorithm is shown to outperform linear filtering methods, extending the detection range of vocalizations.     

Detection of tones in noise and the "severe departure" from Weber's law

Thresholds were measured for the detection of 20-ms sinusoids, with frequencies 500, 4000, or 6500 Hz, presented in bursts of bandpass noise of the same duration and centered around the signal frequency. A range of noise levels from 35 to 80 dB SPL was used. Noise at different center frequencies was equated in terms of the total noise power in an assumed auditory filter centered on the signal frequency. Thresholds were expressed as the signal levels, relative to these noise levels, necessary for subjects to achieve 71% correct. For 500-Hz signals, thresholds were about 5 dB regardless of noise level. For 6500-Hz signals, thresholds reached a maximum of 14 dB at intermediate noise levels of 55-65 dB SPL. For 4000-Hz signals, a maximum threshold of 10 dB was observed for noise levels of 45-55 dB SPL. When the bandpass noises were presented continuously, however, thresholds for 6500-Hz, 20-ms signals remained low (about 1 dB) and constant across level. These results are similar to those obtained for the intensity discrimination of brief tones in bandstop noise [R. P. Carlyon and B. C. J. Moore, J. Acoust. Soc. Am. 76, 1369-1376 (1984); R. P. Carlyon and B. C. J. Moore, J. Acoust. Soc. Am. 79, 453-460 (1986)].     

高精度多脉冲激光测距回波检测技术

针对远距离激光测距机回波信号脉宽强度特征和噪声的统计特性,提出了一种高精度多脉冲激光测距机的回波信号检测技术。采用时钟移相分配方式实现了400 MHz高速信号采集及数字化处理,提高了回波脉冲的检测精度。采用ARM嵌入式高速核处理器进行数据算法处理,完成了滤波降噪、自适应门限检测和多帧相关检测等功能,使回波信噪比提高了7.6 dB,达到远距离精确获取目标距离信息的目的。     

Barker码激励超声导波在长骨检测中的应用

单脉冲激励的超声导波在长骨中传播时,信号的衰减大,导致接收信号的幅度很小,且噪声严重。Barker码激励能有效增大接收信号幅度,提高信噪比(SNR)。将其应用到超声导波长骨检测中,进行仿真和长骨实验,得到的信号分别用加权匹配滤波器和有限冲激响应-最小均方误差(FIR-LS)逆滤波器进行压缩,并与单脉冲激励的结果进行了对比。结果表明,对于13位的Barker码,采用加权匹配滤波器进行解码时,压缩信号幅度是单脉冲激励接收信号的13倍;而FIR-LS逆滤波器则达到-63.59 dB的峰值旁瓣水平(PSL),更好地抑制噪声。说明可以将Barker码激励超声导波应用于长骨的检测中。      

Determination of West Indian manatee vocalization levels and rate

The West Indian manatee (Trichechus manatus latirostris) has become endangered partly because of a growing number of collisions with boats. A system to warn boaters of the presence of manatees, based upon the vocalizations of manatees, could potentially reduce these boat collisions. The feasibility of this warning system would depend mainly upon two factors: the rate at which manatees vocalize and the distance in which the manatees can be detected. The research presented in this paper verifies that the average vocalization rate of the West Indian manatee is approximately one to two times per 5-min period. Several different manatee vocalization recordings were broadcast to the manatees and their response was observed. It was found that during the broadcast periods, the vocalization rates for the manatees increased substantially when compared with the average vocalization rates during nonbroadcast periods. An array of four hydrophones was used while recording the manatees. This allowed for position estimation techniques to be used to determine the location of the vocalizing manatee. Knowing the position of the manatee, the source level was determined and it was found that the mean source level of the manatee vocalizations is approximately 112 dB (re 1 microPa) @ 1 m.     

Evaluation of an auditory model for echo delay accuracy in wideband biosonar

In a psychophysical task with echoes that jitter in delay, big brown bats can detect changes as small as 10-20 ns at an echo signal-to-noise ratio of approximately 49 dB and 40 ns at approximately 36 dB. This performance is possible to achieve with ideal coherent processing of the wideband echoes, but it is widely assumed that the bat's peripheral auditory system is incapable of encoding signal waveforms to represent delay with the requisite precision or phase at ultrasonic frequencies. This assumption was examined by modeling inner-ear transduction with a bank of parallel bandpass filters followed by low-pass smoothing. Several versions of the filterbank model were tested to learn how the smoothing filters, which are the most critical parameter for controlling the coherence of the representation, affect replication of the bat's performance. When tested at a signal-to-noise ratio of 36 dB, the model achieved a delay acuity of 83 ns using a second-order smoothing filter with a cutoff frequency of 8 kHz. The same model achieved a delay acuity of 17 ns when tested with a signal-to-noise ratio of 50 dB. Jitter detection thresholds were an order of magnitude worse than the bat for fifth-order smoothing or for lower cutoff frequencies. Most surprising is that effectively coherent reception is possible with filter cutoff frequencies well below any of the ultrasonic frequencies contained in the bat's sonar sounds. The results suggest that only a modest rise in the frequency response of smoothing in the bat's inner ear can confer full phase sensitivity on subsequent processing and account for the bat's fine acuity or delay.     

Maximum number of connectable laser diode optical switch (LDSW) systems

The maximum number of connectable laser diode optical switches (LDSWs) was studied through the baseband signal-to-noise ratio calculation. For an NRZ 100 Mbits^–1 PCM-IM signal, 60 stages of LDSWs can be connected with a 6-nm optical bandpass filter inserted after the last stage at an input signal level of –30 dB m and an internal gain of 16 dB. A PCM-IM signal of 10 Gbits^–1 can be transmitted through 130 stages of LDSWs at an internal gain of 8 dB and an input signal level of –20 dB m.     

Widely tunable ring-cavity tellurite fiber Raman laser

We report a widely tunable ring-cavity tellurite fiber Raman laser covering the S+C+L+U band. A tunable range (1495-1600 nm, limited by the tunable optical bandpass filter) over 100 nm is obtained by using a single-mode tellurite fiber with high Raman gain coefficients (55 W(-1)km(-1)) and large Raman shift (~22.3 THz) as the gain medium. Furthermore, the free-running 1665 nm Raman fiber laser is achieved from the ring cavity by removing the tunable optical bandpass filter, which shows that such a tellurite fiber has potential for constructing a widely tunable fiber Raman laser covering the S+C+L+U band. A high optical signal-to-noise ratio of over 60 dB for almost all of the tunable range is also demonstrated.     

Photonic microwave bandpass filter with improved dynamic range

A technique to improve the dynamic range of a photonic microwave bandpass filter is proposed and experimentally demonstrated. The filter is implemented based on phase modulation to intensity modulation conversion using fiber Bragg gratings (FBGs) serving as frequency discriminators, with the optical carriers located at the left or right slopes of the FBGs, to generate positive or negative tap coefficients. The dynamic range of the photonic microwave bandpass filter is increased by reducing the optical-carrier-induced shot noise and relative intensity noise at the photodetector, which is realized by placing the optical carriers at the lower slopes of the FBG reflection spectra. A photonic microwave bandpass filter with an improvement in dynamic range of about 10 dB is demonstrated.     

Wavelet based noise suppression technique and its application to ultrasonic flaw detection

The noise suppression techniques with wavelet transform (WT) are widely used in non-destructive testing and evaluation (NDT&E), especially in ultrasonics. Complete reconstruction theory with hard or soft thresholds, reconstruction technique based on the singularities of noise and signal, matched filter with an impulse response, and optimal frequency-to-bandwidth ratio of wavelet technique have all been used to analyze ultrasonic signals for noise suppression. But a more simple and effective technique has been pursued for decades. This paper develops a new technique using WT for the right purpose. In this work, WT is treated as a band-pass filter whose central frequency and frequency bandwidth (CF&FB) are determined by the spectra distribution of an ultrasonic signal captured from real testing situation. For the purpose of matching their CF&FB well, a technique for evaluating the optimal scale of a daughter wavelet is carried out too. By acting this daughter wavelet as a band-pass filter, we can obtain excellent de-noising results, even when the signal to noise ratio (SNR) is below -18 dB. The performance of the technique has been done by ultrasonic signals with computer generated white noises. Finally, the experimental verification is performed on a pipeline specimen with man-made small flaws with good results obtained. The results show that the technique is more suitable for processing heavy noised ultrasonic signals, and it can also be used in automatic flaw detection.     

A release from masking by continuous, random, notched noise

Thresholds for 10-ms sinusoids simultaneously masked by bursts of bandpass noise centered on the signal frequency were measured for a wide range of signal frequencies and noise levels. Thresholds were defined as the signal power relative to the masker power at the output of an auditory filter centered on the signal frequency. It was found that the presentation of a continuous random noise, with a spectral notch centered on the signal frequency, produced a reduction in signal thresholds of up to 11 dB. A notched noise spectrum level of 0-5 dB above that of the masker proved most effective in producing a masking release, as measured by a reduction in masked threshold. A release from masking of up to 7 dB could be obtained with a continuous bandpass noise. The most effective spectrum level of this noise was 5 dB below that of the masker. The effect of the continuous notched noise was to reduce signal-to-masker ratios at threshold to about 0 dB, regardless of the threshold in the absence of continuous noise. Thus the greatest release from masking occurred when "unreleased" thresholds were highest. The release from masking is almost complete within 320 ms of notched noise onset, and persists for about 160 ms after notched noise offset, regardless of notched noise level. The phenomenon is similar in many ways to the "overshoot" effect reported by Zwicker [J. Acoust. Soc. Am. 37, 653-663 (1965)]. It is argued that both effects can be largely attributed to peripheral short-term adaptation, a mechanism which is also believed to be involved in forward masking.     

Detection of multi-frequency weak signals with adaptive stochastic resonance system

Aiming at the poor detection rate of multi-frequency weak signals under a strong background of noise, a novel method based on adaptive stochastic resonance (SR) theory is proposed in this paper. The optimal parameters can be obtained automatically via measurement by establishing an adaptive SR system model and using the reverse location method. After passing through the adaptive SR system, the spectrum values of all eight signals greatly improve, the largest spectrum value gain increases from 12.41 to 2033 when the frequency is 0.01?Hz, which is an improvement of a factor of 162.8, and the signal-to-noise ratio (SNR) gain of the whole system is 10.3134?dB. Under the condition of different input noise intensities and signal amplitudes, the mean SNR of the system increases from –13.1136 to –2.7614?dB, which is a 78.9% increase, and the largest SNR gain is 13.4702?dB when the noise intensity D?=?1.2 and signal amplitude A?=?0.11. Compared to the single optimal spectrum value, when defining multiple optimum spectrum values as the SNR criterion, the detection sensitivity is less than 0.35 when the input noise intensity is between 0.5 and 2.5, and the sensitivity value is 6.29 times higher when D?=?2.5. The system successfully realizes the adaptive detection of twelve weak signals, and the SNR gain is 7.9743?dB, which improves the channel capacity of signal detection. The experimental results demonstrate the high efficiency and strong applicability of the system, improving the signal processing efficiency and speed of signal transmission.     

A novel infrared small target detection method based on BEMD and local inverse entropy

Compared to other targets, it is more difficult to detect infrared small targets due to several aspects such as the low signal to noise ratio, low contrast, small size, the lack of shape and texture information of the targets, especially under complex background. In this paper, a novel infrared small target detection method based on peer group filter (PGF), bi-dimensional empirical mode decomposition (BEMD) and local inverse entropy (LIE) is proposed to overcome these difficulties. The PGF is implemented to remove the noise and improve the signal-to-noise ratio of the initial image. Our proposed BEMD algorithm is able to estimate the background effectively and get the target image by removing the background from the original image and segmenting the Intrinsic Mode Functions (IMFs) making use of the local inverse entropy. Experimental results demonstrate that the novel method can extract the small targets validly and accurately.     

A glimpsing model of speech perception in noise

Do listeners process noisy speech by taking advantage of "glimpses"-spectrotemporal regions in which the target signal is least affected by the background? This study used an automatic speech recognition system, adapted for use with partially specified inputs, to identify consonants in noise. Twelve masking conditions were chosen to create a range of glimpse sizes. Several different glimpsing models were employed, differing in the local signal-to-noise ratio (SNR) used for detection, the minimum glimpse size, and the use of information in the masked regions. Recognition results were compared with behavioral data. A quantitative analysis demonstrated that the proportion of the time-frequency plane glimpsed is a good predictor of intelligibility. Recognition scores in each noise condition confirmed that sufficient information exists in glimpses to support consonant identification. Close fits to listeners' performance were obtained at two local SNR thresholds: one at around 8 dB and another in the range -5 to -2 dB. A transmitted information analysis revealed that cues to voicing are degraded more in the model than in human auditory processing.     

Signal-to-noise ratio for source determination and for a comodulated masker in goldfish, Carassius auratus

The masking effects of white and amplitude comodulated noise were studied with respect to simple signal detection and sound source determination in goldfish. A stimulus generalization method was used to determine the signal-to-noise ratio required to completely determine the signal's characteristics. It was found that the S∕N required for this determination is about 4 dB greater than that required for signal detection, or was about 4 dB greater than the critical masking ratio. This means that the potential harm to fish of a given masking noise is at least 4 dB greater than previously thought, based on critical masking ratios. However, for amplitude comodulated noise between 10 and 50 Hz modulation rate, the potential harmful effects are up to 5.3 dB less than would be predicted from the critical masking ratio for unmodulated noise.     

基于光参量变频与放大的高灵敏红外成像技术

针对可用于微弱红外图像探测的光学参量变频与增强技术,进行了仿真与实验探索.针对高增益光参量放大器(OPA)过程中的参量荧光背景噪声,提出了基于外接圆模型的空间滤波技术,通过仿真优化设计,利用空域、频域滤波与像传递系统相结合的方法,将参量荧光背景的抑制比例超过70%,其增强后的成像质量较之前有明显改善,峰值信噪比提升22%.基于10 Hz,355 nm的大能量皮秒紫外抽运激光,实现了红外波段到可见光波段的参量频率上转换,得到了超过1.3×108(82 d B)的光学图像增益.实验结果表明,采用高增益OPA作为光学预放大级之后,常规非制冷电荷耦合器件可实现微弱红外成像的有效探测,灵敏度可达每像素7.4个光子.该方案有望用于单光子级高灵敏红外成像场合.     

基于CIC的数字正交锁相放大器的甲烷检测实验研究

针对甲烷在大气中背景气体成分复杂、检测难度大、稳定性差等问题,本文基于可调谐二极管激光吸收光谱技术和波长调制光谱技术,将积分梳状滤波器与有限脉冲响应滤波器相结合应用于数字正交锁相放大器,开展大气中甲烷气体的痕量检测实验研究.实验表明,与传统的数字正交锁相放大器相比较,改进的数字正交锁相放大器提取的二次谐波信号的信噪比从...