Comb Filter Research Articles

Improved comb filter for the separation of the voiced speech signals in the case of two speakers

In this paper, we present a method for separating voiced sounds from a composite signal. This method is mainly based on the separation by modified comb filter. This filter is keyed to the average values of the estimated pitch. This estimation is performed through an autocorrelation of multi-scale product analysis to separate the effects of the source and the vocal tract. The “autocorrelation of the multi-scale product” method allows noise elimination and the apparition of a signal periodic structure. Peaks that appear are used to calculate the mean fundamental frequency of the target speaker which will be used in the corresponding comb filters to determine the target speaker contribution. After the subtraction of this contribution from the mixture, we obtain the intrusion speaker. This separation is validated by its application on Cooke database and a part of VCTK database and compared to recent methods as Wang–Brown, Hu–Wang, Zhang–Liu and Quan systems. Results confirm the performance of the proposed approach.

Topological charge conversion and localization in defected heterogeneous multihelicoidal optical fibers

In this paper, we have theoretically studied the processes of topological charge conversion in heterogeneous, that is, with an alternating chirality, multihelicoidal fibers in the presence of twist defects. We have shown that two-part and three-part multihelicoidal fibers of that type can be used as compact topological charge all-fiber inverters and comb filters for optical vortices. Also, we have studied the emergence of topologically charged states localized on twist defects in such fibers. We have demonstrated that such states can emerge only in multihelicoidal fibers comprising three parts with alternating chirality. The obtained results can be readily scaled to the terahertz range.

Generation of Soliton Bursts With Flexibly Controlled Pulse Intervals Based on the Dispersive Fourier-Transform Technique

We propose and demonstrate generation of soliton burst mode pulses based on the dispersive Fourier-transform (DFT) technique. The time-domain soliton burst mode is mapped from a multiwavelength dissipative soliton (MWDS) spectrum obtained from a net-normal dispersion L-band mode-locked fiber laser. The interval between adjacent pulses in the soliton burst can be flexibly tuned by precisely controlling the wavelength spacing of the MWDS, which is realized by employing a Mach–Zehnder interferometer comb filter with a tapered fiber in one arm. Our results demonstrate that the combination of an MWDS laser and the DFT technique may indeed be an effective and flexible way to achieve burst mode soliton pulses.

Native and Denaturing MS Protein Deconvolution for Biopharma: Monoclonal Antibodies and Antibody-Drug Conjugates to Polydisperse Membrane Proteins and Beyond.

Electrospray ionization mass spectrometry (ESI-MS) is a ubiquitously used analytical method applied across multiple departments in biopharma, ranging from early research discovery to process development. Accurate, efficient, and consistent protein MS spectral deconvolution across multiple instrument and detector platforms (time-of-flight, Orbitrap, Fourier-transform ion cyclotron resonance) is essential. When proteins are ionized during the ESI process, a distribution of consecutive multiply charged ions are observed on the m/z scale, either positive [M + nH]n+ or negative [M - nH]n- depending on the ionization polarity. The manual calculation of the neutral molecular weight (MW) of single proteins measured by ESI-MS is simple; however, algorithmic deconvolution is required for more complex protein mixtures to derive accurate MWs. Multiple deconvolution algorithms have evolved over the past two decades, all of which have their advantages and disadvantages, in terms of speed, user-input parameters (or ideally lack thereof), and whether they perform optimally on proteins analyzed under denatured or native-MS and solution conditions. Herein, we describe the utility of a parsimonious deconvolution algorithm (explaining the observed spectra with a minimum number of masses) to process a wide range of highly diverse biopharma relevant and research grade proteins and complexes (PEG-GCSF; an IgG1k; IgG1- and IgG2-biotin covalent conjugates; the membrane protein complex AqpZ; a highly polydisperse empty MSP1D1 nanodisc and the tetradecameric chaperone protein complex GroEL) analyzed under native-MS, denaturing LC-MS, and positive and negative modes of ionization, using multiple instruments and therefore multiple data formats. The implementation of a comb filter and peak sharpening option is also demonstrated to be highly effective for deconvolution of highly polydisperse and enhanced separation of a low level lysine glycation post-translational modification (+162.1 Da), partially processed heavy chain lysine residues (+128.1 Da), and loss of N-acetylglucosamine (GlcNAc; -203.1 Da).

A Method for Measuring the Pitch Frequency of Speech Signals for the Systems of Acoustic Speech Analysis

We developed a new method for measuring the pitch frequency of speech signals with elevated noise immunity. The problem of protection against intense background noise is solved in this method by the frequency selection of vocalized segments of speech signals according to a scheme with comb filter of interperiodic accumulation. The efficiency of the method is analyzed both theoretically and experimentally with the help of a multichannel frequency meter intended for the acoustic speech analysis. It is shown that, for a signal-to-noise ratio of 10 dB and higher, the error of the method does not exceed 2%.

Wavelength Continuously Tunable All-Fiber Flat-Top Comb Filter Based on a Dual-Pass Mach–Zehnder Interferometer

We present a wavelength continuously tunable all-fiber flat-top comb filter. The proposed comb filter is based on a dual-pass Mach-Zehnder interferometer (MZI), constructed with two couplers and a polarization controller (PC), along with a polarizer at the input port. While the interference is created from the optical path difference provided by the two cascaded couplers, the dual-pass structure is based on the feedback of a fiber loop mirror, formed by the second coupler with the PC inserted in the loop, which serves as a reflectivity-variable mirror. The coupling ratio of the first coupler is determined by our optimized results on characterizing a basic dual-pass MZI without a PC. By using Jones matrix formulation, we theoretically analyzed the proposed comb filter. To achieve flat-top spectra, both amplitude condition and phase condition have to be satisfied. Our simulation results revealed that by changing the state of the PC, tunable flat-top spectra can be achieved, and moreover, the flat-top spectra can be continuously adjusted within one free-spectral-range with the help of a rotatable polarizer. The theoretical prediction has been verified by our experimental results. The unique tuning property makes the proposed filter an excellent candidate for applications in dense reconfigurable wavelength division multiplexing networks.

Q-switching near-infrared multiwavelength generation by intracavity tungsten-oxide-induced four-wave mixing in an erbium-doped fiber laser.

We report on the generation of a stable multiwavelength Q-switched erbium-doped fiber laser based on tungsten oxide nanoparticles (WO3 NPs) combined with an intracavity comb filter for the first time, to the best of our knowledge. The prepared WO3-PVA thin film has a modulation depth and saturable intensity of 20% and 100 MW/cm2, respectively. A spectrum of up to 15 peaks with a channel spacing of 0.48nm has been obtained. In the Q-switching regime, a minimum pulse width of 4.24μs and a maximum repetition rate of 52.49kHz were achieved at a maximum pump power of 300mW. The dual effect of WO3 NPs in saturable absorption and high optical nonlinearity has induced pulsed and four-wave mixing effects. Therefore, the intrinsic advantages of WO3 nanomaterial provide a promising source for the realization of a stable multiwavelength fiber laser.

Efficient Implementation of Multichannel FM and T-DMB Repeater in FPGA with Automatic Gain Controller

In this study, we implemented a high-performance multichannel repeater, both for FM and T-Digital Multimedia Broadcasting (DMB) signals using a Field Programmable Gate Array (FPGA). In a system for providing services using wireless communication, a radio-shaded area is inevitably generated due to various obstacles. Thus, an electronic device that receives weak or low-level signals and retransmits them at a higher level is crucial. In addition, parallel implementation of digital filters and gain controllers is necessary for a multichannel repeater. When power level is too low or too high, the repeater is required to compensate the power level and ensure a stable signal. However, analog- and software-based repeaters are expensive and they are difficult to install. They also cannot effectively process multichannel in parallel. The proposed system exploits various digital signal-processing algorithms, which include modulation, demodulation, Cascaded Integrator Comb (CIC) filters, Finite Impulse Response (FIR) filters, Interpolated Second Ordered Polynomials (ISOP) filters, and Automatic Gain Controllers (AGCs). The newly proposed AGC is more efficient than others in terms of computation amount and throughput. The designed digital circuit was implemented by using Verilog HDL, and tested using a Xilinx Kintex 7 device. As a result, the proposed repeater can simultaneously handle 40 FM channels and 6 DMB channels in parallel. Output power level is also always maintained by the AGC.

Chinese medicine injection combined with conventional therapies for acute lung injury: a systematic review and meta-analysis of randomized controlled trials

By incorporating a simple waveplate combination (WPC) set composed of two waveplates, we propose a wavelength-tunable fiber comb filter based on a polarization-diversified loop (PDL). The simple WPC set includes three kinds of waveplate groups such as two quarter-wave plates (QWPs), a set of a QWP and a half-wave plate (HWP), and a set of an HWP and a QWP. The PDL is implemented by making a Sagnac birefringence loop comprised of a four-port polarization beam splitter (PBS), two waveplates, and polarization-maintaining fiber (PMF). In the PDL, one end of PMF is connected to one port of the PBS with its slow axis π/4 (45°) oriented with respect to the horizontal axis of the PBS, and the other end of PMF is concatenated with the waveplates. First, we investigated light polarization conditions required to continuously tune the absolute wavelength location of the proposed filter in terms of input and output states of polarization (SOPs) of a birefringence element, or PMF. Then, three analytic transmittances of the filter were derived for the three WPC sets with arbitrary orientation angles of waveplates through Jones matrix formulation. And eight specific orientation angle sets of two waveplates, which caused phase shifts increasing linearly from 0° to 315° by a step of 45° in a sinusoidal transmittance function, were found for each WPC set. In particular, it has been theoretically proved that an orientation angle set of two waveplates, which can induce an arbitrary phase shift in the sinusoidal transmittance function, always exists for each WPC set. This implies that the comb spectrum of the proposed filter can be continuously tuned within one channel bandwidth by the proper control of the waveplate orientation angles. Finally, the input SOPs of PMF and the wavelength-dependent evolution of its output SOP were examined on the Poincare sphere at the eight specific waveplate angle sets. The relationship between the wavelength tuning and the SOP evolution was also discussed.

A Data-Driven Monitoring Scheme for Rotating Machinery Via Self-Comparison Approach

In the era of big data, a huge amount of monitoring and manufacturing data is generated every hour. As these data are typically measured from different machines and under different working regimes, prior information and domain knowledge are highly required in order to properly analyze and utilize these data. In view of this limitation, a data-driven self-comparison approach is proposed for the monitoring of rotating machinery. In this approach, comb filtering is introduced to extract the concerned signals from multisource background noise. A Gini-guided residual singular value decomposition is then proposed to enhance local anomalies induced by early defects. Finally, an iterative Mahalanobis distance is constructed to measure the statistical deviation of monitored component from a normal state. With the proposed method, health monitoring of rotating machinery could be achieved without prior information and domain knowledge, thereby providing an automatic data processing and condition monitoring tool in big data context.

Investigation on Characteristics of Longitudinal Acoustic Wave Propagation in Coal Mine Drill Rods

In order to investigate the feasibility of drill rods used for wireless data transmission in coal mine, the properties of longitudinal acoustic wave propagation in drill string are studied. The results show that: the spectral properties of drill string for coal mine show like a comb filter with wide passbands; when single drill rod become longer, numbers of passbands become more, the width of total passband become narrower; bigger diameter of drill rod makes the width of total passband narrower and amplitudes lower; for integral type and joint-connection type drill rod with same diameter and length, they have little difference in total passband width; the drill strings with different total length have almost same spectral characteristics. In the drill strings investigated, the drill strings composed by drill rods with 42mm diameter and 1m length has the widest passband which occupy 70% of the whole frequency scope. All results indicate that drill rods can be used as channel in data transmission in coal mine gallery.

Microstrip Tunable Bandpass Filters with Combined Resonators

The paper presents a varicap-tunable fourth-order tunable microstrip bandpass filter with combined resonators: a U-shaped loop resonator and a resonator short-circuited at one end, which improved the symmetry of the frequency response while maintaining a wide rejection band inherent in comb filters with resonators short-circuited at one end. For the first time, the resonance equation for odd resonance frequencies of a U-shaped loop resonator with variable capacitance is obtained, which makes it possible to control the ratio of the first parasitic and the fundamental resonance frequencies of this resonator, f1/f0, on which the rejection band of the filter depends. An experimental sample of a filter, tunable in the frequency range of 225–400 MHz, is presented.

New Perspectives on the Physical Origin of Acoustic Wood Anomalies

Acoustic Wood anomaly is a pulsed ultrasound phenomenon in which sharp spectral anomalies such as tips or dips or both occur in the spectrum of a pulse reflected from or transmitted through a periodically corrugated interface for normal incidence. To date only surface acoustic waves generated on periodic interface through diffraction mechanism have been identified as the physical source of these spectral anomalies, and each anomaly has been proved being matched by specific order(s) of diffraction of related surface wave(s) with high accuracy. However, in this study, we report the observation of several anomalies that are not originated from surface waves, and an anomaly that cannot be matched by any individual order of diffraction. Using a proposed sectional analysis approach, comb filtering effect is proved to be a new source of spectral anomalies, and a multiple surface wave theory is developed not only to provide a satisfactory explanation for the unusual anomaly observed, but also to serve as a general guideline for understanding spectral anomaly formation.

Dual-wavelength thulium/holmium-doped fiber laser generation in 2 μm region with high side-mode suppression ratio

We demonstrate a dual-wavelength laser with emission in the 2 μm wavelength region using a thulium/holmium co-doped fiber and a non-adiabatic tapered fiber as a comb filter. The configuration employed a backward pumping scheme and dual-wavelength laser was generated at 2023.53 and 2032.82 nm. The side-mode suppression ratio (SMSR) is more than 44 dB with overall good stability performance. The proposed configuration presents a simple yet practical scheme of generating 2 μm dual-wavelength laser with good SMSR. This investigation may offer a more efficient yet eye safe dual-wavelength related applications for end users.

Continuous wavelength tuning of the first-order general comb spectrum.

Here we show the continuous wavelength tunability of a general transmittance function (GTF), besides flat-top and narrow band transmittance functions, which is mostly obtainable in the first-order fiber comb filter composed of a polarization beam splitter to form a polarization-diversified loop, four waveplates, and two birefringent fiber segments. On the basis of polarization conditions, which should be satisfied to continuously tune the GTF, waveplate orientation angles (WOAs), which gave an extra phase shift of 1-360° to a specifically chosen GTF, were found using a newly proposed WOA search scheme. Wavelength-tuned comb spectra were calculated at eight WOA sets (i.e., Sets I-VIII) selected from the above WOA sets. In the fabricated filter whose free spectral range was ~0.8 nm, its comb spectrum redshifted step by step by ~0.1 nm, resulting in a total displacement of ~0.7 nm, when the WOA set changed from Set I to VIII. The calculated and measured spectra clearly show that an arbitrary GTF can be continuously wavelength-tuned by properly choosing WOAs.

SVD-Based Separation of Stable and Inertial Cavitation Signals Applied to Passive Cavitation Mapping During HIFU.

Detection of inertial and stable cavitation is important for guiding high-intensity focused ultrasound (HIFU). Acoustic transducers can passively detect broadband noise from inertial cavitation and the scattering of HIFU harmonics from stable cavitation bubbles. Conventional approaches to cavitation noise diagnostics typically involve computing the Fourier transform of the time-domain noise signal, applying a custom comb filter to isolate the frequency components of interest, followed by an inverse Fourier transform. We present an alternative technique based on singular value decomposition (SVD) that efficiently separates the broadband emissions and HIFU harmonics. Spatiotemporally resolved cavitation detection was achieved using a 128-element, 5-MHz linear-array ultrasound imaging system operating in the receive mode at 15 frames/s. A 1.1-MHz transducer delivered HIFU to tissue-mimicking phantoms and excised liver tissue for a duration of 5 s. Beamformed radio frequency signals corresponding to each scan line in a frame were assembled into a matrix, and SVD was performed. Spectra of the singular vectors obtained from a tissue-mimicking gel phantom were analyzed by computing the peak ratio ( R ), defined as the ratio of the peak of its fifth-order polynomial fit and the maximum spectral peak. Singular vectors that produced an were classified as those representing stable cavitation, i.e., predominantly containing harmonics of HIFU. The projection of data onto this singular base reproduced stable cavitation signals. Similarly, singular vectors that produced an were classified as those predominantly containing broadband noise associated with inertial cavitation. These singular vectors were used to isolate the inertial cavitation signal. The R -value thresholds determined using gel data were then employed to analyze cavitation data obtained from bovine liver ex vivo. The SVD-based method faithfully reproduced the structural details in the spatiotemporal cavitation maps produced using the more cumbersome comb-filter approach with a maximum root-mean-squared error of 10%.

Performance analysis of multi-rate signal processing digital filters on FPGA

Multi-rate signal processing, an important part of the design of a digital frequency converter, is realized mainly based on interpolation and decimation, which match the sampling rate between the baseband and high-frequency processing side, especially in down conversion. However, the design of a digital filter is important for realizing multi-rate interpolation and decimation, which is highlighted in this paper. To analyze the digital filter performance in multi-rate signal processing, the ordinary finite impulse response (FIR) filter and more efficient digital filter are discussed respectively. The ordinary FIR filters use a Hamming window to design, while a more efficient digital filter includes a cascaded integrate comb (CIC) and half-band filter. Sampling rate transformation factor is 12 in this design, which is cascaded by three stages. Each stage corresponding to the conversion factor is 3, 2, and 2. Both of these design methods are implemented on the FPGA development board. The hardware resource occupancy and the error rate of the signal amplitude in decimation show that the efficient digital filter is superior to the digital filter designed by the Hamming window in the real-time processing.

Polarization-Independent Wavelength-Tunable First-Order Fiber Comb Filter

Here, we propose a polarization-independent wavelength-tunable first-order fiber comb filter based on a polarization-diversified loop configuration. The proposed filter is composed of a polarization beam splitter, two equal-length high birefringence fiber (HBF) segments, three half-wave retarders (HWRs), and two quarter-wave retarders (QWRs). Among these five wave retarders (WRs), a group of an HWR and a QWR is located in front of each HBF. The third remaining HWR is positioned after the second HBF segment and utilized to adjust the effective orientation angle of this HBF. On the basis of the filter transmittance derived through Jones formulation, orientation angle sets of the five WRs, which can induce an arbitrary phase shift from 0 to 2π in the transmittance function, are found at both flat-top and narrow band transmission spectra. From theoretical transmission spectra obtained at eight selected orientation angle sets of the five WRs, which cause phase shifts increasing linearly from 0 to 7π/4 by a step of π/4, it is confirmed that the flat-top or narrow band comb spectrum can be continuously tuned by properly controlling the WR orientation angles. Then, this theoretical prediction is verified by experimental demonstration. In particular, the spectral evolution of the output states of polarization (SOPs) of the third HWR, the last one of optical components contained within the polarization-diversified loop, is investigated on the Poincare sphere. The relationship of the passband shape and wavelength tuning with the spectral evolution of the SOP is also discussed.

Multiwavelength Fiber Laser Using Erbium-Doped Twin-Core Fiber and Nonlinear Optical Loop Mirror

A multiwavelength fiber laser based on erbium-doped twin-core fiber (ED-TCF) and nonlinear optical loop mirror (NOLM) is proposed and demonstrated in this paper. The ED-TCF is employed as comb filter and gain medium simultaneously. The NOLM serves as an intensity equalizer. A fiber laser with 8 wavelength outputs is achieved at room temperature. The optical signal to noise ratio (OSNR) is larger than 40 dB. The fluctuation of peak power and wavelength is less than 1 dB and within 0.015 nm, respectively. This fiber laser has the advantages of simple structure and stable operation.

A twelve-wavelength Thulium-doped fibre laser based on a microfibre coil resonator incorporating black phosphorus

A novel multi-wavelength continuous Thulium-doped fibre laser incorporating a microfibre coil resonator based on black phosphorus (MCR-BP) material has been successfully fabricated and demonstrated. A twelve-wavelength spectrum with 0.54 nm channel spacing has been achieved by simply adjusting the pump source power. A single peak extinction ratio of more than 40 dB was observed. The superior performance of the MCR-BP described in this article compared to conventional MCR based fibre lasers can be attributed the inclusion of the Black Phosphorous Material. The enhanced lasing is primarily due to the twin properties of the MCR-BP device combining a comb-like filter effect and a four-wave-mixing (FWM) effect caused by the nonlinearity of black phosphorus. The experimental results indicate that the novel device can be successfully used as a comb filter and an FWM source to realize a multi-wavelength continuous fibre laser due to the low cost and ease of fabrication of the MCR-BP device.