Arbitrary Transfer Function Research Articles

Time-Modulated Circuits and Metasurfaces for Emulating Arbitrary Transfer Functions

Time-Modulated Circuits and Metasurfaces for Emulating Arbitrary Transfer Functions

Differentiable Direct Volume Rendering.

We present a differentiable volume rendering solution that provides differentiability of all continuous parameters of the volume rendering process. This differentiable renderer is used to steer the parameters towards a setting with an optimal solution of a problem-specific objective function. We have tailored the approach to volume rendering by enforcing a constant memory footprint via analytic inversion of the blending functions. This makes it independent of the number of sampling steps through the volume and facilitates the consideration of small-scale changes. The approach forms the basis for automatic optimizations regarding external parameters of the rendering process and the volumetric density field itself. We demonstrate its use for automatic viewpoint selection using differentiable entropy as objective, and for optimizing a transfer function from rendered images of a given volume. Optimization of per-voxel densities is addressed in two different ways: First, we mimic inverse tomography and optimize a 3D density field from images using an absorption model. This simplification enables comparisons with algebraic reconstruction techniques and state-of-the-art differentiable path tracers. Second, we introduce a novel approach for tomographic reconstruction from images using an emission-absorption model with post-shading via an arbitrary transfer function.

Frequency domain analysis of optoelectronic oscillators utilizing optical and RF resonators with arbitrary transfer functions

Optoelectronic oscillators (OEOs) have attracted much attention for producing ultra-low phase-noise microwave/millimeter-wave oscillations. Traditional delay-based OEOs usually suffer from strong spurious peaks in their phase noise power spectral densities and possible mode-hopping phenomena. Some methods have been proposed in the literature such as using multi-loop architectures or injection locking to other OEOs or radio frequency (RF) oscillators to reduce these spurious peaks. In other approaches, optical filters/resonators other than optical fibers have been proposed to reduce or suppress these peaks and prevent the mode-hopping phenomenon, such as whispering gallery mode resonators (WGMRs), fiber Bragg gratings, and other forms of microwave photonic filters. Usually, approximate single-purpose approaches have been presented to analyze OEOs utilizing such resonators. Here a general framework for analyzing the performance of OEOs implementing RF and optical filters/resonators with arbitrary linear transfer functions is presented. Consequently, it can consider, for example, the most general dispersion models of the fibers as well as any OEO architecture using a combination of different optical resonators. It can also consider the noise transfer between any sidebands of the RF or optical signals and any kind of amplitude noise to phase noise transfers and vice versa. The non-idealities of the electro-optic modulators such as the chirping and finite extinction ratios can also be taken into account. The validity of the new approach is verified by comparing its results with those previously published in the literature. In particular, the case of a WGMR plus delay line OEO is considered for comparisons.

Temporal analog optical computing using an on-chip fully reconfigurable photonic signal processor

This paper introduces the concept of on-chip temporal optical computing, based on dispersive Fourier transform and suitably designed modulation module, to perform mathematical operations of interest, such as differentiation, integration, or convolution in time domain. The desired mathematical operation is performed as signal propagates through a fully reconfigurable on-chip photonic signal processor. Although a few numbers of photonic temporal signal processors have been introduced recently, they are usually bulky or they suffer from limited reconfigurability which is of great importance to implement large-scale general-purpose photonic signal processors. To address these limitations, this paper demonstrates a fully reconfigurable photonic integrated signal processing system. As the key point, the reconfigurability is achieved by taking advantages of dispersive Fourier transformation, linearly chirp modulation using four-wave mixing, and applying the desired arbitrary transfer function through a cascaded Mach-Zehnder modulator and a phase modulator. Numerical simulations of the proposed structure reveal a great potential for chip-scale fully reconfigurable all-optical signal processing through a bandwidth of 400 GHz.

Switched-current filter structure for synthesizing arbitrary characteristics based on follow-the-leader feedback configuration

A switched-current (SI) filter structure with follow-the-leader feedback configuration has been proposed for synthesizing arbitrary transfer functions. The double-sampling fully-balanced SI bilinear integrator and current mirror are employed as the building cells in order to enhance overall performance and make circuit design more flexible. Also, explicit design formulas are derived by using coefficient matching. The third-order low-pass and band-pass filter are designed as the examples. Simulation results confirm that the presented filter structure is very suitable for the operation with high sampling-to-cutoff frequency ratio, and can synthesize arbitrary filter characteristics precisely and easily while keeping low sensitivity.

A Frequency Domain Design Approach for the Generation of Arbitrary Transfer Function Based on Generalized Optical Transversal Filters

We present a technique to realize arbitrary optical transfer functions based on the use of delay lines and optical waveguide couplers. In particular, this technique is a modification of an optical transversal filter. Following this technique, the transfer function is expressed as a truncated Fourier series with the tap coefficients that are either purely real or purely imaginary. The output ports of an optical waveguide coupler, conveniently and precisely, provide a transmission function that is either purely real or purely imaginary and thus, fit perfectly for the experimental realization of such transfer functions. The use of simple components, such as couplers and delay lines, alleviates the need for bulky electro-optic phase shifters and reduces the complexity of the integration process. To exemplify this approach, an optical field differentiator is implemented. Moreover, the effect of the practical aspects such as frequency dependence of the refractive index and variations of the coupling ratios of optical waveguide couplers, on the resulting transfer function are analytically investigated.

Tera-scale astronomical data analysis and visualization

We present a high-performance, graphics processing unit (GPU)-based framework for the efficient analysis and visualization of (nearly) terabyte (TB)-sized 3-dimensional images. Using a cluster of 96 GPUs, we demonstrate for a 0.5 TB image: (1) volume rendering using an arbitrary transfer function at 7--10 frames per second; (2) computation of basic global image statistics such as the mean intensity and standard deviation in 1.7 s; (3) evaluation of the image histogram in 4 s; and (4) evaluation of the global image median intensity in just 45 s. Our measured results correspond to a raw computational throughput approaching one teravoxel per second, and are 10--100 times faster than the best possible performance with traditional single-node, multi-core CPU implementations. A scalability analysis shows the framework will scale well to images sized 1 TB and beyond. Other parallel data analysis algorithms can be added to the framework with relative ease, and accordingly, we present our framework as a possible solution to the image analysis and visualization requirements of next-generation telescopes, including the forthcoming Square Kilometre Array pathfinder radiotelescopes.

SYNTHESIS, DESIGN AND IMPLEMENTATION OF ULTRA-WIDEBAND IMPULSE RADIO ACTIVE MMIC MATCHED FILTERS

In this paper, we present a comprehensive framework from synthesis to implementation of active matched fllters for UWB Impulse Radio. The method delays and sums UWB pulses coherently to strengthen the signal over white Gaussian noise. Theoretical analysis shows that the signal peak is maximized against noise, and an arbitrary transfer function could be realized by adjusting fllter parameters. To verify the concept, a four-stage matched fllter operating in 3{5GHz with 360 degrees phase delay is demonstrated flrst. It is implemented in a commercial 2-m GaAs HBT process and achieves a power gain of 13.8dB with a 10dB bandwidth of 1.3GHz. Based on a similar architecture, another design is presented but with only half of the delay. It has a power gain of 15.9dB and a 10dB bandwidth of 2.3GHz. An advantage of the proposed method is a precise control of the impulse response that can be matched to either symmetrical or asymmetrical UWB pulses by taking a time domain design approach.

Local Ambient Occlusion in Direct Volume Rendering

This paper presents a novel technique to efficiently compute illumination for Direct Volume Rendering using a local approximation of ambient occlusion to integrate the intensity of incident light for each voxel. An advantage with this local approach is that fully shadowed regions are avoided, a desirable feature in many applications of volume rendering such as medical visualization. Additional transfer function interactions are also presented, for instance, to highlight specific structures with luminous tissue effects and create an improved context for semitransparent tissues with a separate absorption control for the illumination settings. Multiresolution volume management and GPU-based computation are used to accelerate the calculations and support large data sets. The scheme yields interactive frame rates with an adaptive sampling approach for incrementally refined illumination under arbitrary transfer function changes. The illumination effects can give a better understanding of the shape and density of tissues and so has the potential to increase the diagnostic value of medical volume rendering. Since the proposed method is gradient-free, it is especially beneficial at the borders of clip planes, where gradients are undefined, and for noisy data sets.

Long-period-fiber-grating-based filter configuration enabling arbitrary linear filtering characteristics

The filtering scheme proposed here is based on transmission through a dual long-period-fiber-grating (LPFG) configuration and enables implementation of arbitrary spectral transfer functions using available inverse-scattering design algorithms, such as those widely used for fiber Bragg gratings (FBGs) operating in reflection. Besides the important technical advantage of operation in transmission, the proposed device can reach large spectral bandwidths that would be extremely challenging to reach by, e.g., FBG devices. The proposed concept is demonstrated by designing and fabricating a LPFG-based filter for synthesis of transform-limited 1.5-ps-long square-like pulses.

A Study on Adaptive Noise Canceller Using Linear Phase Filter

In this paper, we propose a new adaptive noise canceller using a linear phase filter. The linear model in which the arbitrary signal is defined by the output signal of a linear system at the white signal input is used. The noise is suppressed by the estimated linear system and the signal to noise ratio is improved. At this time, to minimize the distortion of the signal due to the nonlinearity of the phase shift, the linear phase filter has been newly introduced. The transfer function of the linear system is an arbitrary minimum phase rational transfer function that has the poles and zeros. It has the feature of not being specified for all pole model. The adaptive algorithm is a gradient based algorithm with few computational complexities. The feature of the proposed adaptive noise canceller are that the inverse filter of the adaptive filter is stable, the convergence of the algorithm is guaranteed, the distortion of the signal is minimum , and there is no restriction to the transfer function of the linear system.

A Note on Canonic Lattice Realization of Arbitrary FIR Transfer Functions

This note presents some simple alternative solutions to the cases requiring modification of the procedure given in [1] for canonic lattice realization of arbitrary transfer functions.

A New Feedback Configuration for Canonical Realization of IIR Filters and its Application to Lattice Realizations

A feedback configuration is presented for canonical realization of an arbitrary IIR transfer function H(z) = P(z) / Q(z) with M = P(z)0 < N = Q(z)0, which does not appear to have been discussed in the literature. It consists of an IIR transfer function F(z) of degree R in the forward path, where M ≤ R < N, and an FIR transfer function G(z) of degree N – M, In which the first R − M + 1 terms are absent in the feedback path. F(z) and G(z) can be realized by any structure including lattice, if applicable. Application of this method is shown to solve the problem of canonical lattice realization of any IIR transfer function H(z) = P(z) / Q(z) in which M < N and the constant term in P(z) is absent.

Analysis of Second-Order Modes of Linear Discrete-Time Systems under Bounded-Real Transformations

This paper discusses the behavior of the second-order modes (Hankel singular values) of linear discrete-time systems under bounded-real transformations, where the transformations are given by arbitrary transfer functions with magnitude bounded by unity. Our main result reveals that the values of the second-order modes are decreased under any of the above-mentioned transformations. This result is the generalization of the theory of Mullis and Roberts, who proved that the second-order modes are invariant under any allpass transformation, i.e. any lossless bounded-real transformation. We derive our main result by describing the controllability/observability Gramians of transformed systems with the help of the discrete-time bounded-real lemma.

Blind Acoustic Beamforming Based on Generalized Eigenvalue Decomposition

Maximizing the output signal-to-noise ratio (SNR) of a sensor array in the presence of spatially colored noise leads to a generalized eigenvalue problem. While this approach has extensively been employed in narrowband (antenna) array beamforming, it is typically not used for broadband (microphone) array beamforming due to the uncontrolled amount of speech distortion introduced by a narrowband SNR criterion. In this paper, we show how the distortion of the desired signal can be controlled by a single-channel post-filter, resulting in a performance comparable to the generalized minimum variance distortionless response beamformer, where arbitrary transfer functions relate the source and the microphones. Results are given both for directional and diffuse noise. A novel gradient ascent adaptation algorithm is presented, and its good convergence properties are experimentally revealed by comparison with alternatives from the literature. A key feature of the proposed beamformer is that it operates blindly, i.e., it neither requires knowledge about the array geometry nor an explicit estimation of the transfer functions from source to sensors or the direction-of-arrival.

Design of Sigma–Delta Modulators With Arbitrary Transfer Functions

This paper addresses the design of sigma-delta modulators with arbitrary signal and noise transfer functions using a genetic algorithm (GA)-based search method. The objective function is defined to include the difference D between the magnitude of the frequency responses of the designed transfer functions and the ideal one, the quantizer gain lambdacritical for which the poles of the modulator start moving out of the unit circle, and the spread of the coefficients S. Stability can be improved by reducing lambdacritical while a smaller S reduces the implementation complexity. A GA searches for poles/zeros of the transfer functions to minimize the objective function D+w1*lambdacritical+w2*S, where w1 and w2 are two weighing factors. Numerical results demonstrate the effectiveness of the proposed method

A New Canonic Lattice Realization of Arbitrary FIR Transfer Functions

A new lattice structure is presented for realizing a pair of arbitrary FIR transfer functions of the same order, which is canonic in multipliers as well as delays. By a simple modification of the procedure, one can also obtain a canonic realization of a single FIR transfer function. The problem cases have been discussed, along with their solutions, and the limitations of the method have been pointed out. Examples have been worked out to illustrate the theory.

Aperiodic artificial substrates for multipole bandpass filters

Though periodic structures have been the focus of intense research for the past decade, in practice, a truly periodic structure is rarely the proper topology to give a desired or specified transfer function. Traditionally, filtering in periodic substrates was shown by creating defects in an otherwise periodic lattice. However, this approach limits the transfer function of the resulting filter. The use of perturbations in the periodicity in order to create multipole filters with arbitrary transfer functions that are not available inside simple periodic substrates is demonstrated. A methodology is presented to design the aperiodic substrates. Measured and simulated four-pole filter responses are shown for both all-dielectric substrates with high-dielectric-constant titania inclusions and metallodielectric substrates with metallic inclusions inside low-temperature cofired ceramics (LTCC). The measured all-dielectric substrate demonstrates a filter at 21.6 GHz with an insertion loss of 1.69 dB and a narrow bandwidth of only 0.89%, indicating quality factors >1000. Filters with specified Chebyshev transfer functions inside LTCC metallodielectric substrates show consistency within <0.5% at 10.89 GHz due to the mature fabrication process.

A New Canonic Lattice Realization of Arbitrary IIR Transfer Functions

A new lattice realization is presented for any arbitrary stable IIR transfer function, which is always canonic in multipliers as well as delays, except for two degenerate cases, which require one (or more) additional delay(s), but no additional multipliers. In contrast to the existing lattice realization of such a transfer function, which uses an all-pass lattice combined with a ladder, the new structure is a truly lattice one. Also, for achieving multiplier canonicity in the conventional structure, one has to use special single multiplier lattice sections; in the new structure, canonicity evolves automatically.

An Improved Array Steering Vector Estimation Method and Its Application in Speech Enhancement

We propose a robust microphone array for speech enhancement and noise suppression. To overcome target signal cancellation problem of conventional beamformes caused by array imperfections or reverberation effects, the proposed method adopts arbitrary transfer function relating each microphone and target speech signal as array channel model. This is achieved in two ways. First, we propose a method to estimate the array steering vector (ASV) by means of exploiting the nonstationarity of speech signal to combat stationary noise and interference. Next, with the estimated ASV, a robust matched-filter-(MF-) array-based generalized sidelobe canceller (MF-GSC) is constructed to enhance the speech signal and suppress noise/interference. In addition, it also has the capability to reduce the reverberation effects of the acoustic enclosure. Numerical results show that the proposed method demonstrates high performance even in adverse environments.