592 research outputs found
Advanced Quantization Schemes to Increase Accuracy, Reduce Area, and Lower Power Consumption in FFT Architectures.
This paper explores new advanced quantization schemes for fast Fourier transform (FFT) architectures. In previous works, FFT quantization has been treated theoretically or with the sole aim of improving accuracy. In this work, we go one step beyond by considering also the implications that quantization schemes have on the area and power consumption of the architecture. To achieve this, we have analyzed the mathematical operations carried out in FFT architectures and explored the changes that benefit all the figures of merit. By combining or alternating truncation and rounding, and using the half-unit biased (HUB) representation in the different computations of the architecture, we have achieved quantization schemes that increase accuracy, reduce area, and lower power consumption simultaneously. This win-win result improves multiple figures of merit without worsening any other, making it a valuable strategy to optimize FFT architectures.MCIN/AEI/10.13039/501100011033 and âERDF A Way of Making Europeâ under Project PID2021-126991NA-I00, European Union NextGeneration EU/PRTR under Project TED2021-131527B-I00, Fondo Europeo de Desarrollo Regional under Grant UMA20-FEDERJA-059, and MCIN/AEI/10.13039/501100011033 and âESF Investing in Your Futureâ under Grant RYC2018-025384-
Polarization fields: dynamic light field display using multi-layer LCDs
We introduce polarization field displays as an optically-efficient design for dynamic light field display using multi-layered LCDs. Such displays consist of a stacked set of liquid crystal panels with a single pair of crossed linear polarizers. Each layer is modeled as a spatially-controllable polarization rotator, as opposed to a conventional spatial light modulator that directly attenuates light. Color display is achieved using field sequential color illumination with monochromatic LCDs, mitigating severe attenuation and moiré occurring with layered color filter arrays. We demonstrate such displays can be controlled, at interactive refresh rates, by adopting the SART algorithm to tomographically solve for the optimal spatially-varying polarization state rotations applied by each layer. We validate our design by constructing a prototype using modified off-the-shelf panels. We demonstrate interactive display using a GPU-based SART implementation supporting both polarization-based and attenuation-based architectures. Experiments characterize the accuracy of our image formation model, verifying polarization field displays achieve increased brightness, higher resolution, and extended depth of field, as compared to existing automultiscopic display methods for dual-layer and multi-layer LCDs.National Science Foundation (U.S.) (Grant IIS-1116452)United States. Defense Advanced Research Projects Agency (Grant HR0011-10-C-0073)Alfred P. Sloan Foundation (Research Fellowship)United States. Defense Advanced Research Projects Agency (Young Faculty Award
Fundamental Parameters of Massive Stars
We discuss the determination of fundamental parameters of `normal' hot,
massive OB-type stars, namely temperatures, luminosities, masses, gravities and
surface abundances. We also present methods used to derive properties of
stellar winds -- mass-loss rates and wind velocities from early-type stars.Comment: 21 pages, 3 figures, to appear in "Massive Stars: Formation,
Evolution and Environment", eds. Heydari-Malayeri & Zahn (proceedings of 2002
Aussois summer school
An investigation to determine the producibility of a 3-D braider and bias direction weaving loom
The development of prototype machines for the production of generalized braid patterns is described. Mechanical operating principles and control strategies are presented for two prototype machines which were fabricated and evaluated. Both machines represent advances over current techniques for forming composite material preforms by enabling near ideal control of fiber orientation. Furthermore, they overcome both the lack of general control of produced fiber architectures and the complexity of other weaving processes that were produced for the same purpose. One prototype, the modified Farley braider, consists of an array of turntables which can be rotated 90 degrees and returned; hence, they can form tracks in the x and y axis. Yarn ends are transported about the surface formed by the turntables using motorized tractors. These tractors are controlled using an optical link with a control circuit and host computer. The tractors are powered through electrical contact with the turntables. The necessary relative motions are produced by a series of linear tractor moves combined with a sequence of turntable rotations. The movement of the tractors about the surface causes the yarns to produce the desired braiding pattern. The second device, the shuttle plate braider, consists of a braiding surface formed by an array of square elements, each separated from its neighbor by a gap. Beneath this surface lies a shuttle plate, which reciprocates first in one axis and then in the other. As this movement takes place, yarn carrying shuttles engage and disengage the plate by means of solenoid activated pins. By selective engagement and disengagement, the shuttles can move the yarn ends in any desired pattern, forming the desired braid. Control power, and control signals, are transmitted from the electronic interface circuit and host computer, via the braiding surface through electrical contact with the shuttles. Motive power is proved to the shuttles by motion of the shuttle plate, which is passively driven using pneumatic rams. Each shuttle is a simple device that uses only a solenoid to engage the plate and is a simple device that uses only a solenoid to engage the plate and is independently controllable. When compared with each other, the modified Farley braider has the advantage of speed, and the shuttle plate braider the advantage of mechanical control and simplicity
Magnetic and Gravitational Disk-Star Interactions: An Interdependence of PMS Stellar Rotation Rates and Spin-Orbit Misalignments
The presence of giant gaseous planets that reside in close proximity to their
host stars may be a consequence of large-scale radial migration through the
proto-planetary nebulae. Within the context of this picture, significant
orbital obliquities characteristic of a substantial fraction of such planets
can be attributed to external torques that perturb the disks out of alignment
with the spin axes of their host stars. Therefore, the acquisition of orbital
obliquity exhibits sensitive dependence on the physics of disk-star
interactions. Here, we analyze the primordial excitation of spin-orbit
misalignment of Sun-like stars, in light of disk-star angular momentum
transfer. We begin by calculating the stellar pre-main sequence rotational
evolution, accounting for spin-up due to gravitational contraction and
accretion as well as spin-down due to magnetic star-disk coupling. We devote
particular attention to angular momentum transfer by accretion, and show that
while generally subdominant to gravitational contraction, this process is
largely controlled by the morphology of the stellar magnetic field (i.e.
specific angular momentum accreted by stars with octupole-dominated surface
fields is smaller than that accreted by dipole-dominated stars by an order of
magnitude). Subsequently, we examine the secular spin-axis dynamics of
disk-bearing stars, accounting for the time-evolution of stellar and disk
properties and demonstrate that misalignments are preferentially excited in
systems where stellar rotation is not overwhelmingly rapid. Moreover, we show
that the excitation of spin-orbit misalignment occurs impulsively, through an
encounter with a resonance between the stellar precession frequency and the
disk-torquing frequency. Cumulatively, the model developed herein opens up a
previously unexplored avenue towards understanding star-disk evolution and its
consequences in a unified manner.Comment: 18 pages, 7 figures, accepted to Ap
Magnetic fields from low mass stars to brown dwarfs
Magnetic fields have been detected on stars across the H-R diagram and
substellar objects either directly by their effect on the formation of spectral
lines, or through the activity phenomena they power which can be observed
across a large part of the electromagnetic spectrum. Stars show a very wide
variety of magnetic properties in terms of strength, geometry or variability.
Cool stars generate their magnetic fields by dynamo effect, and their
properties appear to correlate - to some extent - with stellar parameters such
as mass, rotation and age. With the improvements of instrumentation and data
analysis techniques, magnetic fields can now be detected and studied down to
the domain of very-low-mass stars and brown dwarfs, triggering new theoretical
works aimed, in particular, at modelling dynamo action in these objects. After
a brief discussion on the importance of magnetic field in stellar physics, the
basics of dynamo theory and magnetic field measurements are presented. The main
results stemming from observational and theoretical studies of magnetism are
then detailed in two parts: the fully-convective transition, and the very-low
mass stars and brown dwarfs domain.Comment: 30 pages, 9 figures. Notes for lectures presented at the Evry
Schatzman school on "Low-mass stars and the transition from stars to brown
dwarfs", September 2011, Roscoff, France. To appear in the EAS Conference
Series, edited by C. Charbonnel, C. Reyle, M. Schulthei
Combined Effects of Rotation and Age Spreads on Extended Main-Sequence Turn Offs
The extended main-sequence turn offs (eMSTOs) of several young to intermediate age clusters are examined in the Magellanic Clouds and the Milky Way. We explore the effects of extended star formation (eSF) and a range of stellar rotation rates on the behavior of the colorâmagnitude diagram, paying particular attention to the MSTO. We create synthetic stellar populations based on MESA stellar models to simulate observed Hubble Space Telescope and Gaia star cluster data. We model the effect of rotation as a nonparametric distribution, allowing for maximum flexibility. In our models the slow rotators comprise the blueward, and fast rotators the redward portion of the eMSTO. We simulate data under three scenarios: nonrotating eSF, a range of rotation rates with a single age, and a combination of age and rotation effects. We find that two of the five clusters (the youngest and oldest) favor an age spread, but these also achieve the overall worst fits of all clusters. The other three clusters show comparable statistical evidence between rotation and an age spread. In all five cases, a rotation-rate distribution alone is capable of qualitatively matching the observed eMSTO structure. In future work, we aim to compare our predicted with observations in order to better constrain the physics related to stellar rotation
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