269 research outputs found
Parametric high resolution techniques for radio astronomical imaging
The increased sensitivity of future radio telescopes will result in
requirements for higher dynamic range within the image as well as better
resolution and immunity to interference. In this paper we propose a new matrix
formulation of the imaging equation in the cases of non co-planar arrays and
polarimetric measurements. Then we improve our parametric imaging techniques in
terms of resolution and estimation accuracy. This is done by enhancing both the
MVDR parametric imaging, introducing alternative dirty images and by
introducing better power estimates based on least squares, with positive
semi-definite constraints. We also discuss the use of robust Capon beamforming
and semi-definite programming for solving the self-calibration problem.
Additionally we provide statistical analysis of the bias of the MVDR beamformer
for the case of moving array, which serves as a first step in analyzing
iterative approaches such as CLEAN and the techniques proposed in this paper.
Finally we demonstrate a full deconvolution process based on the parametric
imaging techniques and show its improved resolution and sensitivity compared to
the CLEAN method.Comment: To appear in IEEE Journal of Selected Topics in Signal Processing,
Special issue on Signal Processing for Astronomy and space research. 30 page
VLSI Circuits for adaptive digital beamforming in ultrasound imaging
Cataloged from PDF version of article.For phased-array ultrasound imaging, alternative
beamforming techniques and their VLSI circuits are studied
to form a fully digital receive frontad hardware. In order
to increase the timiig accuracy in beamforming, a computationally
efficient interpolation scheme to increase the sampling
rate is examined. For adaptive beamforming, a phase aberration
correction method with very low computational complexity is
described. Image quality performance of the method is examined
by processing the non-aberrated and aberrated phased-array
experimental data sets of an ultrasound resolution phantom. A
digital beamforming scheme based on receive focusing at the
raster focal points is examined. The sector images of the resolution
phantom, reconstructed from the phased-array experimental
data by beamforming at the radial and raster focal points, are
presented for comparison of the image resolution performances
of the two beamforming schemes. VLSI circuits and their implementations
for the proposed techniques are presented
Implementation of a flexible frequency-invariant broadband beamformer based on fourier properties
Aperture and operating frequency of a beamformer are generally proportional to its resolution, and inversely proportional to its beamwidth. This paper addresses the design and implementation of a beamformer with a frequency-dependent limitation of its aperture such that the frequency-dependence of its resolution is eliminated. Operating across a number of octaves, firstly an octave-invariance design is achieved by means of a nested array structure. Secondly, within each octave, a frequency-dependent aperture control then removes the remaining frequency-dependency. By exploiting Fourier properties and correspondences between coefficient and beamspace, we show that this design is exact, and can accommodate the inclusion of arbitrary shading and different look directions
High-level synthesis design of scalable ultrafast ultrasound beamformer with single FPGA
Ultrafast ultrasound imaging is essential for advanced ultrasound imaging
techniques such as ultrasound localization microscopy (ULM) and functional
ultrasound (fUS). Current ultrafast ultrasound imaging is challenged by the
ultrahigh data bandwidth associated with the radio frequency (RF) signal, and
by the latency of the computationally expensive beamforming process. As such,
continuous ultrafast data acquisition and beamforming remain elusive with
existing software beamformers based on CPUs or GPUs. To address these
challenges, the proposed work introduces a novel method of implementing an
ultrafast ultrasound beamformer specifically for ultrafast plane wave imaging
(PWI) on a field programmable gate array (FPGA) by using high-level synthesis.
A parallelized implementation of the beamformer on a single FPGA was proposed
by 1) utilizing a delay compression technique to reduce the delay profile size,
which enables both run-time pre-calculated delay profile loading from external
memory and delay reuse 2) vectorizing channel data fetching which is enabled by
delay reuse, and 3) using fixed summing networks to reduce consumption of logic
resources. Our proposed method presents two unique advantages over current FPGA
beamformers: 1) high scalability that allows fast adaptation to different FPGA
resources and beamforming speed demands by using Xilinx High-Level Synthesis as
the development tool, and 2) allow a compact form factor design by using a
single FPGA to complete the beamforming instead of multiple FPGAs. With the
proposed method, a sustainable average beamforming rate of 4.83 G
samples/second in terms of input raw RF sample was achieved. The resulting
image quality of the proposed beamformer was compared with the software
beamformer on the Verasonics Vantage system for both phantom imaging and in
vivo imaging of a mouse brain
Real-time Microphone Array Processing for Sound-field Analysis and Perceptually Motivated Reproduction
This thesis details real-time implementations of sound-field analysis and perceptually motivated reproduction methods for visualisation and auralisation purposes. For the former, various methods for visualising the relative distribution of sound energy from one point in space are investigated and contrasted; including a novel reformulation of the cross-pattern coherence (CroPaC) algorithm, which integrates a new side-lobe suppression technique. Whereas for auralisation applications, listening tests were conducted to compare ambisonics reproduction with a novel headphone formulation of the directional audio coding (DirAC) method. The results indicate that the side-lobe suppressed CroPaC method offers greater spatial selectivity in reverberant conditions compared with other popular approaches, and that the new DirAC formulation yields higher perceived spatial accuracy when compared to the ambisonics method
A Microphone Array System for Multimedia Applications with Near-Field Signal Targets
A microphone array beamforming system is proposed for multimedia communication applications using four sets of small planar arrays mounted on a computer monitor. A new virtual array approach is employed such that the original signals received by the array elements are weighted and delayed to synthesize a large, nonuniformly spaced, harmonically nested virtual array covering the frequency band [50, 7000] Hz of the wideband telephony. Subband multirate processing and near-field beamforming techniques are then used jointly by the nested virtual array to improve the performances in reverberant environments. A new beamforming algorithm is also proposed using a broadband near-field spherically isotropic noise model for array optimization. The near-field noise model assumes a large number of broadband random noises uniformly distributed over a sphere with a finite radius in contrast to the conventional far-field isotropic noise model which has an infinite radius. The radius of the noise model, thus, adds a design parameter in addition to its power for tradeoffs between performance and robustness. It is shown that the near-field beamformers designed by the new algorithm can achieve more than 8-dB reverberation suppression while maintaining sufficient robustness against background noises and signal location errors. Computer simulations and real room experiments also show that the proposed array beamforming system reduces beampattern variations for broadband signals, obtains strong noise and reverberation suppression, and improves the sound quality for near-field targets
A Nested Sensor Array Focusing on Near Field Targets
A nested virtual array subband beamforming system is proposed for applications where broadband signal targets are located within the near field of the array. Subband multirate processing and near field beamforming techniques are used jointly for the nested array to improve the performances and reduce the computational complexity. A new noise model, namely the broadband near field spherically isotropic noise model, is also proposed for the optimization design of near field beamformers. It is shown that near field beamforming is essential for better distance discrimination of near field targets, reduced beampattern variations for broadband signals, and stronger reverberation suppression
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