Browsing by Author "Martins, R."
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- Adaptive deblocking filter for transform domain Wyner-Ziv video codingPublication . Martins, R.; Brites, C.; Ascenso, Joao; Pereira, F.Wyner-Ziv (WZ) video coding is a particular case of distributed video coding, the recent video coding paradigm based on the Slepian-Wolf and Wyner-Ziv theorems that exploits the source correlation at the decoder and not at the encoder as in predictive video coding. Although many improvements have been done over the last years, the performance of the state-of-the-art WZ video codecs still did not reach the performance of state-of-the-art predictive video codecs, especially for high and complex motion video content. This is also true in terms of subjective image quality mainly because of a considerable amount of blocking artefacts present in the decoded WZ video frames. This paper proposes an adaptive deblocking filter to improve both the subjective and objective qualities of the WZ frames in a transform domain WZ video codec. The proposed filter is an adaptation of the advanced deblocking filter defined in the H.264/AVC (advanced video coding) standard to a WZ video codec. The results obtained confirm the subjective quality improvement and objective quality gains that can go up to 0.63 dB in the overall for sequences with high motion content when large group of pictures are used.
- Angular distributions in t(t)over-barH(H -> b(b)over-bar) reconstructed events at the LHCPublication . Santos, S. P. Amor dos; Araque, J. P.; Cantrill, R.; Castro, N. F.; Fiolhais, M. C. N.; Frederix, R.; Gonçalo, R.; Martins, R.; Santos, R.; Silva, J.; Onofre, A.; Peixoto, H.; Reigoto, A.The associated production of a Higgs boson and a top-quark pair, t (t) over barH, in proton-proton collisions is addressed in this paper for a center of mass energy of 13 TeV at the LHC. Dileptonic final states of t (t) over barH events with two oppositely charged leptons and four jets from the decays t -> bW(+) -> bl(+)v(l), (t) over bar -> (b) over barW(-) -> (b) over barl(-)(v) over bar (l) and h -> b (b) over bar are used. Signal events, generated with MadGraph5_aMC@NLO, are fully reconstructed by applying a kinematic fit. New angular distributions of the decay products as well as angular asymmetries are explored in order to improve discrimination of t (t) over barH signal events over the dominant irreducible background contribution, t (t) over barb (b) over bar. Even after the full kinematic fit reconstruction of the events, the proposed angular distributions and asymmetries are still quite different in the t (t) over barH signal and the dominant background (t (t) over barb (b) over bar).
- Large area image sensing structures based on a-SiC : H: a dynamic characterizationPublication . Fernandes, Miguel; Vieira, Manuela; Rodrigues, Isabel Maria Cabrita; Martins, R.In recent works large area hydrogenated amorphous silicon p-i-n structures with low conductivity doped layers were proposed as single element image sensors. The working principle of this type of sensor is based on the modulation, by the local illumination conditions, of the photocurrent generated by a light beam scanning the active area of the device. In order to evaluate the sensor capabilities is necessary to perform a response time characterization. This work focuses on the transient response of such sensor and on the influence of the carbon contents of the doped layers. In order to evaluate the response time a set of devices with different percentage of carbon incorporation in the doped layers is analyzed by measuring the scanner-induced photocurrent under different bias conditions.
- Novel structure for large area image sensingPublication . Fernandes, Miguel; Vieira, Manuela; Martins, R.This work presents preliminary results in the study of a novel structure for a laser scanned photodiode (LSP) type of image sensor. In order to increase the signal output, a stacked p-i-n-p-i-n structure with an intermediate light-blocking layer is used. The image and the scanning beam are incident through opposite sides of the sensor and their absorption is kept in separate junctions by an intermediate light-blocking layer. As in the usual LSP structure the scanning beam-induced photocurrent is dependent on the local illumination conditions of the image. The main difference between the two structures arises from the fact that in this new structure the image and the scanner have different optical paths leading to an increase in the photocurrent when the scanning beam is incident on a region illuminated on the image side of the sensor, while a decreasing in the photocurrent was observed in the single junction LSP. The results show that the structure can be successfully used as an image sensor even though some optimization is needed to enhance the performance of the device.
- Particle agglomeration study in rf silane plasmas: In situ study by polarization-sensitive laser light scatteringPublication . Courteille, C.; Hollenstein, C.; Dorier, J. L.; Gay, P.; Schwarzenbach, W.; Howling, A. A.; Bertran, E.; Viera, G.; Martins, R.; Maçarico, António Filipe Ruas da TrindadeTo determine self-consistently the time evolution of particle size and their number density in situ multi-angle polarization-sensitive laser light scattering was used. Cross-polarization intensities (incident and scattered light intensities with opposite polarization) measured at 135 degrees and ex situ transmission electronic microscopy analysis demonstrate the existence of nonspherical agglomerates during the early phase of agglomeration. Later in the particle time development both techniques reveal spherical particles again. The presence of strong cross-polarization intensities is accompanied by low-frequency instabilities detected on the scattered light intensities and plasma emission. It is found that the particle radius and particle number density during the agglomeration phase can be well described by the Brownian free molecule coagulation model. Application of this neutral particle coagulation model is justified by calculation of the particle charge whereby it is shown that particles of a few tens of nanometer can be considered as neutral under our experimental conditions. The measured particle dispersion can be well described by a Brownian free molecule coagulation model including a log-normal particle size distribution. (C) 1996 American Institute of Physics.
- Probing the CP nature of the Higgs coupling in tth events at the LHCPublication . dos Santos, S. Amor; Fiolhais, M. C. N.; Frederix, R.; Goncalo, R.; Gouveia, E.; Martins, R.; Onofre, A.; Pease, C. M.; Peixoto, H.; Reigoto, A.; dos Santos, Rui Alberto; Silva, J.The determination of the CP nature of the Higgs coupling to top quarks is addressed in this paper, using tt _ h events produced in pffisffiffi 13 TeV proton- proton collisions at the LHC. Dileptonic final states are employed, with two oppositely charged leptons and four jets, corresponding to the decays t. bW_. bl_.l, _ t. _ bW-. _ bl- _.l, and h. b _ b. Pure scalar ( h H), pure pseudoscalar ( h A), and CP- violating Higgs boson signal events, generated with MADGRAPH5_ AMC@ NLO, are fully reconstructed through a kinematic fit. We furthermore generate samples that have both a CP- even and a CP- odd component in the t _ th coupling in order to probe the ratio of the two components. New angular distributions of the decay products, as well as CP angular asymmetries, are explored in order to separate the scalar from the pseudoscalar components of the Higgs boson and reduce the contribution from the dominant irreducible background, t _ tb _ b. Significant differences between the angular distributions and asymmetries are observed, even after the full kinematic fit reconstruction of the events, allowing to define the best observables for a global fit of the Higgs couplings parameters.
- Statistical motion learning for improved transform domain Wyner-Ziv video codingPublication . Martins, R.; Brites, C.; Ascenso, João; Pereira, F.Wyner - Ziv (WZ) video coding is a particular case of distributed video coding (DVC), the recent video coding paradigm based on the Slepian - Wolf and Wyner - Ziv theorems which exploits the source temporal correlation at the decoder and not at the encoder as in predictive video coding. Although some progress has been made in the last years, WZ video coding is still far from the compression performance of predictive video coding, especially for high and complex motion contents. The WZ video codec adopted in this study is based on a transform domain WZ video coding architecture with feedback channel-driven rate control, whose modules have been improved with some recent coding tools. This study proposes a novel motion learning approach to successively improve the rate-distortion (RD) performance of the WZ video codec as the decoding proceeds, making use of the already decoded transform bands to improve the decoding process for the remaining transform bands. The results obtained reveal gains up to 2.3 dB in the RD curves against the performance for the same codec without the proposed motion learning approach for high motion sequences and long group of pictures (GOP) sizes.
- Study of Annealed Indium Tin Oxide-Films prepared by RF Reactive Magnetron SputteringPublication . Meng, L. J.; Maçarico, António Filipe Ruas da Trindade; Martins, R.Tin doped indium oxide (ITO) films were deposited on glass substrates by rf reactive magnetron sputtering using a metallic alloy target (In-Sn, 90-10). The post-deposition annealing has been done for ITO films in air and the effect of annealing temperature on the electrical, optical and structural properties of ITO films was studied. It has been found that the increase of the annealing temperature will improve the film electrical properties. The resistivity of as deposited film is about 1.3 x 10(-1) Omega*cm and decreases down to 6.9 x 10(-3) Omega*cm as the annealing temperature is increased up to 500 degrees C. In addition, the annealing will also increase the film surface roughness which can improve the efficiency of amorphous silicon solar cells by increasing the amount of light trapping.