DTSC - GPM - Artículos de Revistas
http://hdl.handle.net/10016/2315
2015-10-10T05:20:59ZTwo-level sliding-window VBR control algorithm for video on demand streaming
http://hdl.handle.net/10016/21554
Two-level sliding-window VBR control algorithm for video on demand streaming
Frutos-López, Manuel de; González de Suso, José Luis; Sanz-Rodríguez, Sergio; Peláez-Moreno, Carmen; Díaz-de-María, Fernando
A two-level variable bit rate (VBR) control algorithm for hierarchical video coding, specifically tailored for the new High Efficiency Video Coding (HEVC) standard, is presented here. A long-term level monitors the current bit count along a sliding window of a few seconds, comprising several intra-periods (IPs) and shifted on an IP basis. This long-term view allows the accommodation of the naturally occurring rate variations at a slow pace, avoiding the annoying sharp quality changes commonly appearing when non-sliding window approaches are used. The bit excesses or defects observed at this level are evenly delivered to a short-term level mechanism that establishes target bit budgets for a narrower sliding window covering a single IP and shifting on a frame basis. At this level, an adequate quantization parameter is estimated to comply with the designated target bit rate. Recommended test conditions as well as two few minutes long video sequences with scene cuts have been used for the assessment of the proposed VBR controller. Comparisons with a state-of-the-art rate control algorithm have produced good results in terms of quality consistency, in exchange for moderate rate-distortion performance losses.
2015-08-01T00:00:00ZEffect of an external electric field on the propagation velocity of premixed flames
http://hdl.handle.net/10016/21538
Effect of an external electric field on the propagation velocity of premixed flames
Sánchez-Sanz, Mario; Murphy, Daniel C.; Fernández Pello, Carlos
There have been many experimental investigations into the ability of
electric fields to enhance combustion by acting upon ion species present in flames. In
this work, we examine this phenomenon using a one-dimensional model of a lean
premixed flame under the influence of a longitudinal electric field. We expand upon
prior two-step chain-branching reaction laminar models with reactions to model the
creation and consumption of both a positively-charged radical species and free
electrons. Also included are the electromotive force in the conservation equation for ion
species and the electrostatic form of the Maxwell equations in order to resolve ion
transport by externally applied and internally induced electric fields. The numerical
solution of these equations allows us to compute changes in flame speed due to electric
fields. Further, the variation of key kinetic and transport parameters modifies the
electrical sensitivity of the flame. From changes in flame speed and reactant profiles we
are able to gain novel, valuable insight into how and why combustion can be controlled
by electric fields.
2014-06-25T00:00:00ZThe spectra of irreducible matrices over completed idempotent semifields
http://hdl.handle.net/10016/21549
The spectra of irreducible matrices over completed idempotent semifields
Valverde-Albacete, Francisco J.; Peláez-Moreno, Carmen
Motivated by some spectral results in the characterization of concept lattices we investigate the spectra of reducible matrices over complete idempotent semifields in the framework of naturally-ordered semirings, or dioids. We find non-none eigenvectors for every non-none element in the semifield and conclude that the notion of spectrum has to be refined to encompass that of the incomplete semifield case so as to include only those eigenvalues with eigenvectors that have finite coordinates. Considering special sets of eigenvectors brings out finite complete lattices in the picture and we contend that such structure may be more important than standard eigenspaces for matrices over completed idempotent semifields.
2015-07-01T00:00:00Z100% classification accuracy considered harmful: The normalized information transfer factor explains the accuracy paradox
http://hdl.handle.net/10016/21473
100% classification accuracy considered harmful: The normalized information transfer factor explains the accuracy paradox
Valverde-Albacete, Francisco J.; Peláez-Moreno, Carmen
The most widely spread measure of performance, accuracy, suffers from a paradox: predictive models with a given level of accuracy may have greater predictive power than models with higher accuracy. Despite optimizing classification error rate, high accuracy models may fail to capture crucial information transfer in the classification task. We present evidence of this behavior by means of a combinatorial analysis where every possible contingency matrix of 2, 3 and 4 classes classifiers are depicted on the entropy triangle, a more reliable information-theoretic tool for classification assessment. Motivated by this, we develop from first principles a measure of classification performance that takes into consideration the information learned by classifiers. We are then able to obtain the entropy-modulated accuracy (EMA), a pessimistic estimate of the expected accuracy with the influence of the input distribution factored out, and the normalized information transfer factor (NIT), a measure of how efficient is the transmission of information from the input to the output set of classes. The EMA is a more natural measure of classification performance than accuracy when the heuristic to maximize is the transfer of information through the classifier instead of classification error count. The NIT factor measures the effectiveness of the learning process in classifiers and also makes it harder for them to "cheat" using techniques like specialization, while also promoting the interpretability of results. Their use is demonstrated in a mind reading task competition that aims at decoding the identity of a video stimulus based on magnetoencephalography recordings. We show how the EMA and the NIT factor reject rankings based in accuracy, choosing more meaningful and interpretable classifiers.
2014-01-01T00:00:00Z