491 research outputs found
Optimization of Non Binary Parity Check Coefficients
This paper generalizes the method proposed by Poulliat et al. for the
determination of the optimal Galois Field coefficients of a Non-Binary LDPC
parity check constraint based on the binary image of the code. Optimal, or
almost-optimal, parity check coefficients are given for check degree varying
from 4 to 20 and Galois Field varying from GF(64) up to GF(1024). For all given
sets of coefficients, no codeword of Hamming weight two exists. A reduced
complexity algorithm to compute the binary Hamming weight 3 of a parity check
is proposed. When the number of sets of coefficients is too high for an
exhaustive search and evaluation, a local greedy search is performed. Explicit
tables of coefficients are given. The proposed sets of coefficients can
effectively replace the random selection of coefficients often used in NB-LDPC
construction.Comment: First version submitted to IEEE Transactions on Information Theory,
August the 5, 2017. Revised version, May the 5, 201
Global and local synthetic descriptions of the piano soundboard
Up to around 1.1 kHz, the soundboard of the piano behaves like a homogeneous
plate whereas upper in frequency, it can be described as a set of waveguides
defined by the ribs. In consequence: a) The acoustical coincidence phenomenon
is deeply modified in comparison with that occurring in homogeneous plates
since the dispersion curve of a waveguide can present none, one, or two
coincidence frequencies. This may result in a nonuniformity of the soundboard
radiation in the treble range, corresponding to the so-called killer octave,
where a good sustain is difficult to obtain. b) The mobility (mechanical
admittance) in the direction normal to the soundboard can be synthesised with
only a small number of parameters. It compares well with published measurements
(Giordano, JASA, 1998), in particular the step-like falloff of the local
impedance due to the localisation of the waves between ribs. c) The synthesised
mobility has the same features as those which can be derived independantly,
according to Skudrzyk (JASA, 1980) and Langley (JSV, 1994). This approach
avoids the detailed description of the soundboard, based on a very large number
of parameters. It can be used to predict global changes of the driving point
mobility, and possibly of the sound radiation in the treble range, resulting
from structural modifications
Vibroacoustics of the piano soundboard: Reduced models, mobility synthesis, and acoustical radiation regime
In string musical instruments, the sound is radiated by the soundboard,
subject to the strings excitation. This vibration of this rather complex
structure is described here with models which need only a small number of
parameters. Predictions of the models are compared with results of experiments
that have been presented in Ege et al. [Vibroacoustics of the piano soundboard:
(Non)linearity and modal properties in the low- and mid- frequency ranges,
Journal of Sound and Vibration 332 (5) (2013) 1288-1305]. The apparent modal
density of the soundboard of an upright piano in playing condition, as seen
from various points of the structure, exhibits two well-separated regimes,
below and above a frequency flim that is determined by the wood characteristics
and by the distance between ribs. Above flim, most modes appear to be
localised, presumably due to the irregularity of the spacing and height of the
ribs. The low-frequency regime is predicted by a model which consists of
coupled sub-structures: the two ribbed areas split by the main bridge and, in
most cases, one or two so-called cut-off corners. In order to assess the
dynamical properties of each of the subplates (considered here as homogeneous
plates), we propose a derivation of the (low-frequency) modal density of an
orthotropic homogeneous plate which accounts for the boundary conditions on an
arbitrary geometry. Above flim, the soundboard, as seen from a given excitation
point, is modelled as a set of three structural wave-guides, namely the three
inter-rib spacings surrounding the excitation point. Based on these low- and
high-frequency models, computations of the point-mobility and of the apparent
modal densities seen at several excitation points match published measurements.
The dispersion curve of the wave-guide model displays an acoustical radiation
scheme which differs significantly from that of a thin homogeneous plate. It
appears that piano dimensioning is such that the subsonic regime of acoustical
radiation extends over a much wider frequency range than it would be for a
homogeneous plate with the same low-frequency vibration. One problem in piano
manufacturing is examined in relationship with the possible radiation schemes
induced by the models.Comment: Research highlights: - Synthetic modelling of a piano soundboard
overa broad-frequency-range (several kHz). - Quantitative agreement between
predicted and measured apparent local modal density. -Modal density of a
plate with non-special orthotropy and arbitrary contour. -Similar
characteristic impedance between comparable pianos, over several kHz.
-Extension of the subsonic regime of acoustical radiation over a wide
frequency range. Journal of Sound and Vibration (2013)
http://dx.doi.org/10.1016/j.jsv.2013.03.01
Estimations of non-linearities in structural vibrations of string musical instruments
Under the excitation of strings, the wooden structure of string instruments
is generally assumed to undergo linear vibrations. As an alternative to the
direct measurement of the distortion rate at several vibration levels and
frequencies, we characterise weak non-linearities by a signal-model approach
based on cascade of Hammerstein models. In this approach, in a chain of two
non-linear systems, two measurements are sufficient to estimate the non-linear
contribution of the second (sub-)system which cannot be directly linearly
driven, as a function of the exciting frequency. The experiment consists in
exciting the instrument acoustically. The linear and non-linear contributions
to the response of (a) the loudspeaker coupled to the room, (b) the instrument
can be separated. Some methodological issues will be discussed. Findings
pertaining to several instruments - one piano, two guitars, one violin - will
be presented.Comment: 11th Congr\`es Fran\c{c}ais d'Acoustique, Nantes : France (2012
Noisy Gradient Descent Bit-Flip Decoding for LDPC Codes
A modified Gradient Descent Bit Flipping (GDBF) algorithm is proposed for
decoding Low Density Parity Check (LDPC) codes on the binary-input additive
white Gaussian noise channel. The new algorithm, called Noisy GDBF (NGDBF),
introduces a random perturbation into each symbol metric at each iteration. The
noise perturbation allows the algorithm to escape from undesirable local
maxima, resulting in improved performance. A combination of heuristic
improvements to the algorithm are proposed and evaluated. When the proposed
heuristics are applied, NGDBF performs better than any previously reported GDBF
variant, and comes within 0.5 dB of the belief propagation algorithm for
several tested codes. Unlike other previous GDBF algorithms that provide an
escape from local maxima, the proposed algorithm uses only local, fully
parallelizable operations and does not require computing a global objective
function or a sort over symbol metrics, making it highly efficient in
comparison. The proposed NGDBF algorithm requires channel state information
which must be obtained from a signal to noise ratio (SNR) estimator.
Architectural details are presented for implementing the NGDBF algorithm.
Complexity analysis and optimizations are also discussed.Comment: 16 pages, 22 figures, 2 table
Performances of a GNSS receiver for space-based applications
Space Vehicle (SV) life span depends on its station keeping capability. Station keeping is the ability of the vehicle to maintain position and orientation. Due to external perturbations, the trajectory of the SV derives from the ideal orbit. Actual positioning systems for satellites are mainly based on ground equipment, which means heavy infrastructures. Autonomous positioning and navigation systems using Global Navigation Satellite Systems (GNSS) can then represent a great reduction in platform design and operating costs. Studies have been carried out and the first operational systems, based on GPS receivers, become available. But better availability of service could be obtained considering a receiver able to process GPS and Galileo signals. Indeed Galileo system will be compatible with the current and the modernized GPS system in terms of signals representation and navigation data. The greater
availability obtained with such a receiver would allow
significant increase of the number of point solutions and
performance enhancement. For a mid-term perspective Thales Alenia Space finances a PhD to develop the concept of a reconfigurable receiver able to deal with both the GPS system and the future Galileo system. In this context, the aim of this paper is to assess the performances of a receiver designed for Geosynchronous Earth Orbit (GEO) applications. It is shown that high improvements are obtained with a receiver designed to track both GPS and Galileo satellites. The performance assessments have been used to define the specifications of the future satellite GNSS receiver
Russian engagement with the Bologna Process: policies and practices in higher education reform
The Bologna process describes a collaboration of countries from Europe to Central Asia, where state actors and institutions work towards system convergence in higher education and define an international common space of education policy. This thesis provides a critical reflection on international higher education policymaking and reform implementation through the lens of the Russian engagement with Bologna, contributing to literature in the anthropology of “policy as a practice of power”, and contributing to studies of higher education.
Drawing on seven months of fieldwork in Moscow in 2010-11, and from a corpus of European and Russian legislation, education policy documents and university surveys, the thesis explores Bologna as an international sociocultural normative effort, and reveals practices of power that emerge during Russia’s engagement with Bologna.
The chapters offer an ethnographic look at Russian actors’ engagement with Bologna, highlighting their roles inside the institutions, their discursive production, network mobility, and the kinds of agencies that thrive inside the Bologna process. I follow the implementation of the European Credit Transfer System higher education standard by a Moscow university, and illuminate practices of segregation inside the institution that limit the appropriation of Bologna’s Social Dimension policies. Through such explorations the thesis shows regimes of power in the Bologna process, practices that strengthen Bologna’s governance model and establish the legitimacy of its policies, and the emergence of political and institutional hegemonies. I also show negotiation practices that emerge during the appropriation of Bologna’s policies, modifying or challenging these educational norms
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