Repositório RCAAP

Ao fim de 3 anos de actividade, 7 números, cerca de 90 artigos e 800 páginas, a Comissão Editorial decidiu encerrar o 1º volume da revista Electrónica e Telecomunicações. Contámos com a colaboração da quase totalidade do corpo docente do DETUA, com numerosas contribuições de alunos de licenciatura, de mestrado e doutoramento e, ainda de alguns professores e investigadores convidados que estiveram no Departamento. (...)

This paper presents a new coding/compression algorithm for high-quality digital audio signals. The new codercomprehends a nonuniform filter bank, gain-adaptive logaritbmic quantizers, arithmetic coding, and an explicit psychoacoustic model that adapts the quantizers according to perceptual criteria. The new system differs from other perceptual coders in that its quantizers are backward-adaptive: the adaptation is not based on the original signal but on the output signal. We discuss the advantages of backward-adaptation and its application to perceptual coding. Some results are presented.

In this paper we describe the simulation environment for artificial neural networks that we have developed, and also have used, with the aim of obtaining experimental results involving this computational paradigm. This environment is based on the NSL (Neuro-simulalion lanplage), which is an interpreted language for the development and simulation of neural networks.

The objective of this paper is to explain a new technique to simulate non-linear steady-state circuits that does not use time domain, like SPICE[12], but spectral domain to do the simulations. In this paper we develop a new frequency mapping technique to use in the discretization of continuous spectrum and a new tool to do convolution and deconvolution. Finally we prove the validity of this technique.

This paper descrives some experimental work concerning the development of a courseware server. Two versions of this system were implemented (oriented to the ISDN and to the Internet/WWW), that correspond to the evolution in time of the perception, by the authors, of the most appropriate solution to this type of application.

This paper presents a brief description of the "Moliceiro" robot that represented the University of Aveiro in the "Championat du Monde de Robotique Mobile" that took place in La Ferté-Bernard, France, from the 13th to the 19th of May, 1996. Basically, the robot had to follow a line on the floor, about 50m long, and overcome a few more difficulties. The project was based on infra-red analog sensors, low-power DC motors, differential stearing control using a near-proportional controller and a color discrimination mechanism. This way the robot was built with a very low cost (below PTE 50.000 in raw materials and parts), with great simplicity in its mechanics and electronics and still meeting all the contest specifications.

This project implements an autonomous mobile robot platform that performs a series of simple reactive behaviors and adapts to local environmental conditions. Since oscillatory motion under cornered conditions appears to be an inherent characteristic of purely reactive obstacle avoidance mechanisms in autonomous robots, a robot agent will require memory to enhance overall obstacle avoidance efficiency. In this study, a robot remembers previously encountered nearby obstacle sites by means of a selfgenerated local map in its short term memory. The mapprovides sufficient information for the robot to avoid becoming trapped in corners and oscillating endlessly.

In this paper, we describe an application to visualize biological events, particulary suited to neurophisiology environments. The application requirements are based on the poligraphic record. The application was developed using Object-Oriented techniques suported by Borland C++ compiler and its ObjectWindows library and Resource Workshop.

An analysis is performed on the operation of a modified current-source loaded inverter, showing that the proposed modification leads to a more linear characteristic, in terms of delay versus control voltage, than the one obtained with traditional current-starved inverters. A delay cell designed around this inverter is presented. The proposed circuit is intended to be used as a delay of the cell in function of physical parameters. Results of a comparision between the analytically derived delays and the delays obtained from SPICE simulation are presented.

A new wide-band propagation channel model for the millimetre-wave band, based on high frequency ray theory approximation, is presented in this paper. This model consists of a set of simple equations derived from the propagation theory and is able to take into account the propagation environment characteristics as well as the location of the transceiver antennas and their radiation patterns. The results obtained with the developed model agree quite well with the results obtained with a ray tracing tool in a relatively wide range of environments. Also a comparison with some measurements have shown the validity of the proposed model.

This paper introduces the IEEE 802.11 wireless local area network and describes the main contributions given by the University of Aveiro to its specification.

The article discusses the implementation of an optical fiber transmission system, witch suports rates of 622 Mb/s (OC-12). The circuit was developed in a CMOS technology of 1.0μm (ES2). It implements the necessary serial-to-parallel and parallel-to-serial conversions. This ASIC designed in Full Custom is particulary suitable for SONET/SDH (STM-4) based applications.

Nowadays the clock recovery circuits used for frequencies up to 10 Gbit/s use open-loop structures. The band-pass filters used on these structures are implemented with dielectric resonators (DR). This kind of filters introduce spurious modes that degradate the characteristics of the recovered clock signal. So, the clock amplification stage is required not only to provide the desired clock signal levels, but also to reduce the magnitude of the DR filter spurious modes. Next is presented the project, simulation and design of two amplifiers (GaAs MMIC) operating at 10 and 20 Gbit/s, to be integrated in a clock recovery circuit.

This paper describes, in a concise form, part of the Ph. D. work [1] that concerns with modelation of synchronizing systems. To get a better understanding of the Phase Locked Loop with Injection (PLLI) we begin with the description and modelation of a Phase Locked Loop (PLL) and of a Injection Locked Oscillator (ILO). Following this we develop the PLLI model. In a next paper we’ll present a simulated and practical comparasion among these systems.

Neste número iniciamos a publicação de artigos convidados, como é o caso do artigo da autoria do Prof. Valery Sklyarov, professor catedrático visitante do nosso departamento. (...)

A Video Phone application for the MS-Windows platform is a package of software that allow us to talk to another person while viewing his image. This is one of the new applications that can be done by using the facilities of ISDN (Integrated Services Digital Network). Using ISDN and Vídeo Capture cards currently available for PC 'Vídeo Telephony for Windows' gained form.

This paper presents a tool that allows the inclusion of the Boundary Scan test logic in a VHDL description of a given integrated circuit.

This paper presents a method to generate test vectors for an electronic board composed of modules from which the structural description it’s not known. The only information available is the set of test vectors, their coverage on the primary inputs and outputs of each module provided by the manufacturer and the composition of the board. This method provides test vectors to test the board from its primary inputs and outputs and was automatized, resulting in a iterative software tool, developed in C language.

The nonlinear distortion of a PLL frequency discriminator is addressed using the Volterra Series approach. The analysis, made entirely in the frequency domain, allows distortion calculations for periodic and nonperiodic sets of discrete excitation frequencies (harmonic and intermodulation distortion up to 3rd order) and random inputs. It also includes, simultaneously, the two major sources of distortion in a PLL: Phase Detector and VCO. In addiction a method for reduction of non-linear distortionis investigated.

Apresenta-se o projecto dum controlador, que implementa a interface do μP68030 a uma memória dinâmica. A memória permite o acesso de bytes, words e longwords, suportando ainda um modo de acesso rápido (‘Nibble’). Para aumentar a confiabilidade (‘reliability’), a interface inclui ainda um circuito de detecção e correcção de erros.