A Simple Method for Guaranteeing ECG Quality in Real-Time Wavelet Lossy Coding
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Research Article A Simple Method for Guaranteeing ECG Quality in Real-Time Wavelet Lossy Coding ´ Alvaro Alesanco and Jose´ Garc´ıa Communications Technology Group, Arag´on Institute of Engineering Research, University of Zaragoza, 50018 Zaragoza, Spain Received 3 May 2006; Revised 19 October 2006; Accepted 17 December 2006 Recommended by David Hamilton Guaranteeing ECG signal quality in wavelet lossy compression methods is essential for clinical acceptability of reconstructed signals. In this paper, we present a simple and efficient method for guaranteeing reconstruction quality measured using the new distortion index wavelet weighted PRD (WWPRD), which reflects in a more accurate way the real clinical distortion of the compressed signal. The method is based on the wavelet transform and its subsequent coding using the set partitioning in hierarchical trees (SPIHT) algorithm. By thresholding the WWPRD in the wavelet transform domain, a very precise reconstruction error can be achieved thus enabling to obtain clinically useful reconstructed signals. Because of its computational efficiency, the method is suitable to work in a real-time operation, thus being very useful for real-time telecardiology systems. The method is extensively tested using two different ECG databases. Results led to an excellent conclusion: the method controls the quality in a very accurate way not only in mean value but also with a low-standard deviation. The effects of ECG baseline wandering as well as noise in compression are also discussed. Baseline wandering provokes negative effects when using WWPRD index to guarantee quality because this index is normalized by the signal energy. Therefore, it is better to remove it before compression. On the other hand, noise causes an increase in signal energy provoking an artificial increase of the coded signal bit rate. Clinical validation by cardiologists showed that a WWPRD value of 10% preserves the signal quality and thus they recommend this value to be used in the compression system. ´ Alesanco and J. Garc´ıa. This is an open access article distributed under the Creative Commons Attribution Copyright © 2007 A. License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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INTRODUCTION
ECG signal recording and further interpretation constitute the first tool for cardiopathy diagnosis. Almost all new ECG acquisition devices offer the possibility to digitally store the acquired ECG signals. Taking into account that a standard acquisition device usually works with a sampling frequency of 512 samples per second with a resolution of 16 bits and records 8 leads, the output information rate obtained is about 65 Kbit/s. Although the storage capacity in information systems as well as network bandwidth increases quickly, an efficient storage and transmission method is still required because it leads to an efficient use of the available resources. Moreover, this is specially critical in telecardiology systems, where the available network capacity
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