Compression of ECG signals by optimized quantization of discrete cosine transform coefficients.

This paper presents an ECG compressor based on optimized quantization of Discrete Cosine Transform (DCT) coefficients. The ECG to be compressed is partitioned in blocks of fixed size, and each DCT block is quantized using a quantization vector and a threshold vector that are specifically defined for each signal. These vectors are defined, via Lagrange multipliers, so that the estimated entropy is minimized for a given distortion in the reconstructed signal. The optimization method presented in this paper is an adaptation for ECG of a technique previously used for image compression. In the last step of the compressor here proposed, the quantized coefficients are coded by an arithmetic coder. The Percent Root-Mean-Square Difference (PRD) was adopted as a measure of the distortion introduced by the compressor. To assess the performance of the proposed compressor, 2-minute sections of all 96 records of the MIT-BIH Arrhythmia Database were compressed at different PRD values, and the corresponding compression ratios were computed. We also present traces of test signals before and after the compression/decompression process. The results show that the proposed method achieves good compression ratios (CR) with excellent reconstruction quality. An average CR of 9.3:1 is achieved for PRD equal to 2.5%. Experiments with ECG records used in other results from the literature revealed that the proposed method compares favorably with various classical and state-of-the-art ECG compressors.

Compressão de sinais eletrocardiográficos baseada em transformada discreta do cosseno e codificação de entropia / Compression of electrocardiographic signals based on discrete cosine transform and entropy coding

O presente trabalho relata uma técnica, baseada no padrão JPEG, que combina a Transformada Discreta do Cosseno com técnicas genéricas de compressão de dados visando aumentar a taxa de compressão do sinal de ECG. Dada à grande eficiência do padrão JPEG, buscou-se aplicar pelo menos parte de suas estratégias de compressão ao caso do ECG. A técnica de compressão proposta (CP) permitiu alcançar boas taxas de compressão, com distorções aceitáveis na morfologia do ECG. A razão de compressão média obtida com a CP seguida pela codificação de Huffman foi de 10,2:1 e com a CP seguida pela codificação LZW foi de 9,4:1, para uma PRD = 5 por cento. Traçados de alguns ECGs são mostrados antes e após o processo de compressão/descompressão, permitindo avaliar visualmente a distorção introduzida pelo processamento.