LABORATORY HARDWARE IMPLEMENTATION OF NON-UNIFORM SAMPLING ECG RECORDER Piotr Augustyniak + + University of Mining and Metallurgy, Institute of Automatics, al. Mickiewicza 30, 30-059 Kraków, Poland, august@biocyb.ia.agh.edu.pl, http://galaxy.uci.agh.edu.pl/~august Abstract: This paper concerns the prototype of ECG recording system using variable sampling rate. The signal-dependent non-uniform sampling is currently an interesting alternative for the effective discrete signal representation. Author is reporting the implementation of main concepts of non-uniform space sampling in a microcontroller-based recording system that uses the fixed-point data representation. The experiences gathered during this implementation are particularly useful for future development of real-world devices based on micropower circuitry. Main result, however, is the proof of feasibility for implementation of advanced mathematical concepts in the energy-saving hardware of today. 1 Introduction Electrocardiogram is the most frequently performed electrophysiological test. Its accessibility, however, is often limited by the cost of data transfer or storage. An emerging solution, developed currently in our laboratory, makes use of variable density of diagnostic information in the cardiac signal that is obvious to the expert but difficult to express in a mathematical way. Our proposal is an alternative to the compression of electrocardiogram data that has great practical significance and is widely used in clinical practice despite the disscussion about the reliability of the restored signal [1]. In many discussions the topic of the adequate sampling frequency for the ECG data was concerned. In the stand-alone 12-lead standard recorders the rule is very simple but the long-term cardiac recordings (i. e. Holter techniques) always involve a compromise between the amount of the stored data and the precision of signal representation. Usually, digital recorders comply with the general assumption made on signal analysis saying that the occurrence of any probable signal component is possible at any time, hence full bandwidth of the transmission channel is to be provided continuously [2]. This approach is widely used for its generality and careless use of technical resources, however the output data stream is overestimated. It guarantees that the parameters of the channel throughput are time-invariant and thus the transmission features, such as distortions, are related to the amplitude and frequency characteristics of input signal components and not to their occurrence in time. It is worth a remark here, that when the bandwidth of the digitized signal changes in time, the sampling frequency may be locally adapted for satisfy the Shannon theorem. This remark is the foundation of sampling signals at the variable rate and is developed further in this chapter. Certainly, for the ECG recorder sampling at the variable rate, the most important issue is the definition of the local sampling interval based on the signal features. Fortunately, the ECG has several properties of high relevance when considering the local optimization of sampling frequency: - The full bandwidth is used for short time intervals only representing the ventricles' contraction (i. e. the QRS complex) - these intervals are identified with high reliability by commercially available software and by hardware detectors. - For a large amount of time the local bandwidth is significantly (to four times) lower [3], [4]. - The medical point of view that the diagnostic information is distributed irregularly in the signal converges with the technical notion of information throughput expressed by the local bandwidth. - Some extend of regularity may be anticipated, and the variety of possible co-occurrences of signal components is limited by the physiology. The prototyped recorder uses the memory resources in more efficient way, but preserves the signal diagnostability typical for devices sampling continuously at the high rate. Another advantage resulted from the irregular sampling is the immunity to