ANNUAL JOURNAL OF ELECTRONICS, 2009, ISSN 1313-1842 50 Simple High-Q Comb Filter for Mains Interference and Baseline Drift Suppression Dobromir Petkov Dobrev, Tatyana Dimitrova Neycheva and Nikolay Tsvetanov Mudrov Abstract - This paper presents a simple digital high-Q comb filter for baseline wander and power-line (PL) interference suppression. The filter concept relies on a first difference – a discrete version of the signal first derivative, resulting in a high-pass roll-off in combination of the so called a comb frequency response. The presented filter is evaluated by Matlab simulations with real ECG signal contaminated with high amplitude PL interference. The made simulations show that this filter has minimal influence on processed ECG signal. Due to its high-pass characteristic and high-Q notches only at PL harmonics the presented filter is appropriate for almost all biosignal acquisition applications where PL interference and baseline drift suppression is needed. The filter is suitable for real-time operation with popular low-cost microcontrollers. Keyword s - power-line interference, comb filter, baseline drift, FIR filter, IIR filter I. INTRODUCTION One of the common problems in almost all biosignal acquisition applications is a baseline wander due to imperfect electrode-skin interface. The difference in electrode polarization potentials, and its time variation, produces a baseline drift which is usually suppressed by a high-pass analog filter after the first amplification stage. For a standard ECG processing the implemented high-pass filter should have a time constant of 3s, corresponding to a high-pass cut-off frequency of 0.05Hz. In some cases a cut- off frequency of 0.05Hz is too low, and the baseline drift suppression should be enhanced by additional high-pass filtering in a digital domain. Power-line (PL) interference (hum) is another common problem in almost all biosignal acquisition applications. Because the body serves as a capacitively coupled antenna, a part of the picked up PL interference currents traverses the electrodes and produces a common mode voltage over an amplifier common mode input impedance. At the amplifier output some AC noise remains as a consequence of electrode impedance imbalance and/or due to the finite value of the amplifier CMRR [1], even when special signal recording techniques are applied (shielding, driven right leg, body potential driving, etc.). A further reduction of the interference should be implemented by either post-digital or post-analog filters. The most common option for mains interference suppression is to use a low-pass averaging digital filters with firs zero at the PL frequency. The name of such a filter is known also as an averager, smoother, moving- average, rolling-average or running-mean filter. Because of introduced additional signal bandwidth limitation these filters significantly attenuate important frequency components. A special FAS (Filtration-Addition- Subtraction) algorithm can partially improve their high- frequency response [2]. Smart approaches such as adaptive noise cancellation [3], subtraction procedure [4] and various lock-in techniques [5] require sophisticated software organization. Recently an interesting linear phase high-pass, and at the same time high-Q comb frequency response was presented [6]. By an averaging of several first differences, each one using the current and the delayed in multiple PL periods sample, a high-Q response is achieved, with a price of ripples in the pass-band, and a large amount of data (PL periods) that have to be processed. Although in presence of many different approaches, the problem of interference removal still exists and the researchers continue to find a simple solution to result in a ‘high fidelity’ and ‘clean’ recordings. This paper presents a simple digital high-pass high-Q comb filter for PL interference suppression. Based on a recursive (IIR) feedback extension [7] of a simple first difference comb filter, a very narrow band (high-Q) frequency response can be achieved. The main advantage of the presented filter is that it rejects the baseline drift and all harmonics of the rated PL interference. II. FILTER CONCEPT The comb filters are widely used in digital signal processing. They are widely spread in either audio signal processing to achieve special sound effects (echo, flanging, etc.), or in TV signal processing for separating the luminance (black & white) and chrominance (color) signals from composite video signal [8]. In a nutshell, a simple first difference comb filter is achieved when incoming signal samples are subtracted from their delayed copy. Thus a high-pass roll-off and alternating constructive and destructive spectrum interference are produced as a function of the time delay between original and delayed signals. A simple first difference feed forward (FIR) comb filter is shown in Fig. 1. D. Dobrev is with Centre of Biomedical Engineering “Ivan Daskalov” – Bulgarian Academy of Sciences, Bl. 105 Acad G. Bontchev Str., 1113 Sofia, Bulgaria, e-mail: dobri@clbme.bas.bg T. Neycheva is with Centre of Biomedical Engineering “Ivan Daskalov” – Bulgarian Academy of Sciences, Bl. 105 Acad G. Bontchev Str., 1113 Sofia, Bulgaria, e-mail: tatiana@clbme.bas.bg N. Mudrov is with Centre of Biomedical Engineering “Ivan Daskalov” – Bulgarian Academy of Sciences, Bl. 105 Acad G. Bontchev Str., 1113 Sofia, Bulgaria, e-mail: mudrovn@clbme.bas.bg