Power-line Interference Suppression Research Articles

An Approach to Successful Power-line Interference Suppression in ECG Signals

The ECG signals acquisition is usually corrupted by presence of Power-Line Interference (PLI) induced by the electromagnetic field around us. Many methods for PLI suppression/elimination have been developed over the years. The easy to apply traditional notch filters suppress unacceptably the ECG spectrum around the rated PL frequencies of 50 or 60 Hz and their deviations, which are restricted by the standards within the range of ± 0.5 Hz. The changes are very slow but the current PL frequency has to be continuously checked to allow start and performance of adequate PLI suppression during any ECG recordings including the 24 hours Holter monitoring. According to the proposed approach, the corrupted ECG recording is bi-directional band-pass (BP) filtered. The resulting sinusoidal BP waves differ in amplitude from the PLI but their zero crossing points remain identical. The two out-sample distances located at both ends of each current sinusoidal curve are calculated and aided to the inter-sample distances. The obtained fractal wave period is converted into current PL frequency and used for bi-directional notch filtration with narrow stop-band. The results obtained demonstrate a very successful PLI suppression in ECG signals. The errors committed are within a few μV, except for the edges of the recordings due to the transition processes.

Noise detection and suppression in ECG using adaptive filter algorithm

A novel Power Line Interference (PLI) detection and suppression algorithm is proposed to preprocess Electrocardiogram (ECG) signals. A distinctive feature of this algorithm is the ability to detect the presence or absence of PLI in the ECG signal before applying the PLI suppression algorithm in the ECG signal. Only after the detection of PLI, it can be suppressed. We propose a PLI detector employing a classifier designed based on observations of the Root Mean Square (RMS) values of noise and reference ECG signals. An efficient recursive Least Mean Square (LMS) adaptive notch filter was also developed to serve the purpose of PLI suppression. An adaptive filter consists of a digital filter with adjustable coefficients and an LMS algorithm to modify the coefficient values to filter each sample. Experimental results demonstrate superior performance of this proposed algorithm.

Synchrosqueezing Transform Based Powerline Interference Reduction in ECG Recording

This paper presents an adaptive ECG enhancement procedure based on Synchrosqueezing Transform (SST) to eliminate Powerline interference (PLI) from ECG signal. This work also incorporates the principles of modified discrete cosine transform (MDCT) and wiener filter. PLI is a major source of artifacts in the ECG signal which can affect its interpretation. Separating PLI from ECG signal poses a great challenge in the ECG analysis. The existing PLI removal techniques suffer from two major drawbacks such as Mode Mixing, inability to deal with non-stationary characteristics of signal. In this paper, we propose SST based wiener filtering approaches which can overcome the limitation of existing PLI suppression techniques. The proposed approaches undergo three stages of operation: mode decomposition, mode determination and peak restoration to filter out PLI from ECG recording. The mode decomposition uses SST to decompose the corrupted ECG signal into a sum of well separated intrinsic mode functions (IMFs). The objective is to filter out PLI from these IMFs. To do so, mode determination step which is based on Kurtosis and Crest factor is applied to categorize decomposition result into groups such as signal mode and noisy mode. Direct subtraction of the noisy mode from the corrupted ECG observation results in ECG signal with reduced peak since noise mode carries part of signal components in addition to interference. Hence, to restore the peak, wiener filter is applied on noisy mode to estimate actual PLI component. Finally, Noise free ECG signal is reconstructed by subtracting estimated PLI from the corrupted ECG signal. This paper discusses four possible PLI suppression methods which are derived by combining SST domain with wiener filter in various ways. Simulations are carried out to test the effectiveness of proposed methods. It is evident from the simulation results that the proposed methods can remove PLI of 50 Hz and its harmonics. The proposed techniques effectively removed PLI in both real and artificial ECG signals and to test its performance they are compared with state of the art methods. The SST based filtering methods outperformed other methods under the condition of PLI frequency variations. The experimental results also suggest that the SST based wiener filtering with modified reference approach offers better PLI suppression than all other methods.

A novel PLI suppression method in ECG by notch filtering with a modulation-based detection and frequency estimation scheme

In this paper, a new algorithm composed of detection, estimation, and filtering has been implemented to suppress power-line interference (PLI) in the contaminated electrocardiogram (ECG) signal. In the proposed system, the detector can transform the measured signal into a complex signal, whose absolute value is compared with a threshold to decide whether PLI is present or not. If PLI is detected, then the estimate of its frequency computed from the estimator is sent to tune a coefficient of the notch filter for precise suppression of PLI without the requirement of any reference input. The proposed approach has been verified by artificially generated sinusoids and practical PLI for ECG signals. The experimental results have shown the efficacy of the proposed method.

Sensor Configuration and Algorithms for Power-Line Interference Suppression in Low Field Nuclear Magnetic Resonance.

Low field (LF) nuclear magnetic resonance (NMR) shows potential advantages to study pure heteronuclear J-coupling and observe the fine structure of matter. Power-line harmonics interferences and fixed-frequency noise peaks might introduce discrete noise peaks into the LF-NMR spectrum in an open environment or in a conductively shielded room, which might disturb J-coupling spectra of matter recorded at LF. In this paper, we describe a multi-channel sensor configuration of superconducting quantum interference devices, and measure the multiple peaks of the 2,2,2-trifluoroethanol J-coupling spectrum. For the case of low signal to noise ratio (SNR) < 1, we suggest two noise suppression algorithms using discrete wavelet analysis (DWA), combined with either least squares method (LSM) or gradient descent (GD). The de-noising methods are based on spatial correlation of the interferences among the superconducting sensors, and are experimentally demonstrated. The DWA-LSM algorithm shows a significant effect in the noise reduction and recovers SNR > 1 for most of the signal peaks. The DWA-GD algorithm improves the SNR further, but takes more computational time. Depending on whether the accuracy or the speed of the de-noising process is more important in LF-NMR applications, the choice of algorithm should be made.

Power-line interference suppression of MT data based on frequency domain sparse decomposition

Power-line interference is one of the most common noises in magnetotelluric (MT) data. It usually causes distortion at the fundamental frequency and its odd harmonics, and may also affect other frequency bands. Although trap circuits are designed to suppress such noise in most of the modern acquisition devices, strong interferences are still found in MT data, and the power-line interference will fluctuate with the changing of load current. The fixed trap circuits often fail to deal with it. This paper proposes an alternative scheme for power-line interference removal based on frequency-domain sparse decomposition. Firstly, the fast Fourier transform of the acquired MT signal is performed. Subsequently, a redundant dictionary is designed to match with the power-line interference which is insensitive to the useful signal. Power-line interference is separated by using the dictionary and a signal reconstruction algorithm of compressive sensing called improved orthogonal matching pursuit (IOMP). Finally, the frequency domain data are switched back to the time domain by the inverse fast Fourier transform. Simulation experiments and real data examples from Lu-Zong ore district illustrate that this scheme can effectively suppress the power-line interference and significantly improve data quality. Compared with time domain sparse decomposition, this scheme takes less time consumption and acquires better results.

Joint power line interference suppression and ECG signal recovery in transform domains

Abstract This work addresses the electrocardiogram (ECG) recovery problem in the presence of power line interference (PLI) that corrupts the signal quality if it is not effectively suppressed. In this paper, the PLI is modeled as a linear superposition of sinusoidal signals, which has a sparse representation in the frequency domain. To accurately reconstruct the ECG, the time, second-order difference, and wavelet domains are exploited to sparsely represent the ECG. From the reformulations conducted, a novel joint optimization estimation is devised to simultaneously perform the ECG recovery and PLI suppression in the transform domains. Moreover, in order to solve the optimization problem, two efficient schemes based on the greedy algorithm together with the basis pursuit (BP) are developed. Finally, numerical studies demonstrate that the performance of the joint estimation algorithm is superior to the state-of-the-art approaches.

Adaptive suppression of power line interference in ultra-low field magnetic resonance imaging in an unshielded environment

Power-line harmonic interference and fixed-frequency noise peaks may cause stripe-artifacts in ultra-low field (ULF) magnetic resonance imaging (MRI) in an unshielded environment and in a conductively shielded room. In this paper we describe an adaptive suppression method to eliminate these artifacts in MRI images. This technique utilizes spatial correlation of the interference from different positions, and is realized by subtracting the outputs of the reference channel(s) from those of the signal channel(s) using wavelet analysis and the least squares method. The adaptive suppression method is first implemented to remove the image artifacts in simulation. We then experimentally demonstrate the feasibility of this technique by adding three orthogonal superconducting quantum interference device (SQUID) magnetometers as reference channels to compensate the output of one 2nd-order gradiometer. The experimental results show great improvement in the imaging quality in both 1D and 2D MRI images at two common imaging frequencies, 1.3 kHz and 4.8 kHz. At both frequencies, the effective compensation bandwidth is as high as 2 kHz. Furthermore, we examine the longitudinal relaxation times of the same sample before and after compensation, and show that the MRI properties of the sample did not change after applying adaptive suppression. This technique can effectively increase the imaging bandwidth and be applied to ULF MRI detected by either SQUIDs or Faraday coil in both an unshielded environment and a conductively shielded room.

Development of an ultra low noise, miniature signal conditioning device for vestibular evoked response recordings

BackgroundInner ear evoked potentials are small amplitude (<1 μVpk) signals that require a low noise signal acquisition protocol for successful extraction; an existing such technique is Electrocochleography (ECOG). A novel variant of ECOG called Electrovestibulography (EVestG) is currently investigated by our group, which captures vestibular responses to a whole body tilt. The objective is to design and implement a bio-signal amplifier optimized for ECOG and EVestG, which will be superior in noise performance compared to low noise, general purpose devices available commercially.MethodA high gain configuration is required (>85 dB) for such small signal recordings; thus, background power line interference (PLI) can have adverse effects. Active electrode shielding and driven-right-leg circuitry optimized for EVestG/ECOG recordings were investigated for PLI suppression. A parallel pre-amplifier design approach was investigated to realize low voltage, and current noise figures for the bio-signal amplifier.ResultsIn comparison to the currently used device, PLI is significantly suppressed by the designed prototype (by >20 dB in specific test scenarios), and the prototype amplifier generated noise was measured to be 4.8 nV/Hz @ 1 kHz (0.45 μVRMS with bandwidth 10 Hz-10 kHz), which is lower than the currently used device generated noise of 7.8 nV/Hz @ 1 kHz (0.76 μVRMS). A low noise (<1 nV/Hz) radio frequency interference filter was realized to minimize noise contribution from the pre-amplifier, while maintaining the required bandwidth in high impedance measurements. Validation of the prototype device was conducted for actual ECOG recordings on humans that showed an increase (p < 0.05) of ~5 dB in Signal-to-Noise ratio (SNR), and for EVestG recordings using a synthetic ear model that showed a ~4% improvement (p < 0.01) over the currently used amplifier.ConclusionThis paper presents the design and evaluation of an ultra-low noise and miniaturized bio-signal amplifier tailored for EVestG and ECOG. The increase in SNR for the implemented amplifier will reduce variability associated with bio-features extracted from such recordings; hence sensitivity and specificity measures associated with disease classification are expected to increase. Furthermore, immunity to PLI has enabled EVestG and ECOG recordings to be carried out in a non-shielded clinical environment.

Time-efficient removal of power-line noise from EMG signals using IIR notch filters with non-zero initial conditions

Power-line interference is always a problem when biopotential signals are recorded. This paper presents a technique for time-efficient power-line interference suppression from EMG signals using digital IIR (Infinite Impulse Response) notch filters with reduced transient response. The reduction of the transient response is obtained by finding optimal non-zero initial conditions for the considered notch filters. Simulations verifying the effectiveness of the proposed technique are presented and compared with the performance of the traditional notch filters with zero initial conditions using EMG signal with unwanted sinusoidal interferences as a study case.

Robust suppression of nonstationary power-line interference in electrocardiogram signals

It is a challenge to suppress time-varying power-line interference (PLI) with various levels in electrocardiogram (ECG) signals. Most previous attempts of tracking and suppressing the nonstationary PLI signal are based on the least-squares (LS) algorithm. This makes these methods susceptible to QRS complex in suppressing a low-level PLI signal which is frequently coupled in battery-operated ECG equipment. To address the limitation of LS-based methods, this study presents a robust PLI suppression system based on a robust extension of the Kalman filter. In addition, we used an improved version of empirical mode decomposition to further attenuate the QRS complex. Experiments show that our system could effectively suppress the PLI while preserving meaningful ECG components at various interference levels.

Power-Line Interference Detection and Suppression in ECG Signal Processing

A novel power-line interference (PLI) detection and suppression algorithm is presented to preprocess the electrocardiogram (ECG) signals. A distinct feature of this proposed algorithm is its ability to detect the presence of PLI in the ECG signal before applying the PLI suppression algorithm. No PLI suppression operation will be performed if PLI is not detected. We propose a PLI detector that employs an optimal linear discriminant analysis (LDA) algorithm to make a decision for the PLI presence. An efficient recursive least-squares (RLS) adaptive notch filter is also developed to serve the purpose of PLI suppression. Experimental results demonstrate superior performance of this proposed algorithm.

Suppression of powerline interference in the ECG signal using a bilinearly transformed null phase notch filter

Suppression of powerline interference in the ECG signal using a bilinearly transformed null phase notch filter

Automatic power-line interference canceller for geophysical instruments

Power-line interference is one of the main interferences for many geophysical measurements. Especially for instruments operating in the low frequency range, possessing broad frequency bandwidths and being very sensitive to phase changes, such as time-domain electromagnetic (TEM) instruments, any conventional linear filter used to suppress the power-line interference will cause distortion to the desired signals because of its unwanted phase (and amplitude) contribution(s). Normally, the 'comb' filtering effect of a TEM instrument provides excellent suppression of power-line interference. But in the vicinity of an industrial area, the power-line interference is sometimes so strong that it may saturate the linear circuits of the instrument or modulate the desired signal, and thus spoil the 'comb' filtering effect. The completely self-contained power-line interference canceller introduced in the following is developed to cancel the strongest frequency component, very often the fundamental one, of the power-line interferer. Two or more such cancellers can be used together, if more than one frequency component of the interferer need to be cancelled.