Timing Error Correction Research Articles

Timing errors in auditory event-related potentials

Electronic problems of electroencephalographic (EEG) system may occur in even the best-managed laboratories. Timing error may happen in the coupling of computers from various manufactures, resulting in the misalignment of event markers that signal the onset of stimuli. In one system, an impedance check desynchronized a computer and thus caused misalignment of events in EEG signals. Thus, the aim of this study was to develop a method to identify and correct such timing errors that contaminated 114 raw data of EEG/auditory event-related potentials (ERPs) recorded in one of our longitudinal studies. A two-step procedure was introduced in the correction of timing errors. First, the time delay was roughly estimated by identifying a P 150 component in two ERP blocks. Second, a small phase-locked positive wave was identified for fine estimation. Reliability within and among evaluators was examined using ERP data with simulated timing errors. Concordant results were obtained in 104 (91.2%) of the 114 raw EEG/ERP data sets. Our results showed that the method presented here is reliable and can be used for correcting timing errors without introduction of experimenter bias.

Self-validating presentation and response timing in cognitive paradigms: how and why?

With the proliferation of commercial experiment generators and custom software within cognitive psychology and the behavioral sciences, many have assumed that issues regarding millisecond timing accuracy have been largely solved. However, through empirical investigation of a variety of paradigms, we have discovered numerous sources of timing error. These can range from poor scripting practices, to incorrect timing specifications, to hardware variability. Building upon earlier research, we have developed a commercial device and associated software that enables researchers to benchmark most computer-based paradigms in situ and without modification. This gives them the opportunity to correct timing errors where practicable, increase replicability, and reduce variability by altering onset times for stimuli, by replacing inaccurate hardware, or by post hoc statistical manipulation should the source of error be constant. We outline the features of the device and accompanying software suite, stress the importance of such independent validation, and highlight typical areas that can be subject to error.

Removal of time-varying gradient artifacts from EEG data acquired during continuous fMRI

Objective : Recording low amplitude electroencephalography (EEG) signals in the face of large gradient artifacts generated by changing functional magnetic resonance imaging (fMRI) magnetic fields continues to be a challenge. We present a new method of removing gradient artifacts with time-varying waveforms, and evaluate it in continuous (non-interleaved) simultaneous EEG–fMRI experiments. Methods : The current method consists of an analog filter, an EEG–fMRI timing error correction algorithm, and a temporal principal component analysis based gradient noise removal algorithm. We conducted a phantom experiment and a visual oddball experiment to evaluate the method. Results : The results from the phantom experiment showed that the current method reduced the number of averaged samples required to obtain high correlation between injected and recovered signals, compared to a conventional average waveform subtraction method with adaptive noise canceling. For the oddball experiment, the results obtained from the two methods were very similar, except that the current method resulted in a higher P300 amplitude when the number of averaged trials was small. Conclusions : The current method enabled us to obtain high quality EEGs in continuous simultaneous EEG–fMRI experiments. Significance : Continuous simultaneous EEG–fMRI acquisition enables efficient use of data acquisition time and better monitoring of rare EEG events.

Optimized interpolator filters for timing error correction in DMT systems for xDSL applications

Discrete multitone (DMT) modulation is a multicarrier technique that allows the transmission of high speed data over band limited channels. This type of system is very sensitive to synchronization errors when used in digital subscriber loop applications (xDSL), due to the high number of carriers and high density constellations involved. This paper addresses the topic of all-digital timing error correction in a DMT system for xDSL applications, where the timing error correction procedure is based entirely on signal interpolation. An analytical study of the interpolator filter performance is carried out, arriving at an expression for the signal-to-distortion ratio (SDR) at the output of the receiver, as a merit figure. In this derivation, the fixed frequency domain equalizer (FEQ) plays an important role since it compensates for a great part of the distortion introduced by the interpolator. From this study, the design of the optimal interpolator filter in terms of SDR, based on a multirate approach with Kaiser window, is presented. Specific designs for ASIC and for DSP-based implementations are obtained. Performance results are excellent, yielding SDR values above 50 dB for all carriers while keeping the computational cost low.

Timing Accuracy in Modern ATE

LSI test systems built in the 1970s had compact, 60-pin test heads that presented devices under test with open wires leading to lumped-capacitive loads. Today's VLSI testers have 256-pin test heads with transmission lines leading from the DUTs to driver-comparator circuits that are as far away as 50 cm. Even though these automatic testers are adjusted to subnanosecond accuracy, reflections within the transmission lines can cause timing measurement errors up to 10 ns for MOS devices whose output impedances are not matched to the transmission lines. The authors offer the Advice circuit simulator (an AT&T proprietary version of SPICE) as one way to analyze errors due to transmission line problems before testing. They also discuss ways to correct timing errors. Finally, they recommend that very high speed ICs be designed to drive transmission lines that are terminated with matched resistors at the comparators.

Correction of timing errors in photomultiplier tubes used in phase-modulation fluorometry

The measurement of fluorescence lifetimes is known to be hindered by the wavelenght-dependent and photocathode area-dependent time response of photomultiplier tubes. A simple and direct method is described to minimize the effects in photomultiplier tubes for phase-modulation fluorometry. Reference fluorophores of known lifetime were used in place of the usual scattering reference. The emission wavelenghts of the reference and sample were matched by either filters or a monochromator, and the use of a fluorophore rather than a scatter decreases the differences in spatial distribution of light emanating from the reference and sample. Thus photomultiplier tube artifacts are minimized. Five reference fluorophores were selected on the basis of availability, ease of solution preparation, and constancy of lifetime with temperature and emission wavelenght. These compounds are p-terphenyl, PPO, PPD, POPOP and dimethyl POPOP. These compounds are dissolved in ethanol to give standard solutions that can be used over the temperature range from −55 to +55°C. Purging with inert gas is not necessary. The measured phase and modulation of the reference solution is used, in conjunction with the known reference, lifetime, to calculate the actual phase and modulation of the exictation beam. The use of standard fluorophores does not require separate experiments to quantify photomultiplier effects, and does not increase the time required for the measurement of fluorescence lifetimes. Examples are presented which demonstrate the elimination of artifactual photomultiplier effects in measurements of the lifetimes of DADH (0.4 ns) and indole solutions quenched by iodide. In addition, the use of these reference solutions increases the accuracy of fluorescence lifetime measurements ranging ranging to 30 ns. We judge this method to provide more reliable lifetime measurements by the phase and modulation method. The test solutions and procedures we describe may be used by other laboratories to evaluate the performance of their phase fluorometers.

The correction of timing errors due to tape speed variation in the tape recording of physiological data

A high degree of system accuracy is required for the recording and analysis of heart rate and beat-to-beat variability in newborn infants. Many tape recording systems at present in use do not provide this. A technique is described using a recorded clock signal of known frequency to correct the inter-beat intervals for inaccuracy due to tape speed variations. This has improved the resolution of the system from 7·5 ms to 1·5 ms, and reduced the overall error from 1·5% to 0·3%.

The effect of timing errors on the intelligibility of deaf children's speech

At the 93rd meeting of the Acoustical Society of America, we presented a preliminary report on a technique to correct deviant timing errors in deaf children's speech by computer processing of recorded speech samples. Timing modifications were accomplished by direct manipulation of the waveform in which whole pitch periods were deleted from steady‐state portions of speech segments, leaving all other aspects of the speech unchanged. The following six‐stage approximation procedure was used to correct the deviant timing patterns: (1) original, unaltered utterance, (2) correction of pauses only, (3) correction of relative timing (stressed/unstressed ratio), (4) correction of absolute syllable duration, (5) correction of relative timing and pauses, and (6) correction of absolute syllable duration and pauses. This report will present preliminary data on the changes in intelligibility that occurred as a result of the systematic timing corrections Intelligibility measures were obtained by playing the original sentences and the computer‐modified versions to persons not familiar with the speech of the deaf. The measure of intelligibility was the proportion of words correctly understood by the listeners. The data showed that, on the average, intelligibility improved slightly when relative timing errors only were corrected. A decrease in intelligibility was observed for the other forms of timing modification. There were marked individual differences and the correction of deviant timing patterns was relatively more important for improving the intelligibility of some children's speech than others. [Research supported by PHS Grant NS 09252.]

The effect of timing errors on the intelligibility of deaf children’s speech

The purpose of this study was to quantify the effect of timing errors on the intelligibility of deaf children's speech. Deviant timing patterns were corrected in the recorded speech samples of six deaf children using digital speech processing techniques. The speech waveform was modified to correct timing errors only, leaving all other aspects of the speech unchanged. The following six-stage approximation procedure was used to correct the deviant timing patterns: (1) original, unaltered utterances, (2) correction of pauses only, (3) correction of relative timing, (4) correction of absolute syllable duration, (5) correction of relative timing and pauses, and (6) correction of absolute syllable duration and pauses. Measures of speech intelligibility were obtained for the original and the computer-modified utterances. On the average, the highest intelligibility score was obtained when relative timing errors only were corrected. The correction of this type of error improved the intelligibility of both stressed and unstressed words within a phrase. Improvements in word intelligibility, which occurred when relative timing was corrected, appeared to be closely related to the number of phonemic errors present within a word. The second highest intelligibility score was obtained for the original, unaltered sentences. On the average, the intelligibility scores obtained for the other four forms of timing modification were poorer than those obtained for the original sentences. Thus, the data show that intelligibility improved, on the average, when only one type of error, relative timing, was corrected.

An Optical Video Disc Player for NTSC Receivers

This paper reports on a team effort to develop an optical player based on a thin flexible disc and the aerodynamic disc stabilizer of Thomson-CSF. The signal is encoded by a method which minimizes the effect of microscopic non-linearities within the disc surface: luminance and chroma together modulate an FM carrier of relatively high frequency. Radial tracking is achieved by a method which keeps the focused laser beam centered on the recorded track, requires no auxiliary beams and permits unusually high track density. Timing errors, caused by eccentricity and distortion of the disc, are suppressed by an electromechanical servo which is combined with the tracking servo. Improved timing error correction opens up an interesting possibility of simplifying the decoder.

Automatic Correction of Timing Errors in Magnetic Tape Recorders

A basic problem in magnetic recording is the preservation of accurate time relationships between reproduced bits of information. If information is recorded at a given rate, the reproduced signal rate will tend to deviate from this rate. Servo systems for controlling the mechanisms of the machine can maintain the reproduced signal rate equal to the recorded signal rate, averaged over a period of a few milliseconds. Mechanical servos cannot, however, prevent short-term deviations in reproduced signal rate, because of their relatively slow response. Electronically variable delay lines (EVDLs) have been developed which, because of their relatively fast response, can further reduce timing errors. This paper presents a mathematical analysis of the problem and of the requirements imposed on the EVDLs. Two devices for the accomplishment of electronically variable delay are then described, one for high-frequency signals, and one for low-frequency signals. Experimental results obtained by using these two delay lines are also presented.