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Signal Processing

Autocorrelation
The autocorrelation function gives a measure of the extent to which a signal correlates with a displaced version of itself, as a function of the displacement.
Bandpass Filter
Bandstop Filter
Bandwidth-Time Product
The bandwidth-time product is used in calculating the confidence limit of a measurement. Confidence limits describe the uncertainty in measuring the level of random signals in finite periods of time.
Bartlett Window
Bessel Filter
Cepstrum
A cepstrum is a time history defined as the power spectrum of the logarithm of the power spectrum. This you can interpret as the spectrum of a spectrum. It extracts periodicities in the spectrum, such as harmonic families and sidebands. The imaginary part is the Hilbert transform.
Chirp
A chirp is a sound that has a rapid increase in frequency.
Coherence
A two channel function which is used when an output (e.g. displacement) is caused by more than one input (e.g. two unrelated forces at different positions). The coherence function shows how much of the output is caused by each of the inputs, as a function of frequency.
Colourmap
This is a method of plotting noise or vibration versus engine speed and frequency simultaneously. Normally frequency is plotted along the x-axis, engine speed along the y-axis and the level is shown by the colour (the rpm and frequency axes can be plotted the other way).
Correlation
If two variables are in some way dependent on each other or related to each other, they are said to be correlated.
Covariance
Covariance functions define the degree of alikeness between two time histories as a function of time shift (cross covariance) or the degree to which a function is correlated with itself as a function of time delay (autocovariance).
Crest Factor
Peak value divided by the R.M.S. value, used as a measure of the severity of a segment of data.
Cross CorrelationThe cross-correlation function gives a measure of the extent to which two signals correlate with each other as a function of the time displacement between them.
Cross SpectrumThe cross spectrum is the forward Fourier transform of the Cross Correlation function.
DFTThe digital (sampled) version of the fourier transform. Any block size can be used. This technique involves long computations, and normally the Fast Fourier Transform is used instead due to its quicker computation.
Down Sample
Elliptic Filter
FFTA method developed in 1965 which allows a Fourier Transform to be computed very quickly. This method relies on the number of data points in the time series being a power of 2 (e.g. 512, 1024, 2048). Most modern frequency analysers now use this process.
FilterA device which removes or attenuates certain frequencies present in a voltage signal. Low pass filters cut out high frequencies, high pass filters cut out low frequencies and bandpass filters cut out frequencies above and below a selected frequency band.
Frequency Domain
Hamming WindowAn amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
Hanning WindowAn amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
Spectrogram
Time HistoryA continuous record of the variation of a physical quantity (e.g. displacement, acceleration, force, etc.) with time. The record may be stored in a chart recorder, magnetic tape, computer disc or any other storage medium.
Up Sample
WaterfallA method of plotting engine vibration or noise data in three-dimensional form.
WaveletThe wavelet allows a rapidly changing time history (e.g. door slam noise) to be investigated in the frequency domain.
WindowingAn amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
Science & Engineering Encyclopaedia Version 2.3 © 2001-2008 Dirac Delta Consultants Limited