Nyquist artifact

Gratton et. al. in [34] describes a least squares regression method to model heartbeat artifacts and to filter it out adaptively. Since the heartbeat rate is approximately 1 Hz, it is necessary that data is sampled at a sufficiently high rate (e.g. above the Nyquist rate) so as to represent and filter the artifacts out correctly and have minimal impact on the signal of interest itself [98]. 

Figure 8.7 Digital Sine function - the frequency response for a zero order hold interpolator sample rate converter with L = 4, which puts the original Nyquist frequency at 0.25 7t. We can see rolloff in the passband of about -3.9 dB and very poor rejection of images outside of the passband, which result in artifacts perceived as pitch shifting distortion.

Such a method was used in the literature for the determination of impedances [99-101], and a commercial apparatus [102] applying a current step was described. Taking the FT of the derivative of the potential and current versus time gives the impedance as a function of frequency. However, some authors [100, 101] tried to extrapolate the obtained results to low frequencies beyond the experimental values. If the data are acquired during time T = NAt, the information in the measured signal is obtained for frequencies from l/T up to the Nyquist frequency l/2At, where At is the sampling time. It has been shown [103] that extrapolating impedances to frequencies lower than l/T introduces artifacts. In addition, the measured... 

To minimize data acquisition time, the overall magnification of this system was made as small as possible, consistent with the spatial resolution desired. Since the image is sampled by the discrete pixels of the CCD camera, the Nyquist sampling criterion must be obeyed to avoid the generation of artifacts. There is an explicit relationship describing how the number of counts per pixel depends on the optical design. We know from Equation (1.2) that the radius of the Airy disk r is equal to 0.61A/NA (since 2r). If the size of the CCD pixel is p, then the Nyquist sam-... 

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