![]() ![]() ![]() These techniques could also be used in, but they were not specially mentioned there. Along with the methods considered above, there are alternative techniques of demodulation and extraction of undistorted biometric signals. To eliminate this drawback in a video-pulse sensor directly measuring the delay, the linear section of the discriminator aperture should be at least several times longer. 7 suggests that, in a video-pulse sensor with a narrow-aperture discriminator, even small permanent offsets of the delay (distance) from the zero point of the discrimination characteristic may give rise to the second harmonic of breathing and heartbeat, as occurred in a monochromatic sensor with amplitude processing. The solid line shows the initial phase fluctuations, which are proportional to the delay fluctuations. Average-centered output realizations of breathing and heartbeat calculated at a nonzero offset of the delay ∆τ 0 = 0.25 ns from the zero point in a narrow-and wide-aperture discriminators are given in Figs. The increments in the delay due to breathing and heartbeat have been calculated using formulas (2) and (5). ![]() Figures 5 and 6 show the outputs of discriminators with broad and narrow discrimination characteristics, D(∆τ) and D 00 (∆τ), respectively. The situation can be improved, for example, by using broader strobe pulses and longer delay 2∆θ between them i.e., by substantially extending the linear section of the discrimination characteristic compared to the case shown in Fig. However, in the case of monochromatic signals, the quadrature processing and phase demodulation entirely eliminate nonlinear distortions of a biometric signal, a result that can hardly be achieved with video-pulse signals. For one quadrature of a monochromatic signal. ![]()
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