1. A filter for which the phase shift is zero for all frequencies. Zero-phase filters are anticipatory and hence are not physically realizable, i.e., half of the energy arrives before the time reference so that one gets output before the input arrives. If the input to a zero-phase filter is symmetric, then the output will also be symmetric. Zero-phase filtering can be approximated by using a linear-phase filter, a mixed-phase filter which shifts component frequencies proportional to their frequency, and then delaying the time reference. A zero phase filter produces no phase distortion. See Figure P-2 and phase characteristics. 2. A wavelet symmetric about zero time. It is the shortest possible wavelet for any given spectrum. The SEG standard for display of a zero-phase wavelet is shown in Figure P-6.