368 15 Causalitv arid the Hilbert ‘fiansform input arid output signal exists if two input signals sl(t)arid rZ(t) are the same up t o time t o , then they will give rise to the same outpiit signals u l ( t ) and y2(C): rift) = ~ ( )for t < t o =.$ yl(f) = y ( t ) for t