From Eq. (B.2), we that the impulse response is alwayszero for . Any filter having a zero impulse response prior to time is said to be causal. Thus, a tapped delay line such as that depicted in Fig. B.1 can only implement causal FIR filters. In software, however, we may easily implement non-causal filters as well based simply on the definition ofconvolution. However, noncausal filters are never precisely physical, of course, since events are causal in the real world (disregarding certain effects in quantum mechanics).