From Eq. (5.6), we see also that the impulse response is always zero for . Recall from §5.3 that any LTI filter having a zero impulse response prior to time 0 is said to be causal. Thus, a tapped delay line such as that depicted in Fig.5.5 can only implement causal FIR filters. In software, on the other hand, we may easily implement non-causal FIR filters as well, based simply on the definition of convolution.