7.1 Linear-phase FIR transfer function Likewise, for a Type 3 or 4 filter H(1)=-H(1) implying H(z) must have a zero at z=1 On the other hand, only the type 3 FIr filter is restricted to have a zero at z=-1 since here the degree n-1 is even and hence H(-1)=-(-1)H(-1)=-H(-1)7.1 Linear-phase FIR transfer function