# Who first coined "Direct Form I" and "Direct Form II"?

I see "Direct Form I" and "Direct Form II" commonly in literature to refer to FIR / IIR implementation from the filter transfer function, and reference it myself, but where did that terminology first originate so that I can properly credit that person?

For example:

• No response yet, so for now I assume it must have been "Dr. Direct" Jun 13 at 14:10
• i think the "first" direct form was the "direct" realization of the current output in terms of what is known: the current and past input and the past output. with no intermediate variables. the DF2 is "canonical" (meaning the number of states is exactly what the order of the filter requires) but requires an intermediate variable which is the input to the first $z^{-1}$ state. Jun 14 at 5:16
• the first time i saw the terms was in m. e. van valkenburg's Network Analysis from the 1970s. Jun 14 at 5:19
• there is this reference. Jun 14 at 5:27
• the M.E. Van Valkenburg thing is just a memory, Dan. it is not authoratative. Jun 15 at 0:03