PURPOSE: To make the spectrum of an input time series flat by adopting the three-braid constitution for an inverse filter used to make the spectrum of a discrete time series flat.

CONSTITUTION: The 2nd-order systems of the three-braid form are combined to constitute the titled filter. A caption D in figure signifies a delay element by one word. When the order (p) of the three-braid type digital inverse filter is an even number, the transfer function expressed in equation is realized. A coefficient c1 is a value limited between plus 2 and minus 2 and contributes equally to the flat spectrum because of the symmetry of the structure of the inverse filter. Thus, effective expression is attained by a few number of bits than that of a conventional inverse filter.