[Xy-pic] Diagrams of digital LTI filter

[Nicolas Ratier]

I'm preparing a course on Digital Signal Processing.
I have to draw diagrams of digital LTI filter on the form :

                           z
                X o-->--o-->--o
                        |     |
                      h |     |
                        o-->--o-->--o Y

I don't succeed in doing 3 things :
       + Write the letter X to the left of the node
         (idem for Y to the right of the node)
       + Write an array at the center of the line : i.e.  -->--
         (horizontally and vertically)
       + Write black and white circle and set the radius

With such an example, I can draw almost all the LTI filter.
(And release a LaTeX/xy-pix filter library)

Many thanks,
Nicolas

-------------------------------------------------------
\documentclass{article}

\usepackage[all,2cell,ps]{xy}

\begin{document}

\newcommand{\Xr}{\ar@{-}[r]}
\newcommand{\Xd}{\ar@{-}[d]}
\newcommand{\Xv}{*\txt{ }}

\entrymodifiers={+[o][F-]}
\xymatrix{
\Xr & \Xr^{z^{-1}} \Xd^{h_{0}} & \Xd^{h_{1}} & \Xv    \\
\Xv & \Xr                      & \Xr         & \, \\
}

\end{document}