# difference equation block diagram delay or advance confusion

When does the unit delay, become unit advance essentially.

Goal is to deduce difference equation from block diagram. Green text is notes about what the value should be in that node of the diagram.

It appears that the difference equation should be equal to

\begin{align} y[n+2]&=x[n]&-&0.9y[n]\\ x[n]&= 0.9y[n]&+&y[n+2] \end{align}

Green text denotes my personal notes.

$$x[n] \longrightarrow \boxed{ z^{-1} } \longrightarrow y[n]$$
Then either $$y[n] = x[n-1]$$ or $$x[n] = y[n+1]$$ are both correct and equivalent expressions.
• At the summing node you have $y[n+2] = x[n] - 0.9 y[n]$ and yes the derivation is correct. You can best check this by sending a simple test signal to the system... – Fat32 Nov 15 '18 at 22:31