# Why is $y(n)=x(n)-x(n−1)+x(n-2)$ a low-pass filter?

I know that it's a low-pass FIR filter with impulse response of length equal to $$3$$, but I don't know how to explain exactly why it's low-pass.

• Might want to revert title to original; there was room for confusion hence the earlier comment. Aug 7 '21 at 20:03

Observe the magnitude of frequency response (rescaled to 0 to 1):

where

\begin{align} H(\omega) &= e^{-j 0\omega} - e^{-j 1\omega} + e^{-j 2\omega} \\ &= 1 - e^{-j 1\omega} + e^{-j 2\omega} \tag{1} \end{align} following the time-shift property:

$$x(t - t_0) \Leftrightarrow e^{-j t_0 \omega} X(\omega) \tag{2}$$

Though it isn't particularly good, it's more "high-pass" than "low-pass" if we must choose. Example on White Gaussian Noise:

We can see the frequency response for all possible $$t_0$$ up to $$x$$'s length:

The more terms we add following the pattern, the better the high-passing. (Note: plots use DFT with dc bin at sample 0 and Nyquist at 16, so toward center = higher freqs).

### Explanation

$$x(n) - x(n - 1) + x(n + 2)$$ is simply a transformation of $$x$$. Whether it's "lowpass" or "highpass" is a frequency-domain description, so we describe the transform directly in frequency domain.

Here we're simply adding time-shifted and sign-flipped version of $$x$$ to itself. In frequency domain, this is described by $$(2)$$. So we have:

\begin{align} x(n) & \Leftrightarrow X(\omega) \\ x(n) - x(n - 1) & \Leftrightarrow X(\omega) - e^{-j 1 \omega}X(\omega) \\ x(n) - x(n - 1) + x(n - 2) & \Leftrightarrow X(\omega) - e^{-j 1 \omega}X(\omega) + e^{-j 2 \omega}X(\omega) \\ & ... \end{align}

To see the effect this has on any $$x$$, independent of $$x$$, we simply divide it out - which gives the frequency response:

\begin{align} H(\omega) &= Y(\omega) / X(\omega) \\ &= \left( X(\omega) \cdot (1 - e^{-j1\omega} + e^{-j2\omega})\right) / X(\omega) \\ &= \boxed{1 - e^{-j1\omega} + e^{-j2\omega}} \end{align}

and thus, for any $$x$$ with frequencies $$X(\omega)$$, the effect of the transform is $$X(\omega) \rightarrow H(\omega)X(\omega)$$.

### Code

Available at Github.

• @OverLordDragon: Nice answer and I follow how you got $H(\omega)$ but I didn't follow where you used the time shift property ? Thanks. Aug 7 '21 at 4:17
• @markleeds Time-shift + linearity: $x(t-1) \Leftrightarrow e^{-j1 \omega} X(\omega)$, so $\mathcal{F}(\text{stuff} + x(t-1)) = \mathcal{F}(\text{stuff}) + e^{-j1 \omega} X(\omega)$. Aug 7 '21 at 4:27
• I'm sorry for my denseness but are you saying that you are using that relation to calculate the last animated plot where you plot the result for different values of $t_{0}$. Thanks for your patience. Aug 7 '21 at 13:56
• @markleeds Right, it's just $x(t-2), x(t-3), ...$ and signs alternated, $+, -, ...$, which corresponds to $+e^{...2...} - e^{...3...} ...$ when taken as impulse response. Impulse response is $H(\omega) = Y(\omega) / X(\omega)$ and we factor the common $X(\omega)$ in $Y(\omega) = X(\omega) e^{...0...} - X(\omega) e^{...1...} ...$ which cancels and just leaves $e^{...}$'s. Aug 7 '21 at 16:49
• Thanks. Your patience and explanation is much appreciated. And very kool plot. Aug 7 '21 at 21:38