These graphs depict the same transfer function as <a href="//commons.wikimedia.org/wiki/File:Frequency_response_of_3-term_boxcar_filter.gif" title="File:Frequency response of 3-term boxcar filter.gif">File:Frequency response of 3-term boxcar filter.gif</a>.  But here, the amplitude is a signed quantity.  And where it is negative, the quantity π has been added to the phase plot (before computing the <a href="https://en.wikipedia.org/wiki/principal_value" class="extiw" title="w:principal value">principal value</a>).  The purpose is to illustrate the linear-phase property of the FIR filter.
original: <a href="//commons.wikimedia.org/wiki/User:Bob_K" title="User:Bob K">Bob K</a><br> vector version: <a href="//commons.wikimedia.org/wiki/User:Krishnavedala" title="User:Krishnavedala">Krishnavedala</a>
2017-08-16
<span class="int-own-work" lang="en">Own work</span>