+This defines the input or output as a periodic value with a phase width of 360, like
+the compass rose. Any value reaching the filter's input or leaving the filter at the
+output will be transformed to fit into the given range by adding or substracting one phase
+width of 360. Values of -270, 90 or 450 applied to this periodical element will allways
+result in +90. A value of 630, 270 or -90 will be normalized to -90 in the given example.
+<period>
+ <min>-180.0</min>
+ <max>180.0</max>
+</period>
+
+