Editing Distortion (section)

===Group delay distortion===
Can be found only in [[dispersion (optics)|dispersive media]]. In a [[waveguide]], [[phase velocity]] varies with frequency. In a filter, group delay tends to peak near the [[cut-off frequency]], resulting in pulse distortion.  When analog long distance trunks were commonplace, for example in [[12 channel carrier]], group delay distortion had to be corrected in [[repeater]]s.