In a general sense, ringing refers to an undesirable oscillation exhibited in a signal. This video artifact is common to recordings created using less sophisticated, early model cameras and VTR equipment (particularly early U-matic equipment). It can be accentuated by over-enhancement or sharpening of the image using processing hardware or CRT monitor controls. When recorded in the signal on tape, it becomes part of the image.
Can it be fixed?
No, there is no remedy if the ringing has been recorded into the source signal.
If not present in the recording but ringing is detected on playback, then the system needs routing adjustment or the VTR, cabling or TBC may need adjustment or maintenance.