This is due to the black burst signal (video reference/house sync) either not being received properly by the video card (AVID Video Board/ABVB, or Meridian Digital Media Board), or being bad.In one case, the black burst signal was being distorted when going through a BNC "T" connector, that fed the video card and the USD. Reconnecting all cables resolved the problem. Trace the entire signal path of the black burst signal, check and re-seat all connections. If this doesn't resolve the problem, try a different black bust generator.