a、 Whether the video cables short or open circuit, or is it connected to a bad case of weld and video.
b、 The video cables suffered from strong electrical interference, video cables and high voltage cables can not be walking in the same the line, at the same time, please select shielded cable with good quality.
c、 In the whole system, only use a single point of center device to connect to grounding, you can not use multi-point to grounding, otherwise it will cause common mode interference. Please access the screw in the rear of DVR to earth grounding.
d、 Please check whether the monitor, the camera and wires exist the problem of aging lines.