It may be the DVR recognizes the input signal source incorrectly and needs to plug and unplug the BNC cable again. 2. It may be the problem of coaxial line, so the coaxial line needs to be replaced 3. The DVR may not support the input source of the coaxial HD camera (including signal mode, resolution, frame rate). It is necessary to switch the signal mode to CVBS through the five-way switch of the camera, and then switch to the correct input source through the OSD menu of the camera.