Video display has scrolling lines or static bars

Issue

Ground Loops cause hum bars viewed on a monitor display. They look like a horizontal band or bar across the video monitor that slowly moves up the video screen. These bars can be barely noticeable, or can be so bad that the video monitor loses lock and breaks up the picture. If the video camera is Line-Locked to the 60 cycle main power, the bars may stand still in the picture, but they still obscure picture definition.

Product Line

Pelco Cameras

Environment

All Pelco camera products

Cause

The source of the 60 cycle bar originates from the power industries use of local grounds to balance their power grid. Everywhere 60 cycle power is used, a local ground is attached to the power grid to return all unbalanced current flow to ground. Also, the source and destination of a video signal can be at differing ac or dc earth potentials. Earth loop currents flow and cause longitudinal hum to be introduced into the video signal. Video hum is low frequency (50 or 60 Hz mains frequency or it's harmonics) noise from the ground lines which has influenced the video signal, causing degradation of the displayed signal.

Resolution

Isolating video signal is typically achieved by installing a (Pelco GIT100 if available) Ground Loop Isolator in-line on the video coaxial at the head end or at the camera. Also the camera input with floating ground works nicely for small ground potential differences. Differential inputs are also used in applications where a video signal is transmitted through twisted pair wiring (some CCTV applications which use twisted pair interfacing equipments).

Fiber-optic isolation works well in applications where complete electrical isolation is necessary.