Video hum bars often look like a light or dark bar, moving from the bottom of the picture to the top, then repeating about every four seconds. This occurs when several pieces of equipment connect to ground, but are actually at different potentials.
Often while dubbing video or audio tapes, several pieces of electrical equipment are involved. Each is powered by the A.C. line, but they are also connected to one another by RCA or s-video connectors. This, in effect, confuses the ground circuit. Equipment tries to connect to ground via other equipment, as well as it's own ground. This exhibits nasty side effects, such as an audio hum, or a video "hum bar" where a dark (or light) bar rolls up through the picture.
Try powering everything from one A.C. outlet. Also, try unplugging select pieces of equipment in your capture chain. When the hum bar disappears you've found the offending equipment. Isolation transformers, optical couplers and other devices are available that can correct the ground loop.
Disclaimer: This article is for informative purposes only. TGrant Photo & TGP Sales is not responsible for any damaged cause to your equipment or to yourself by changing the AC configuration of your equipment
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