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Author Topic: Chasing hum bars? Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 06-03-2009 10:42 AM                        In RF land, a shield that isn't tied to ground at both ends is an antenna, whether it's an audio or video signal (or some other type of data). So any mucking with the shield connections (usually used to work around audio equipment design flaws) is definitely not for all applications. I've assembled plenty of complex audio systems, and I've never needed to lift a shield unless I was working around a problem in the equipment. The hum is usually caused by ground loops, where current flowing through the grounding system is causing multiple ground points to be at different potentials, and those differences are superimposed on the low level signals that are referenced to ground. You can fix this by either removing the dependency on the ground reference, or by tying every ground point together with very low resistance connections so that they're all at the same potential (this gets hard at RF). To remove the dependency on ground referencing, you'd use differential transmission (twisted-pair Ethernet is differential, and so is pro audio), or isolation transformers to provide galvanic isolation. This isolation isn't perfect, but the induced noise is usually at a much lower level than the signal to start with, so the extra noise rejection the isolation adds brings the noise down to an acceptable level. Another cause of hum is inductive or capacitive coupling of noise into the signal lines. Because the impedance of the interconnect cable is not zero, a noise voltage can be imposed on the signal voltage. The shielding of a cable mostly helps with capacitive coupling (electrical charge), but inductive coupling (current-induced magnetic fields) remains an issue. Differential transmission helps greatly eliminate noise that's inductively coupled into a signal line by canceling out signal added to both lines of a pair in the receiver. You can use a balun to take single-ended signals and transmit them differentially over, for example, Cat. 5. If your VGA cables have a long run in close proximity to AC current they will pick up noise regardless of any shielding. A little bit of physical separation helps greatly.  |  IP: Logged Jonathan Bodge Film Handler Posts: 83 From: East Dorset, VT Registered: Nov 2006 posted 06-08-2009 11:03 AM                     Whenever we have hum bar issues on video shoots (usually when a engineering truck shows up and there is a mix of AC between truck & location) I knock on the door and ask for a humbucker. A small heavy passive box with a BNC in/out. This has always fixed it in the past but we know gremlins will always try to change it up in the future!  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 06-08-2009 11:59 PM                        I've had RF problems in cases where several audio sources, some fed from different power circuits, terminated in a common mixer. The different wiring lengths from the breaker panel to the source equipment caused all kinds of problems before we figured that one out.  |  IP: Logged

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