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frequently asked questions

FREQUENTLY ASKED QUESTIONS


What causes hum and ground loops?

Hum and ground loops are typically generated two ways 1) differences in voltage between equipment grounding points and 2) magnetic coupling. A long cable run may suffer from both problems, video equipment is not usually connected to the same ground terminal in different parts of a building and the cable may be run alongside other wires and cable.  
Where do you locate the hum eliminator for best results?
The best place is at the end of the cable run near the monitor, however, you can experiment with more convenient locations. Remember, a hum eliminator can handle a large ground loop current so it doesn't need to be placed in the most ideal spot. A single unit usually should eliminate the problem, a second unit can be added in series for severe problems with little effect to the video.
How does a hum eliminator work?
Ground loop current which flows through the shield of coax video cable produces a magnetic field in the core of the hum eliminator. This creates an inductor, or choke. At power frequencies, the high impedance of this choke in the ground path effectively blocks the hum from passing through the unit. The video signal, on the center conductor, is shielded from this choke (by the cable's ground shield) and passes through the unit unaffected.
Why use a hum eliminator instead of a video isolation transformer?
The major advantage is high bandwidth. The low frequency response extends to dc, avoiding an effect called field-rate tilt seen when using isolation transformers. A high frequency response of over  100 MHz makes hum eliminators suitable for even high-definition and projection TV systems. An isolation transformer typically has a bandwidth no greater than 10 MHz.
Can a hum eliminator be used with PAL and 50 cycle systems?
Yes. 1) PAL and NTSC video need approximately a dc to 6 MHz bandwidth to operate. A hum eliminator has a bandwidth of dc to over 100 MHz, making if suitable for PAL and NTSC as well as HDTV and projection TV signals. 2) The impedance of the choke in the ground path is made large enough to block hum from both 50 and 60 cycle sources.
 

2002 Empire State Filter Co., Inc.   1559 Lincoln Avenue, Holbrook, NY   11741
  Phone: 631-589-0950   888-345-8370   www.empirefilter.com