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
|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
|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
|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.