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WHAT ARE GROUND LOOPS ?

Ground loops generally occur when there is a difference in potential between the various grounding points in a home theater audio and video system. When potentials like this occur, 60hz ground currents and high frequency noise can flow around the system and cause hum in both audio and video signals. Audio hum is the most obvious evidence of 60 hz ground loop currents as one can hear it directly from the speakers.

Video hum can be more frustrating to locate it source. It usually
manifests itself as a series of faint lines that rise up through the video image. Sometimes they are very faint but, occasionally, can be quite severe.

The difference in ground potentials in an A/V system can be caused by a number of factors. One of the most notorious involves the grounding of the cable TV coax where it enters the building.

The cable grounding block should be securely grounded to the electrical service ground as per the National Electrical Code (NEC). If this grounding is poor or nonexistent, external 60 Hz currents can circulate throughout the shield of the A/V system interconnect cables.

If the system is properly grounded and video hum still exists (which is very common, incidently), it is probably caused by grounding problems in from the local house AC wiring.

A correct method of bonding and/or grounding a cable TV or satellite installations is shown in the diagram below;

DIAGRAM I
EXAMPLE OF CORRECT GROUND BLOCK BONDING & GROUNDING AT THE SERVICE ENTRY TO A DWELLING

Figure I Ground Block Installation


GROUND LOOP INDUCED VIDEO & AUDIO HUM.

Multiple paths to ground formed when electronic devices using three wire power cords are powered by line voltage and connected together by signal cables. In a properly functioning system, the only signal carried on the coax is the independent audio and video. If an imbalance in the AC power ground system occurs, a small AC current may flow through the video (or audio) cabling and manifest itself as audible hum or a scrolling horizontal bar on the television screen. Review the diagram II.

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AC power is regulated at the power supplier's generation station to 60 cycles. Video uses a refresh rate of 59.94 cycles. This is harmonically related to the video color burst frequency and horizontal sync rate. This slight difference in frequency means that any AC induced hum bar will never stay in one place but will scroll from the bottom to the top of the screen.

Both the signal source chassis and the display chassis will be grounded (we are presuming these are state of the art video monitors). When the coaxial signal cable is connected between the two devices, essentially, the two chassis are electrically bonded. If both devices are plugged into the same outlet, there is very little chance of a ground loop problem occurring. The problem usually arises when the source and display are not plugged into the same outlet.

If everything is connected
properly and there is a low impedance ground at all the outlets, there should be no problem.


GROUND LOOP INDUCED VIDEO & AUDIO HUM (con't).

If the hum bar appears or is heard in the audio, this means that there is a difference in ground potential between two or more AC outlets. In addition to the electrical outlets, the source or display device itself may improperly cause current to be drawn through the ground connection. When this happens, it causes a voltage to appear on the ground relative to a properly functioning outlet. This elevated voltage potential can cause ground loop current to flow and it will show up on screen as a scrolling hum bar.

The hum in the system might also be induced into the video cabling by being routed too closely to an AC power cable.
The signaling and AC power cables should never be bundled together.

POTENTIAL CAUSES AND CURES TO MINIMIZE GROUND LOOP INDUCED HUM.

A video "hum" bar is a dark, horizontal bar in a television picture caused by hum interference in the video signal. For standard NTSC video displays, a disturbance which creeps upward on the video monitor. The movement is caused by the slight difference in frequency between the NTSC video refresh rate at 59.94 Hz, and the 60 Hz of the power line. The 0.06 Hz frequency difference means it takes about 16 seconds to move from bottom to the top of the screen.

As explained earlier, ground voltage differences are often generated by the parasitic transformer in building wiring and are a function of branch circuit load currents. Since many, if not most, loads draw their power-line current as pulses at each peak of the ac cycle (i.e., at 120 Hz), a pair of disturbances is most commonly seen.

The shield of coaxial video cable is the return path for the video signal current but, as shown in the drawing (diagram II below), it also becomes a path for power-line ground current.

The magnitude of the current in this loop depends on the system ground voltage difference and the total resistance in the loop, in accordance with Ohms Law. This also means that a voltage drop, proportional to the resistance of the shield, will appear across the length of the cable.

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Because driver (appliance I) and receiver (appliance II) impedances are equal, half of this voltage is added to the signal as seen by the receiver. As in all unbalanced interfaces, the shield impedance (resistance) is common to both the signal and the ground current paths, creating this mechanism called common-impedance coupling.

DIAGRAM II
SCHEMATIC OF GROUDING DIFFERENTIALS RESULTING IN HUM DISTORTION

DIAGRAM II, SCHEMATIC OF GROUDING DIFFERENTIALS RESULTING IN HUM DISTORTION

At power frequencies, about half of the voltage drop over the length of a coaxial cable's shield is added directly to the video signal. Therefore, the objective of any solution is to reduce this voltage drop. Obviously, it makes sense to reduce system ground voltage differences as much as possible, but this often requires expensive utility power modifications or rewiring. Likewise, use of shorter cables or types with lower shield resistance will reduce the coupling impedance. But, if none of these measures are practical, the general solution is to reduce shield current by inserting a device in the signal path that has high common-mode impedance. There are three basic kinds of devices to do this:



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6/22/2017