Thinking in Isolation – A Primer on Ground Loops
Your new plasma HDTV is beautifully hung on the wall. The cables are run behind the drywall and under the floor to an equipment rack built into an unused closet space. The speakers are a recessed bit of invisible magic. Everything looks like a photo spread from Architectural Digest. Until you turn it on…
There is a humming sound coming from the speakers. A faint dark bar rolls from the bottom of the image to the top, changing colors and distorting the picture as it goes. What’s this? How is this possible? All this equipment is new and your home is only a few years old. Welcome to the nefarious neighborhood of the ground loop! The simple fact is that your installation is perfect but your electrical connections… well… not so much.
What is a Ground Loop?
Signal interconnects carry a very low current, often measured in millivolts (thousandths of a volt). Low voltage and low current connections are susceptible to ground loops (EMI, aka Electro-Magnetic Interference). If two pieces of gear are plugged into different power outlets there will often be a difference in their respective ground potentials. This is because the ground path from one outlet to the breaker box may be longer or shorter than the other. Or perhaps the two grounds are connected to different ground points in a home with multiple service panels. Or maybe the electrical wiring is fine, but the ground connection of your cable television, satellite TVRO or antenna system, which is properly connected to a copper rod driven eight feet into the ground close to the cable ingress) is forming the ground loop with your electrical service, which is grounded to a cold water pipe one-hundred feet away. Whatever the situation, there is an electrical potential difference between the two spots and that difference is going to manifest itself as a ground loop problem.
If a signal is passed from one component to another via an unbalanced connection, this electrical potential difference (electrical potential difference is measured in volts, by the way) causes a spurious current to flow through the cables. This current flow can create an audible buzz at the AC mains base frequency (50 or 60 Hz) and the harmonics thereof (120 Hz, 240 Hz, and so on), called mains hum. In a video signal the current flow can cause diagonal lines on the screen (hum bars and herring-bone) or the wide, barely visible bar that rolls from the bottom to the top of the screen (AC Beat interference).
Ground loops can even be dangerous. For example, the electrical potential at different points on the surface of the Earth can vary by thousands of volts, primarily from the influence of charged clouds. Such an occurrence can be hazardous to technicians working on long metal conductors such as long runs of power cable, even when the cables aren’t carrying their intended load! While it’s highly unlikely that we’ll face the dangers of electrocution in our home theater system due to a ground loop, it is almost a guarantee that we’ll have to struggle through the puzzle in order to peak the performance of our investment.
Ground loop noise is most often in the form of 60 Hz hum. Horizontal lines slowly moving up the monitor are the characteristic interference pattern of hum. These are frequently seen with Video projectors and displays where the display device has its case grounded via a 3-prong plug, and the other components have a floating ground connected to the CATV coax. In this case the video cable is grounded at the display end to the home electrical system, and at the other end to the cable TV's ground, inducing a current through the cable which distorts the picture.
Ground loops also make a transmission line susceptible to crosstalk and stray pickup from high frequency sources, creating the diagonal lines mentioned above. Once a video signal has been infected by hum or other noise it is impossible to filter it out because the noise frequency falls within the video band. Galvanic isolation is the best solution.
How do I Fix a Ground Loop?
Sometimes frustrated installers or system owners may try removing the grounding pin from the A/C cord connecting a device to the power outlet. Be forewarned! This can create an electrocution risk! The proper solution is to ensure that all metal chassis are interconnected, and the system is connected to the electrical service at ONE point. When this is impossible the only other solution is to use isolation transformers and implement a system of galvanic isolation.
Galvanic (magnetic) isolation is the principle of isolating functional sections of electric systems so that charge-carrying particles cannot move from one section to another. In other words, there is no electrical current flowing directly from one section to the next. Energy and/or information can still be exchanged between the sections by other means, however, such as by capacitance, induction, electromagnetic waves, or mechanical means. It is this very concept of galvanic isolation and electromagnetic coupling that is implemented in an isolation transformer. An isolation transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits. An isolation transformer allows a signal to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. In this way they also block interference caused by ground loops.
Isolating a video signal is more complicated than isolating audio or antenna signals, because the DC level of the video signal is important and video signals have very high frequency spectrum. Composite standard definition video can have bandwidth from 50 Hz to 6 MHz. Component HDTV can have a bandwidth to nearly 27MHz! A wideband isolation transformer is needed for this task. A transformer that can transfer the whole video frequency spectrum without adding distortion is very hard to produce. And just to add a bit of spice to the stew, specialized isolation transformers need to be developed for composite video, s-video, component video and RF antenna signals. Each is highly specialized.
Solution for Composite Video
Composite video, as you know, is the most common of all video connections. Older video formats such as analog laser disc, 8mm, VHS and Beta should be connected to the display or A/V Receiver using a composite interface for maximum performance. With a bandwidth extending from a few tens of Hertz to a maximum frequency of about 6 MHz and allowing for some 480 lines of interlaced resolution, the demands of this transformer design are well documented and tested. In systems where your VCR is connected directly to a cable TV feed, ground loops are often formed by the interaction of your home’s A/C power grounding and the remote grounding of the cable company’s transmission gear. Inserting a composite video isolation transformer (and sometimes and audio isolation transformer) between the VCR and the A/V Pre-Pro can eliminate many causes of EMI interference. Installing an isolation transformer is easy. Unplug the cable from the device under question and plug it into the isolation transformer. Then use a second, identical interconnect to connect the output of the isolation transformer back to the device being isolated. It’s as easy as that!
Solution for S-Video
Super Video has become known by the euphemism S-Video. S-Video is an interface protocol first introduced by JVC with the very first S-VHS video decks in the early part of the 1980’s. S-Video sends an analog video signal on two 75-ohm coaxial cables. One conductor delivers a luminance signal, which is a black and white wide-bandwidth television signal. The other delivers a chrominance (color) signal which would normally be a composite signal riding “under” the luminance information at a frequency below the 3.58MHz ‘color burst’ frequency.
Many sources transmit a video signal using this two-part interface. Contrary to common belief, S-Video does not improve resolution. That is a function of the bandwidth of the connection. S-Video is the equivalent of two Composite video connections and uses a four-pin connector called a mini-din. S-Video is actually a form of Component Video, but because of its composite roots it is incapable of transferring HDTV information. The difficulty in manufacturing a high quality S-Video isolation transformer resides in issues of proximity. Two separate transformer assemblies must be placed in a very small space and the connection of those transformers to a single mini-din connector must avoid signal cross-talk and leakage. Tight manufacturing tolerances and a good understanding of transmission line theory and EMI interactions are brought into play. Like the Composite Isolation Transformer above, connection is simple and requires only an additional length of interconnect.
Solution for Component Video
Component video is a type of analog video information that is transmitted or stored as three separate signals. Component video can be contrasted with composite video (such as NTSC or PAL) in which all the video information is combined into a single signal such as a TV broadcast. Component video comes in a number of configurations including the ubiquitous computer format, VGA, which uses five conductors to transmit Red, Green, Blue, Horizontal sync and Vertical sync. European SCART uses four conductors, matrixing the horizontal and vertical sync signals onto one conductor. For most consumer electronics enthusiasts Component means one thing, though. Three wires and a high definition signal are characteristics of the CE Component video transmission standard.
Component (also referred to as "YPbPr", "YPrPb", "PrPbY") is a color space (a mathematical construct) used in video electronics. YPbPr is the analog version of the YCbCr color space; the two are numerically equivalent, but YPbPr is designed for use in analog systems whereas YCbCr is intended for digital video. YPbPr is converted from the RGB video signal, which is split into three components, Y, Pr and Pb where Y carries luma information, Pb carries the difference between blue and luma (B - Y), and Pr carries the difference between red and luma (R - Y). In American HDTV systems the signal, when in the analog domain, is transmitted over three conductors, one for each of the Y, Pb and Pr signal components. A Component Video Isolation Transformer must address three separate connections. To make matters worse, it is vital that the precise timing relationship between the three signals that make up the full Component Video signal not be damaged. Therefore, each leg of a Component Video Isolation Transformer must be identical in electrical latency and bandwidth. Manufacturing tolerances become very, very demanding.
When employing a Component Video Isolation Transformer it is vital to use quality interconnects to avoid introducing RFI (Radio Frequency Interference) while you are combating the EMI (Electro-Magnetic Interference) ground loop? As with the above listed Isolation Transformers it is also a good idea to use the same interconnect quality and brand for the input and output of the Isolation Transformer to avoid slight impedance mismatches or internal signal reflection that could compromise the signal you are working so hard to improve.
In conclusion, ground loops and electro-magnetic interference can compromise the signal quality of even the very best devices. Ground loop interference is introduced into a system when two parts of the system, typically the source and the monitor, are plugged into different AC outlets. Electrical outlets that have different ground points, and cable television or satellite reception gear that is grounded in such a manner as to produce an electrical potential between multiple grounding points, cause ground loops. Because ground loop interference is in the signal’s bandwidth, filtering is not an option. The path of the interference must be broken. The correct way to break a ground loop is through the use of a high quality Isolation Transformer, and NOT by lifting the ground connection on a product designed to be properly grounded for safe use! High quality Isolation Transformers place significant demands on manufacturing tolerances. To get the best results its important to use high quality, properly shielded interconnects and to use the same style of interconnects on the input and output of the Isolation Transformer to prevent compromising the delicate signal through impedance mismatches or internal signal reflection.
Many thanks to Impact Acoustics for this article contribution.
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