“HDTV and Your AC Power Grid”

Reprinted from Audio Video International magazine
"Industry Leaders Speak Out" November, 2006


You have probably noticed that most HDTV sets now come with a 3-pin AC power plug versus a 2-pin polarized plug. This transformation is based primarily on safety concerns and more stringent electrical codes. Generally, a single HDTV set will work just fine with a 3-pin AC plug in a residential environment, but in a retail setting, a 3-wire power cord may cause problems such as “hum bars” or “flag-waving.” Remember HDTV sets are designed for the home, not for a retail environment where ground loops are (usually) inevitable and where many HDTV sets are plugged in to the same electrical circuit…and, the problem can grow when there are multiple AC power circuits fully loaded.

A ground loop occurs when the grounds of two pieces of electronic equipment are connected together through two paths, forming an electrical loop. When a ground loop includes the electrical wiring of a building through three-wire power cords, trouble often ensues because 60 Hertz can produce visible artifacts on a video display. In that case, imperfections in the building's electrical grounds cause 60 Hz current to flow in the ground loop, adding interfering signals to the video input.

We get many calls from retailers who are exasperated by power-line related picture problems with their HDTV sets. When we visit retail stores, we find problems that can usually be traced down to several issues, sometimes in combination. Satellite receivers and/or master antenna systems are often used as a source for HD content; however, they can be primary causes for electrical interference in a 3-wire electrical system by creating ground loops. Analog inputs on HD sets can also be susceptible to ground loops. If you have a really well-grounded electrical system, three-wire cords help maintain clean power input and provide electrical safety as well; but if you have differences in AC potential between the grounds of electrical outlets, then power-line related ground current may flow between the televisions, potentially corrupting the video. You may wish to have a qualified electrician measure the voltage potential between grounds at several AC receptacles on the same circuit…ideally, it should measure 0 volts, but anything greater than a few tenths of a volt could cause problems with picture performance. 

We also often find electrical circuits overloaded from the sheer number of devices the circuits are trying to power. This problem is very prevalent in older retail stores since it would have been unimaginable that we’d be plugging in so many new devices to the store’s electrical system when the store was built. In many cases, we have measured line voltages well below the safe harbor of 110 volts. Many of today’s HDTV sets won’t operate properly (or at all) if the line voltage is too low. While your electrician is there, make sure everything you wish to demonstrate is turned on and have the electrician measure the circuit’s available line voltage. If the measurements indicate anything below 110 volts, it’s a good idea to have several properly grounded new circuits installed.
 
Finally, you may need to examine your HD A/V signal distribution system, especially if picture problems have increased with an expanded HDTV SKU count. Generally, the best component video distribution systems employ a buffered differential signal bus using Category 5 UTP cable to carry high definition video and audio signals from the source to the HDTV units on display. Cat 5 balanced line distribution systems tend to reject interference generated by AC grounding problems, yielding better video performance in your showroom. However, no distribution system can help improve the picture if electrical circuits are overloaded to the point of low AC voltage. My point is simple…these are complex problems that need careful attention; but, with some effort you can build a solid electrical and signal distribution system that will provide razor-sharp HD content which should enhance your opportunity to sell more HD Televisions…feel free to call us anytime for help.

Author: Bob Sollee
President
Audio Authority Corporation
Lexington, KY