Recently we have been experiencing more phone calls about HDMI drop out issues, some being noise related. Noise is always bad for any high-performance signal transmission. The noise that we will be discussing today is from the interference category.

This kind of noise can be of two flavors, ingress (entering) or egress (exiting). In HDMI we are concerned about both. Any time a product is built for high frequencies or some internal oscillator over, say, 1Mhz, the FCC has rules you must abide by. One rule is FCC Part 15, which essentially limits how much your product can spew out in the form of emissions or trash. Since HDMI is operating in the stratosphere of the spectrum, it also must abide by the Part 15 rules. So all the digital stuff (TMDS video) that is traveling down stream has to be contained inside the cable. Otherwise, if it is leaking out, it could very likely cause problems in other devices throughout that environment.

The same rules apply for ingress. What goes out can also come in.

HDMI does have built-in safeguards to fight off ingress noise. One way it does this is by using a balance line cable assembly (differential signaling). This is a wonderful way to provide noiseless cable transmissions. HDMI also insisted on a thin-shielded wrap around the ever-so-critical TMDS lanes. This is to prevent high-frequency emissions and ingress from disturbing the signals. At the same time, however, HDMI also insisted on a braided type of shield around the entire assembly. In fact, we have had some cable products come in with two or three braided shields. This was included to the spec to reduce ingress leakage into the signal at frequencies below 1MHz.

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The majority of noise issues in the field are related to ingress, with nearly all of them being Cat 5 and 6 – no shield there to speak of. In this case we need the braided shield. The foil that generally comes with shielded Cat 5 and 6 only works in the higher frequencies. All of these problems appear in the low portion of the spectrum.

What causes these issues? To name just a few: SCR (Silicon Control Rectifiers) dimmers; security alarm systems; microcontrollers (I/O); compressors; motors; neon signs; thermostats; touch lamps; AC power distribution systems; ceiling fans and controls; street lighting and electrical transformers.

Combating HDMI Noise

So how do you stop this? How much equipment do you need, how much does it cost, and what preventive maintenance can be done?

One sure way to reduce the chances of interference is to use conduit (pipe) as the chase you send your cable through. It can’t be plastic; it must be made of some metal and is typically called RMC or (rigid metal conduit).  is type of material is usually found in aluminum, coated steel, or even stainless steel. Any cabling that is chased through RMC is pretty much protected from any outside electrical ingress.  e pipe should be grounded at least at both end marks.

{pagebreak}Speaking of Grounds …
Poor grounding can also be reasons for ingress to overtake the signal and cause intermittent HDMI A/V. When running long lengths of cable there will always be some potential difference along these wires from beginning to the end. I don’t care if it is video, audio, DC, data, or ground. By the time you reach your end destination there will always be some resistance in the line and this resistance can raise havoc with any type of signal. One of the nice things about RMC is that it provides a wonderful return path for ground because it is so massive in size.

But many installs don’t have that luxury of using pipe so you must proceed with some caution as you meander your way through the congestion of other cables that may be in the walls already, especially those that are prone to emitting noise. Most of the noise-emitting devices I mentioned previously are AC related, which means you need to stay as far away as possible from AC lines such as Romex.

Don’t be as concerned with BX since it’s shielded and grounded already. But again, we don’t typically see this in residential applications. If you must go near these cables don’t run them side-by-side; cross over and continue. Perpendicular crossing is the best and will reduce the amount of noise induced to the cable.

Category 5 and 6
Most traditional ATDs (baluns) that get a fair amount of use can be purchased with a single or dual power supply (wall warts). Because of convenience, single supply systems are gaining more popularity. With only one power supply to deal with, it just makes the installs neater and easier. However, in many cases these products have electronics on both sides (transmitting and receiving). So when you power up with just one power supply the DC and the ground wires must be carried over the length of the cable to support the opposite end. The DC level will drop just by way of load and distance.

But there is little that can be done with the ground. It must return not only the DC component but must also act as the return for signal lines. As this resistance goes up, it reduces the effectiveness of the ground that connects source to sink. This high impedance ground can also add to the susceptibility for ingress interferences. The more wires used for ground the better.

Cut it Off at the Source
Is there a way to stop these problems at the source if you can’t stop the noise entering the cable? Yes and no. The first thing to do is find the hardware that is causing the noise. You should know the major culprits that cause these problems to begin with.

If intermittent HDMI occurs, you need to include these interference transmissions as part of your troubleshooting list. First, by way of trial and error, eliminate the ones that could be causing the problem. Just turn it off and check it again. By pure elimination you may be able to find the problem. If you need more help, get a cheap AM radio. Tune to mid band and turn the volume up. Since these noises are usually AC related, the radio will pick up low frequencies. If the screen drops out as you hear loud noise or static, you’ve probably found your problem.

Now use it as a field strength meter and locate the hardware that is causing the problem. The closer you are, the louder it will get. When you find it, determine what kind of noise it is and put some AC suppression on it. You may want to bring in a licensed electrician to limit your liability.