HDMI noise is bad no matter what context. Imagine, spending all the time and money into setting up and wiring an elaborate HDMI system for yourself or a client only for there to be a buzz, whine, hiss or chatter that ruins an otherwise perfect experience.
A lot of what causes HDMI noise boils down to what is referred to as interference, which can be further broken down into two flavors: ingress (entering) or egress (exiting).
Any time a product is built for high frequencies or has some internal oscillator over, say, 1Mhz, the FCC has rules you must abide by. One rule, FCC Part 15, essentially limits how much your product can spew out in the form of electronic emissions.
Since HDMI is operating in the stratosphere of the spectrum, it must abide by the Part 15 rules, so all the digital frequencies (TMDS video) that is traveling down stream has to be contained inside the cable.
If it starts leaking out, it could very likely cause problems in other devices throughout that environment.
These same rules also apply for ingress, which HDMI also has a set of built-in safeguards to fight off ingress noise. One way it does this is by using a balance line cable assembly (a.k.a. differential signaling), and is a wonderful way to provide noiseless cable transmissions.
HDMI also uses on a thin-shielded wrap around the ever-so-critical TMDS lanes to prevent high-frequency emissions and ingress from disturbing the signals.
At the same time, however, HDMI also uses a braided type of shield around the entire assembly. In fact, some cable products use two or three braided shields. This helps reduce ingress leakage into the signal at frequencies below 1MHz.
That being said, the majority of noise issues in the field are related to ingress, with nearly all of them involving Cat 5 and 6 cables, which have no shielding to speak of. Even the foil that generally comes with shielded Cat 5 and 6 only works in the higher frequencies.
As such, any problems related to ingress interference generally appear at the lower end of the spectrum.
But what’s causing the interference? 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 can all cause ingress interference which then leads to HDMI noise.
Combating HDMI Noise
So how do you stop HDMI noise? How much equipment do you need, how much does it cost and what preventive maintenance can be done? There’s a variety of methods in which this can be achieved.
Using a Conduit Pipe (or RMC)
One sure way to reduce the chances of interference is to use a conduit (pipe) as the chase you send your cable through. Don’t try to substitute it with plastic; it must be made of some type of metal, as the metal is what shields the line from interference. Typically, these conduits are referred to as RMCs (or Rigid Metal Conduits) and use materials such as aluminum, coated steel or even stainless steel
Any cabling that is chased through RMC is pretty much protected from any outside electrical ingress. The pipe should also be grounded at least at both end marks. Which, speaking of grounding…
Ensuring Proper Grounding of the Cable
Poor grounding can also be reasons for ingress to overtake the signal and cause intermittent HDMI noise. When running long lengths of cable there will always be some potential difference along these wires from beginning to the end.
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 RMCs is that they provide a wonderful return path for grounding because it is so massive in size.
However, many installs don’t have the luxury of using pipe. In this case, you must proceed with some caution as you meander your way through the congestion of other cables that may be in the walls already. This is especially true for cables that are prone to emitting noise.
Most of the time, these ‘noise-emitters’ are AC related, which means stay as far away from AC lines as possible if you want to avoid HDMI noise.
On the other hand, BX lines are less of a concern, as they are shielded and grounded already, though these aren’t commonly found in residential applications.
A good rule of thumb is that if there’s no other way of getting around these cables: do not run them side-by-side. Simply cross over, and continue onward. Perpendicular crossing is the best and will reduce the amount of noise induced to the cable.
Addressing Category 5 and 6 Cables
Most traditional ATDs (baluns) that get a fair amount of use can be purchased with a single or dual power supply (also affectionately referred to as wall warts).
For convenience reasons, single supply systems are gaining more popularity, and as such, with only one power supply to deal with, it ends up making 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.
There is little that can be done with the ground in this instance.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 and thus HDMI noise. Here, the more wires used for grounding the better.
Cutting it off at the Source
All of these issues so far involve shielding or protecting against any ingress noise with HDMI, but is there a way to simply stop the problem at the source of emission? Yes and no. The first thing to do is find the hardware that is causing the noise.
If intermittent HDMI noise 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. A simple turn it off, check the quality, then turn it on again before moving to the next.
By process of elimination you may be able to find the problem. However, if you need more help, a cheap AM radio will do the trick. Tune to mid band and turn the volume up.
Since noises affecting HDMI 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.
At that point, use the radio 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 also want to enlist the help of a licensed electrician to help eliminate any potential liability.
