Understanding & Fixing HDMI Noise
HDMI noise issues can be of two flavors, ingress (entering) or egress (exiting). In HDMI we are concerned about both.
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.
There are basically two types of ingress: narrowband noise and impulse noise.
Narrowband noise is typically RF signals that are very narrow in bandwidth. These frequencies can be as small as 5KHz to 25KHz in width and are sometimes modulated at several different frequency bands.
Impulse noise is a form of RF noise that tends to vary in time. This can be a real SOB to fix due to its random timing. The odds of snagging these impulses at the precise time they propagate into the assembly are virtually zero. In HDMI, we really don’t have the luxury of TDMA (Time Division Multiple Access), or OFDMA (Orthogonal Frequency Division Multiple Access). So you have to use brute force: Shield the SOB.
This is why HDMI cables have so much shielding. One day I grabbed one of our DPL member’s cables - with permission, of course - and started digging into the cable to expose the TMDS lanes. It only took 200mv of amplitude and 1μs of time to totally kill the HDMI signal both for video and audio.
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.
Jeff Boccaccio is president of DPL Labs. Jeff can be reached at firstname.lastname@example.org. Have a suggestion or a topic you want to read more about? Email Jeff at email@example.com
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