HDMI and ground loops

In my Audio and video signal susceptibility classes I wrote that digital video interfaces like DVI and HDMI can also have ground loop noise problems as they use a combination of balanced and unbalanced signals. Ground loop problem is one very possible source of intermittent problems over long distance in HDMI installations. This article is a continuation to this article giving more details on HDMI interface and how it reacts to ground loops.

HDMI interface uses low level very high speed differential signals (TMDS signal with +-780 mV signal at few Gbit/s speed) for video data and single-ended TTL level signals for control signals. Disturb either one of them, and you have problems in video and audio output. The high bandwidth video signals use differential signaling without ground reference, so they are pretty insensitive as such to ground loop problems as long the ground noise is less than what the receiver can handle (common mode range, CMRR). Too high voltage on the ground will still disturb the signal. Susceptibility class for this signal could be somewhere between low and partial immunity.

There is one important signal in an HDMI cable which is unbalanced (and therefore considerably less immune to noise). This is the DDC/CEC signal. It has a much lower bandwidth than the TMDS signals, but this doesn’t make it less immune to noise. DDC/CEC signal is very important because HDCP key exchange is done over the DDC (Display Data Channel) that is basically an I2C serial bus, living on pins 15 (clock) & 16 (data) with ground on pin 17. The I2C bus communications belongs to as data communications interface to Susceptibility class: Low immunity.

Based on this I would pretty much put HDMI to audio and video signal susceptibility class Low Immunity (same class as composite video, VGA, USB etc.). This classification applies also to DVI interface (it is very close to HDMI).

A common HDMI problem is that Picture Comes On, Then Goes Off. In most cases, it is one of two things: video TMDS channels has a high bit error rate or HDCP key refresh problems. Video TMDS channel usually has high error rate because of too long or marginal quality cable (but also severe ground noise could cause this I think). The second case is the DDC line is just not quite making it and HDCP is not getting a new refresh key. This could be caused by too long cable or ground loop disturbing DDC line signals.

One of the first things you must keep in the back of your noggin is that the longer you go, the more susceptible you are in real-world conditions. There are always ways to overcome signal noise induced from ground loops. Many pieces of consumer AV equipment are NOT connected to mains “earth”, to avoid the ground loops. Sometime these ground loop and other HDMI problems require careful design of both electrical and AV installations and may also require additional equipment.

3 Comments

  1. tabletki poronne says:

    There is certainly a lot to know about this subject.
    I love all the points you made.

    Reply
  2. Tomi Engdahl says:

    https://www.techhive.com/article/3063590/home-audio/how-to-get-rid-of-hum-and-eliminate-other-noises-from-your-audio-and-video-systems.html

    There are three methods for removing USB (and HDMI) cable noise. One is to use a cable with a ferrite noise suppressor sleeve (that big round slug at one end. You can also buy a clip-on ferrite noise suppressor). These are sometimes called a ferrite bead.

    The second method is to run a wire that’s less resistive than the USB/HDMI cable shielding from the case of the USB audio interface or HDMI-connected audio component to the case of your computer.

    The third method is to get a USB noise filter (I’ve never seen one for HDMI, but an HDMI adapter could work), which is actually a USB re-transmitter that splits the shield connection. These cost around $50 and are said to indeed eliminate the noise

    Reply
  3. Tomi Engdahl says:

    Understanding & Fixing HDMI Noise
    https://www.cepro.com/article/understanding_fixing_hdmi_noise

    HDMI noise issues can be of two flavors, ingress (entering) or egress (exiting). In HDMI we are concerned about both.

    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.

    The only way to prevent ingress and egress noise from HDMI cabling is shielding.

    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

    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). s 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.

    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.

    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.

    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.

    Reply

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