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.

1 Comment

  1. tabletki poronne says:

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

    Reply

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