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Just had a "spectrum engineering officer" from OFCOM (the UK's communications regulator) visit our office investigating alleged interference from our building with a local cell phone tower. After waving an aerial attached to what looked like a portable spectrum analyser around he decided that the HDMI cable feeding a set of four TVs was causing the problem.

This seems extremely unlikely to everyone in our office and we are wondering of their is any validity in his claim. We are a ground floor office in a modern concrete building and the phone tower is on the the opposite side of the street on the top of a multi-story car park (about 4 stories high).

Is it possible for a leaky HDMI to cause such interference?

(He did assure us that we haven't broken any regulations and that no action would be taken.)

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  • \$\begingroup\$ Googling your title brings up a few anecdotes of others having an HDMI cable knocking out wireless networks. I have a hard time believing there is a sufficient amount of power to interfere with the tower, but I don't know enough about it. \$\endgroup\$ Mar 20, 2020 at 15:47
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    \$\begingroup\$ HDMI = 300mW total (of which most is consumed by the TV set for receiving the signal, and maybe, possibly, 5-10% are in the air when the leaky cable is an "antenna"). Whereas cell phone tower = 3-5kW. So... \$\endgroup\$
    – Damon
    Mar 21, 2020 at 13:15
  • \$\begingroup\$ @Damon you're overestimating cell phone tower power, I'd presume. Think about it – if all city cell towers would be consuming 3kW continously, and there's one for every ~500 people, what that would cost in energy! \$\endgroup\$ Mar 21, 2020 at 15:02
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    \$\begingroup\$ @Damon the thing you're trying to imply (but not quite saying) is false. The power output of the cell tower isn't relevant here. The power received at the cell tower by an interference source, as compared to the power it receives from a handset, is. It's not about milliwatts vs kilowatts, it's about nearby milliwatts vs. a distant maybe 1 watt, in an inverse-square situation. \$\endgroup\$
    – hobbs
    Mar 21, 2020 at 17:48
  • \$\begingroup\$ I also got a visit from local comm regulator asking to replace the hdmi cable at pc, which interferes with a gsm tower nearby (250 meters) also found this: independent.ie/business/technology/… \$\endgroup\$
    – google
    Jul 8, 2021 at 17:07

2 Answers 2

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Ok, first of all, that wasn't an OFCOM official, probably, in all likelihood. Best case, you just failed some security penetration testing.

Other than that, if the guy with the directive antenna, proper spectrum analyzer and training says "that's where the radiation comes from", I'd trust him.

HDMI symbol rates are, depending on HDMI generation used, 165, 340 or 600 MHz, meaning that at these frequencies or typically odd multiples thereof, you'll see harmonics, with power strongly decaying with the multiple.

UK uses cellular bands at 800, 900, 1400 and 1800, 2100, 2300 and 2600 MHz; aside from the unlikely 600 MHz·3 interference, those are not likely victims.

Then again, OFCOM does have a phone number for spectrum interference investigation: 020 7981 3131 (option 2); so ask them whether someone was dispatched to your location. It's a legitimate concern.

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    \$\begingroup\$ The HDMI cable can't interfere with cell tower across the street. Sounds like a scam to me. The clock and symbol rate however depend on the resolution and bit depth, so they are not fixed, there is just a maximum that the devices can use. For example BD player in menu can use different link rate than watcing a movie. And for symbol rates exceeding 340 Msps, the clock rate is actually divided by four. \$\endgroup\$
    – Justme
    Mar 20, 2020 at 16:02
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    \$\begingroup\$ Also seems very unlikely to me. Across the street is way too far. And the tower is way too powerful. If anything, it's the tower which could interfere with the HDMI not the opposite. \$\endgroup\$
    – Fredled
    Mar 20, 2020 at 22:54
  • \$\begingroup\$ Any thoughts on what the likely scam/attack was? It seems like they would have offered a "replacement cable", threatened fines, or something else sketchy if they were actually nefarious... \$\endgroup\$ Mar 21, 2020 at 16:40
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    \$\begingroup\$ @R..GitHubSTOPHELPINGICE Or they wanted physical access to the computers, to perhaps slip a wireless rubber ducky in one of the machines or to nick a hard drive or something. They could've done lots of things, not just computery ones. \$\endgroup\$
    – wizzwizz4
    Mar 21, 2020 at 17:07
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    \$\begingroup\$ @R..GitHubSTOPHELPINGICE An unsupervised 5 seconds would be enough. \$\endgroup\$
    – wizzwizz4
    Mar 21, 2020 at 19:44
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Many HDMI cables are garbage, improperly connected shields being a main shortcoming. Try a different, known-good-quality cable.

Another thing to try: a clamp-on common-mode ferrite on the cable.

By the way, the most common clock rates for HDMI are 74.25MHz (720p60, 1080i30), 148.5 MHz (1080p60) and 297 MHz (2160p30).

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    \$\begingroup\$ Your clock rates assume 8-bit colors at 4:4:4 chroma sampling, or 12-bit 4:2:2 color sampling. For example most sources like BD players use more bits per color these days. E.g. 2160p24 uses 445MHz TMDS symbol rate with 12-bit colors - but the clock will be one quarter of that. \$\endgroup\$
    – Justme
    Mar 20, 2020 at 18:16
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    \$\begingroup\$ 8 bit RGB is the default for HDMI - that's what 'the most common' phrase means. Extended color depth, HDR, etc are optional features that may or may not be supported by both the HDMI source and sink. \$\endgroup\$ Mar 20, 2020 at 18:33
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    \$\begingroup\$ Not according to my experience they don't limit to 8 bits RGB. Source devices such as BD players, Apple TV 4K, etc, tend to select highest and best settings that the display supports, including 4k, high color depths, YCbCr colorspace and HDR/DolbyVision as default, so consumers don't have to muck around with settings. \$\endgroup\$
    – Justme
    Mar 20, 2020 at 19:11
  • \$\begingroup\$ I imagine the cheap HDMI cables are built as poorly as cheap USB cables. A work mate was having interference issues on his software defined radio system in his office. Turned out to be USB cables. Almost every USB cable he had was defective (friction connection of the cable shield to the outer metal part). He finally found a manufacturer that built good quality cables which eliminated the interference issues. \$\endgroup\$
    – qrk
    Jul 8, 2021 at 17:22

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