HDMI cable causing wireless mic interference - Page 1
Forum section: electronics / repair

author: andtfoot
author_rank: Unknown
timestamp: August 11, 2014, 09:43:02 am
content: Had a job today where a bunch of wireless microphones (Sennheiser EW 135 G3 1G8) go berzerk when video was pushed through a HDMI cable near the receiver.The microphones are of the 1.8GHz variety (1792.3MHz was the channel the most susceptible one was on). The noise was crackling, bursts of white noise. This was while the microphone transitter was off. Turning the mic on would help things.I tried using my mini spectrum analyser (RF Explorer) but could pick up anything on top of 1792MHz.Ended up fixing it by swapping in different HDMI leads until it stopped. 2 of the 4 I tried had the issue. I cut open one of the 'bad' ones, and it seemed fine with plenty of shielding.Anyone have any ideas what could be happening, and why changing HDMI leads might have helped?Maybe ideas on testing the remaining HDMI lead that I took back with me?

author: wraper
author_rank: Unknown
timestamp: August 11, 2014, 10:32:48 am
content: Is outer shielding connected to the connector body? Did you measure resistance between body of the both ends? I have seen crappy cables where it was not connected at all, only internal shieldings (over twisted pairs) were connected to connector pins.

author: Rasz
author_rank: Super Contributor
timestamp: August 11, 2014, 10:36:17 am
content: was that computer or video equipment? it computer:-too bad you didnt try changing resolution or refresh rate-there is also option in drivers for low bandwidth DVI modeif ~1800mhz was clear then maybe receiver is sensitive to ~120-160MHz?

author: andtfoot
author_rank: Unknown
timestamp: August 11, 2014, 10:50:02 am
content: Yup. I chucked a multimeter and measured bugger-all resistance between the metal on the ends.The shielding had braid on the outside, then a bare earth/drain wire, then foil. Each differential pair had foil and an insulated earth wire. I tried both 1m cable and 0.5m cable.One thing I was thinking of was wrapping a coil of wire around the outside of the HDMI cable to use as an antenna for the RF Explorer. I'll give that a go tomorrow.Unfortunately I don't have any wireless microphones back at the office to reproduce the issue.The cable was from ascaling HDBaseT (HDMI over twisted pair) receiverto the input of avideo conference codec.I didn't have access to change the resolution that the scaler was set to (1920x1080@60).

author: allikat
author_rank: Contributor
timestamp: August 11, 2014, 03:01:51 pm
content: Certainly sounds to me like the cable itself is adequately shielded. You may want to look for grounding issues at either end.Possibly stupid thought...Alternatively if the signal source and destination are on different segments of the main, you could be seeing the shielding becoming a transmitter itself, the linked grounds possibly leading to a voltage differential across the cable. That would depend on how the building is wired. In that situation, a poorly grounded cable may actually work better than a well grounded one.

author: avcom
author_rank: Newbie
timestamp: March 15, 2015, 10:54:51 pm
content: Quote from: andtfoot on August 11, 2014, 09:43:02 amI did a search today for "HDMI RF interference with wireless mics" and was led to this thread. I wanted to comment on this as it is almost the exact scenario I'm having.Our church recently did an upgrade which included two LCD TV's at the front, driven by an HDMI DA back in the sound booth. We noticed almost immediately that our three Sennheiser wireless receivers (the lone Shure was unaffected) were getting interference just as was described in the OP. We knew it went away when the DA was unplugged so we ordered a better one. No difference. I too tried to "see it" with my RF Explorer but could not and the RF affected 3 receivers 150MHz apart. Long story short we came to the conclusion it is a combo of things including the DA and cables. Unfortunately the HDMI cables are long and run through the ceiling. Not easy to replace.My "quick" solution was to go into the receiver settings and raise the squelch level from the default of "low" to "medium". So far, so good. If this works, we plan to install the DA inside a steel electrical box and mount it when we build the new sound booth. I'll seal the cable openings with copper tape. Hopefully this will shield the RF.So thanks for the thread. It's good () to know we're not alone having this issue.

author: wraper
author_rank: Unknown
timestamp: March 16, 2015, 12:45:15 pm
content: You could try to change refresh rate, for example, from 60 Hz to 50 or 100 Hz if TV supports that or change resolution. That would change pixel clock frequency, therefore interference would be radiated at different frequencies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 16, 2015, 01:05:03 pm
content: Quote from: andtfoot on August 11, 2014, 10:50:02 amBut this isn't indicative of proper shielding at the connectors -- the shield must be contiguous from connector shell to braid/foil. If there is a slot or gap there, you wouldn't be able to measure it, but very high frequency signals can still leak out.QuoteThis probably won't do much -- the fields will tend to be polarized lengthwise, because the wires within run lengthwise. A single turn inductive pickup loop might still be illuminating, but rather than placing it over the cable, hold it near the connector ends, at various angles, and see if anything pops up in the 1-2GHz range. Repeat the measurement with the HDMI connection disabled or disconnected, so you can try to isolate HDMI as a source, from whatever trash the equipment itself may be generating.HDMI signals are high bandwidth digital logic, so they should have plenty of harmonics out there. Between that and the low voltage used in the logic circuits, these cables must be shielded -- to keep harmonics in, and to keep external interference (RFI as well as ESD transients) away. Of course, good shielded cable isn't cheap, and fully shielded connectors aren't easy to apply, so... you can find lots of products out there that work, under lots of normal conditions, but which simply don't meet spec. You may very well have such a cable.Tim

author: amyk
author_rank: Super Contributor
timestamp: March 16, 2015, 03:42:22 pm
content: Quote from: T3sl4co1l on March 16, 2015, 01:05:03 pmSo there is some reason for audiophiles to buy expensive HDMI cable after all?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 16, 2015, 03:52:15 pm
content: Correctly made cables, yes. Or more appropriately, professionals will."Blessed" cables, well, you know...Tim

author: lpc32
author_rank: Frequent Contributor
timestamp: March 16, 2015, 04:43:25 pm
content: Anyone compared cables with and without ferrite beads?

author: Rasz
author_rank: Super Contributor
timestamp: March 16, 2015, 05:15:43 pm
content: Quote from: lpc32 on March 16, 2015, 04:43:25 pmyou can add ferrite beads yourself, just salvage/buy few and experiment

author: lpc32
author_rank: Frequent Contributor
timestamp: March 16, 2015, 10:56:01 pm
content: Yeah, just curious what results others are seeing.