Intermittent Signal Drop and Distortion - RF, phantom, or electrical issue?

Chris Jolocon

Member
Joined
Feb 7, 2018
Location
New York
I recently did a live event where the audio was clean 97% of the time except for a few short segments - anywhere from 5 seconds to 30 seconds, perhaps 5 or 6 instances altogether - where the signal suddenly dropped in level, approx. 15-20 db, and distortion introduced, then the signal would return to clean and healthy level. This was video recorded, so I was able to compensate for the level drop in post, in video editing, but not the distortion. When I tested the same equipment after the event but in an indoor space I could not recreate any of the issues experienced at live event. My initial sense is that this was either an electrical issue, possibly causing artifacts or a temporary drop of phantom power, or there was some sort of RF or EM interference. For the audio samples below the level drop has been compensated, but you can still hear the distortion:

Audio Sample 1: Audio Sample 2:
Here are some details of setup.
Outdoor event at football field.
Audio Technica shotgun mic at lectern, approx. 150ft. of XLR cable to a 75' snake, over metal bleachers up to a press box (metal structure), to an Allen and Heath ZED 12fx mixer. Mixer provided the phantom power. Audio signal sent back down to field to loudspeakers and a video camera.
For some stretches audio and power cables were next to each other (I know this is a no-no).
The mixer was within a foot of an iMac computer.
We were close (200 yards) to high-powered electrical lines.
audio cables were run on top of metal bleachers and mixer/computer were within a metal "press box".
Since mixer was feeding a live stream, a video camera on field, and the loudspeakers on the field, I realize this essentially ties all these components together. I wonder whether some sort of malfunction or interference with any of the connected devices caused the audio issue.

Other oddities occurred at same event:
I have boxes that send HDMI signal over longer stretches of Cat6 cable. One set of such boxes did not seem to work at all, and with another set the signal was at first intermittent, but thankfully for the event itself worked consistently. The Cat 6 cable was not shielded, but I am not certain that was the issue. We were within specifications for the length of the Cat6 runs and the converter boxes.
The Video camera recorded 2-3 blank frames at one point (all green). This has never happened before. The audio was also out of sync by a few frames, which has never been a problem in previous events.
One of the 4 loudspeakers on field died at some point between testing the day prior to the event, and the setup time day of event. I was able to replace it with a different loudspeaker, but original unit still does not power on (it was a fairly new QSC k series speaker).
Everything was on Furman power strips, offering basic surge protection and RF /EMI rejection, but not the more robust features of their power conditioners.
During the event I set up a wireless mic in case the issue got worse, but it actually went away for the 2nd half of event. When I scanned for open frequencies there were barely any found. Sennheiser evolution 100 series, normally finds many available frequencies.

I would like to identify the cause. If I had to do the event again I would try these things, but I'm not 100% sure they would prevent the same issue:
Use AA battery on board the condenser shotgun mic, to avoid running phantom through a long XLR run.
Use shielded cat 6 cable
Use an RF inline filters (XLR barrels, Sescom and Shure make such a product)
Keep mixer away from iMac computer
Separate all audio and power cable runs.

Any ideas whether this seems like a phantom power issue or electrical issue, or RF or other type of interference?
 

FMEng

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Fight Leukemia
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Mar 31, 2008
Location
Tacoma, WA
I don't hear any clues that would lead to a smoking gun. It clearly is not noise from power cables or RF. 225' of cable is really nothing, and phantom power isn't picky. My guess, based on many years of experience, would be some oxidized pins on an XLR. Perhaps a cable that isn't used often and stored in less than ideal conditions? There are a lot of knock-off XLRs with questionable pin plating, these days. Add in some humidity or dew, and you've got a recipe for a bad connection.

Caig DeoxIT D5 is a great for connector maintenance. It's hard to do, but I try to avoid cables with connectors that aren't Neutrik's or Switchcraft's.

The wireless mic problem could well have been from the power lines. All of the HDMI extenders I have seen specify shielded cable only. UTP may work, but extra noise could cause bit errors that would be visible in the video. Computers sit next to digital mixers all the time without trouble.
 

MNicolai

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Mar 30, 2008
Location
Sarasota, FL
@FMEng could be onto something with corroded pins. Whether it's that or a cable that's got a wire internally shorting or intermittently faulty would produce a major gain drop and distortion. I would probably start by inspecting cable ends and opening up the ends of the connectors. Usually it's pretty easy to see if you've got connectors that are poorly terminated. In particular, I would look for cases where the cables are stripped back too far and the ground wire could come in contact with the other pins, or where pins 2 and 3 might be open.

Ultimately I'm not sure what I'm listening to in terms of how this signal relates to the video extenders and such. A signal flow diagram would help.

How hot was it outside? HDMI extenders definitely can run hot and I could see them having issues in the direct sunlight.

Also, what was the power source?
Length and AWG of power cables?
If connected to a convenience receptacle, how far is the receptacle from circuit breaker panel and did you put a meter on it? If you're teetering under 110 volts, I could see that producing issues.

If you have the K.2 speakers as opposed to the original K series, I would check out what firmware they're running. QSC has released updated firmware to improve the speaker protection features. Worth taking a look at here.
 

Chris Jolocon

Member
Joined
Feb 7, 2018
Location
New York
Thank you for the suggestions, I will inspect cable ends and try deoxit. It may have even been the XLR connectors on the mixer inputs - is it safe to use deoxit there as well?
The XLR cables I used had never given me problems prior to this, though previously they were used indoors probably 95% of the time. They were all Audio Technica "premium" with neutrik connectors. Snake was a ProCo sound.
I will check on the firmware for K8.2 speakers. I found it odd that only 1 out of the 4 speakers died, as their signal was daisy-chained (it was a mono setup) and hooked up to the same electrical source.

Power source was a quad box, about 50 ft. away from a circuit panel that was under bleachers.
From there about 100-150ft of extension cord to loudspeakers, camera. 16awg.

I think there was some dew on the field which may have been a factor. Next time I can try placing something waterproof underneath any of the xlr and power connections, to separate from the field.
It got up to 80 degrees, perhaps even hotter on the ground at the field, so that could have been a factor with the HDMI extender. Next time I can look into creating a small box to provide it some shade and separate from the field. I will also upgrade to a heavy duty, shielded cat6 cable.
 

macsound

Well-Known Member
Joined
Jun 15, 2018
Location
San Francisco, CA
That's an interesting type of distortion there. It would be interesting to see if it's the microphone. Since we don't have the whole recording, what happened immedietly before and after the distortion? Was it the same each time? Like a click or hiss or fade in or out?

I've had 1 microphone that had similar issues, an Audix VX10, condenser vocal mic.
Inside the microphone, there's a wire that connects to the body of the mic which was heat shrunk, not soldered. Ground I assume.
The movement of that wire within the heat shrink would cause random bits of distortion but it wasn't always when the wire was immediately disconnected (like if you shook the mic) because of a capacitor I assume, slowing the response, which sounded like a fade in of the distortion.
 

Chris Jolocon

Member
Joined
Feb 7, 2018
Location
New York
Hi, I will check inside the mic to see if any evidence. It had a windscreen on it for the entire event and the people speaking at the event were wearing face masks at all times, so I don't think it was an issue of condensation. The way the issue came and went was somewhat strange - signal would pop in and then get weak almost instantaneously but there were also times it would fade, as in get gradually weaker and more distorted, then return to normal gradually, over a second or two. It's as if something were sort of clamping down on the signal, then releasing it. It was windy, but the issue did not correlate with strong gusts of wind. It was an Audio Technica 8035 shotgun mic.

HDMI to Cat6 boxes I used were Ocean Matrix: https://www.bhphotovideo.com/c/prod..._omx_hdmi_hdbit_1080p_hdmi_extender_over.html

Any recommendations for the best cat6 cable to use in this case, for future events? The cheaper, cat6 cable I used on previous events did not cause me any issues until now.

I see comprehensive makes a Cat6 cable that is Shielded, "solid", twisted pair,
https://www.comprehensiveco.com/sto...s-Solid-Shielded-Black-Patch-Cable-100ft.html

but also a more expensive one that is "heavy duty", twisted pair but does not mention shielding or being solid.
https://www.comprehensiveco.com/sto...vy-Duty-Snagless-Patch-Cable-Black-100ft.html
 
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macsound

Well-Known Member
Joined
Jun 15, 2018
Location
San Francisco, CA
I've always been happy with monoprice. The price is usually great too.
For these type of unpredictable runs, I usually use pre-terminated stuff like this

When it's install, I'm confident in terminating myself, but shielded CAT6a is a beast and requires more specialized rj45s because the cable itself is so thick.