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Author Topic: Intermittent signal drop and distortion during live event  (Read 804 times)

Chris Jolocon

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Intermittent signal drop and distortion during live event
« on: July 20, 2020, 02:16:59 pm »

Hello,
I recently did a live event where the audio was clean 96% 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. 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: https://drive.google.com/file/d/17Su7f8Mtx4OuqAccCzwY6ZPmSl23mW7E/view?usp=sharing
Audio Sample 2: https://drive.google.com/file/d/1uNBy8WoHOCvd9l5W_hZ0PjS3wj0zYaNa/view?usp=sharing

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
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?
« Last Edit: July 20, 2020, 03:34:35 pm by Chris Jolocon »
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Tim McCulloch

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Re: Intermittent signal drop and distortion during live event
« Reply #1 on: July 20, 2020, 03:43:50 pm »

I do not believe the audio problem in the linked audio clips is related to an AC power problem.  It has more of a "bad mic cable" sound to me, but that would not explain the various details (intermittent, 15-30 sec duration, etc)... the problem is ahead of whatever other fault conditions your encountered as the failure was present in the live sound and to all recordings and streams.  Was that recorded at the mixer or by a camera?  Was the level difference seen by the mixer's meters and audible in the mixer headphone output?  A loss of +48VDC usually creates a very loud POP.  The audio may stop instantly or fade out over several seconds as the capacitors discharge.

That said, I think a clue is in the results of your wireless mic freq scan.  Were you using SDI - especially an SDI/HDMI converter?  If not, I suspect your HDMI/CAT cable and/or converters.  Something is spewing lots of noise and it was probably fairly close (<10 ft) to your receiver.  The other possibility - was your e100 in the Senny "A" band?  B and C have been auctioned off to the cellular carriers (600mHz band transition is mostly complete) and you may have been in T-Mobile territory.

Power and audio cable separation?  Yeah, in an ideal world.  In my non-ideal world we have 8/5 208v/120v 30 amp service bundled with the 12 channel drive snake, which is laid right next to the 54 input mic snake.  Classical recording level of quiet?  Eh, maybe not.  For everything else, any induced noise is the least of my show day worries.  I don't think cable proximity is an issue in your cascading failures.

The one thing you haven't mentioned is how you had all of this powered up, and it may relate to the powered speaker failure.  All large facilities have power that comes from a variety of breaker boxes (which may be fed by different transformers in the venue's power system).  If you were running extension cords from a variety of sources, using whatever outlet was closest, read on...

It's possible that you had equipment grounds "not at equipotential" - that means you measure voltage between the grounds of different devices and have more than the theoretical Zero volts.  An XLR 'pin 1 lift' will break this without compromising electrical safety.  The possibility that there was 5v or more on the *signal* ground under typical circumstances is bad enough, but with one mis-wired outlet, extension cord or plug, you could have 120V on the signal ground.  Nobody would ever know because the way stadiums and arenas are mostly used, almost never do multiple AC sources meet at a common device.  If you open the QSC up and find a burned ground trace or similar damage...

The video camera recording blank frames is odd, but if it was happening the other way - the receive side blanking out - it would indicate loss of HDCP authentication.  Were you using the HDMI/CAT converters at the cameras?

The HDMI converter failure could be from a number of causes, and I tend to point at cable and termination quality.  One of the AV shops I sometimes freelance for carries spools of new CAT6 and employees are trained to terminate and test...
« Last Edit: July 20, 2020, 03:48:36 pm by Tim McCulloch »
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Mike Caldwell

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Re: Intermittent signal drop and distortion during live event
« Reply #2 on: July 20, 2020, 04:11:52 pm »

You had a lot of parts and pieces (maybe kind of thrown together) to keep working all at the same time!

Tim already hit a lot of the major points.

I have seen the HDMI Baluns in systems where they are fairly close
to wireless receivers and just all but make the wireless unusable with
all the RF noise they put out.

What connection is on the video camera for the audio input?

Did you just split off the feed to the speakers to feed the camera or have some
form of transformer isolation, actually with feeding power speakers from a mixer in
the press box having isolation on that feed would not be a bad idea.

How did you connect the iMac to mixer, did you use the USB audio out on the ZED.

In addition to the distortion the audio sound like there was a lot phase issues.
By chance were you connecting to an 1/8 stereo input with a balanced line audio
connection?

Do all the recordings have the same sound issues?
Did you notice the issues in the live PA system?

As for the K12 speakers are they a permanent install of just set up for the event?

If there a permanent install are they really weather protected?
Even if they are even semi weather protected I would not install
a speaker like that at a football field.

Chris Jolocon

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Re: Intermittent signal drop and distortion during live event
« Reply #3 on: July 20, 2020, 04:38:19 pm »

Hi,
Thank you for the replies.

QSC speakers were set up on field, just for event. 80 degree heat but no rain or adverse weather.

The signal path of sound was: shotgun mic to mixer, mixer main out back down to field to feed the loudspeakers. aux send from same mixer also sent back down to the field to the camera's audio inputs (camera has a built-in 2-xlr input interface, can select mic/line level...). Then that camera's hdmi (video and audio) sent back up to the video switcher at press box. The video switcher fed the audio and video of the livestream. I was always using the HDMI from camera 1 as the audio source, regardless of whether camera 1 or 2 video was active. The only audio sources were the shotgun mic, and a separate laptop that had some pre-recorded music. These were the inputs to the mixer, sent to main mix and aux send to feed both the loudspeakers and then to the camera/live stream. I could look into transformer isolators on the outputs.
 
From what I remember, yes the audio issue occurred everywhere - headphones monitoring off of mixer, the livestream, and in the loudspeakers.

Yes, 1 culprit could be the fact that I was drawing electrical power from 2 different places, probably different circuits.
The loudspeakers and camera #1 were drawing power from a quad box under the bleachers, close to the track and field.
The mixer, computer, camera #2, video switcher, were from outlets up in the press box.

In future would it be better to run everything off of 1 outlet to power conditioner, then to all pieces of gear?

I'll see whether an electrician could test the outlets themselves for proper grounding or other issues.
Another interesting piece of evidence is that there is a power sequencer for a different sound system also located in the press box. The voltage readout seems normal, but there is also a current meter and it is displaying "e" when turned on. I can also remember audio problems, signal dropouts, at this location (track/field) with previous systems and before I started running A/V for live events. This would seem to point to an electrical issue or a site-specific issue.

 I could also look into testing the extension cords I used, due to the physical setup they were long runs, I was probably using about 300 - 400 ft. worth of extension cords in total to the loudspeakers and camera position.

For the camera #1 signal - It was an HDMI direct output from the video camera, into an ocean matrix box that converts to RJ45 connector, then a cat 6 cable run, about 125', at the other end of the cat6 cable is a similar box that converts it back to HDMI, then into a Roland video switcher. The Sennheiser receiver was fairly close to the hdmi receiver box when I did the scan. It is G-band (high 500 mhz).
Camera #2 was close to the video switcher, so that was an HDMI cable direct from camera to the video switcher.

QSC loudspeaker still under warranty, so once I take it to a service center I will see what evidence can be gleaned from that failure.




I do not believe the audio problem in the linked audio clips is related to an AC power problem.  It has more of a "bad mic cable" sound to me, but that would not explain the various details (intermittent, 15-30 sec duration, etc)... the problem is ahead of whatever other fault conditions your encountered as the failure was present in the live sound and to all recordings and streams.  Was that recorded at the mixer or by a camera?  Was the level difference seen by the mixer's meters and audible in the mixer headphone output?  A loss of +48VDC usually creates a very loud POP.  The audio may stop instantly or fade out over several seconds as the capacitors discharge.

That said, I think a clue is in the results of your wireless mic freq scan.  Were you using SDI - especially an SDI/HDMI converter?  If not, I suspect your HDMI/CAT cable and/or converters.  Something is spewing lots of noise and it was probably fairly close (<10 ft) to your receiver.  The other possibility - was your e100 in the Senny "A" band?  B and C have been auctioned off to the cellular carriers (600mHz band transition is mostly complete) and you may have been in T-Mobile territory.

Power and audio cable separation?  Yeah, in an ideal world.  In my non-ideal world we have 8/5 208v/120v 30 amp service bundled with the 12 channel drive snake, which is laid right next to the 54 input mic snake.  Classical recording level of quiet?  Eh, maybe not.  For everything else, any induced noise is the least of my show day worries.  I don't think cable proximity is an issue in your cascading failures.

The one thing you haven't mentioned is how you had all of this powered up, and it may relate to the powered speaker failure.  All large facilities have power that comes from a variety of breaker boxes (which may be fed by different transformers in the venue's power system).  If you were running extension cords from a variety of sources, using whatever outlet was closest, read on...

It's possible that you had equipment grounds "not at equipotential" - that means you measure voltage between the grounds of different devices and have more than the theoretical Zero volts.  An XLR 'pin 1 lift' will break this without compromising electrical safety.  The possibility that there was 5v or more on the *signal* ground under typical circumstances is bad enough, but with one mis-wired outlet, extension cord or plug, you could have 120V on the signal ground.  Nobody would ever know because the way stadiums and arenas are mostly used, almost never do multiple AC sources meet at a common device.  If you open the QSC up and find a burned ground trace or similar damage...

The video camera recording blank frames is odd, but if it was happening the other way - the receive side blanking out - it would indicate loss of HDCP authentication.  Were you using the HDMI/CAT converters at the cameras?

The HDMI converter failure could be from a number of causes, and I tend to point at cable and termination quality.  One of the AV shops I sometimes freelance for carries spools of new CAT6 and employees are trained to terminate and test...
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Mike Caldwell

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Re: Intermittent signal drop and distortion during live event
« Reply #4 on: July 20, 2020, 06:57:00 pm »

Good to hear the K12's were just set up for the event, what sounded like a rescheduled graduation.

When the system was powered up with the mic open was there any background hum or noise
in the system?

I'm guessing you had the camera XLR inputs set to line level, any chance they could have been set to mic level with the phantom power turned on?
Some equipment will act strange when it's outputs get hit with phantom power.

Scott Holtzman

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Re: Intermittent signal drop and distortion during live event
« Reply #5 on: July 20, 2020, 07:06:31 pm »

Hi,
Thank you for the replies.

QSC speakers were set up on field, just for event. 80 degree heat but no rain or adverse weather.

The signal path of sound was: shotgun mic to mixer, mixer main out back down to field to feed the loudspeakers. aux send from same mixer also sent back down to the field to the camera's audio inputs (camera has a built-in 2-xlr input interface, can select mic/line level...). Then that camera's hdmi (video and audio) sent back up to the video switcher at press box. The video switcher fed the audio and video of the livestream. I was always using the HDMI from camera 1 as the audio source, regardless of whether camera 1 or 2 video was active. The only audio sources were the shotgun mic, and a separate laptop that had some pre-recorded music. These were the inputs to the mixer, sent to main mix and aux send to feed both the loudspeakers and then to the camera/live stream. I could look into transformer isolators on the outputs.
 
From what I remember, yes the audio issue occurred everywhere - headphones monitoring off of mixer, the livestream, and in the loudspeakers.

Yes, 1 culprit could be the fact that I was drawing electrical power from 2 different places, probably different circuits.
The loudspeakers and camera #1 were drawing power from a quad box under the bleachers, close to the track and field.
The mixer, computer, camera #2, video switcher, were from outlets up in the press box.

In future would it be better to run everything off of 1 outlet to power conditioner, then to all pieces of gear?

I'll see whether an electrician could test the outlets themselves for proper grounding or other issues.
Another interesting piece of evidence is that there is a power sequencer for a different sound system also located in the press box. The voltage readout seems normal, but there is also a current meter and it is displaying "e" when turned on. I can also remember audio problems, signal dropouts, at this location (track/field) with previous systems and before I started running A/V for live events. This would seem to point to an electrical issue or a site-specific issue.

 I could also look into testing the extension cords I used, due to the physical setup they were long runs, I was probably using about 300 - 400 ft. worth of extension cords in total to the loudspeakers and camera position.

For the camera #1 signal - It was an HDMI direct output from the video camera, into an ocean matrix box that converts to RJ45 connector, then a cat 6 cable run, about 125', at the other end of the cat6 cable is a similar box that converts it back to HDMI, then into a Roland video switcher. The Sennheiser receiver was fairly close to the hdmi receiver box when I did the scan. It is G-band (high 500 mhz).
Camera #2 was close to the video switcher, so that was an HDMI cable direct from camera to the video switcher.

QSC loudspeaker still under warranty, so once I take it to a service center I will see what evidence can be gleaned from that failure.


You didn't answer the question about using any 600Mz devices.





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Chris Jolocon

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Re: Intermittent signal drop and distortion during live event
« Reply #6 on: July 20, 2020, 08:39:34 pm »

No wireless devices were used in the 617-698 mhz spectrum. I had 1 wireless mic on standby, tuned to 567mhz I believe, but ended up not using it as the problem did not present itself again during 2nd half of event. The wireless mic was of the Sennheiser "G" range (566-608 mhz).

1 other thing I just thought of.  There is no wired internet hookup at the press box but shortly before this event our IT dept. installed some sort of Wi-Fi receiver device - it looks like a square panel affixed on the outside of the press box that points to and communicates with a similar looking panel on a neighboring building, about 300 feet away. The panel on the building is hooked up in some way to our wired network at large. This created excellent wi-fi reception, but I wonder whether this could be another possible source of the audio artifacts. The audio mixer was perhaps 8-10ft. from this panel.
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Chris Jolocon

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Re: Intermittent signal drop and distortion during live event
« Reply #7 on: July 20, 2020, 09:02:22 pm »

Mike - correct, rescheduled graduation, and yes camera audio input was set to line level, as I was sending it line level signal from the aux send.

There was one point when the audio was at its worst that both a hum and hiss were just barely audible, given I had to crank the signal 25db or so in post production to match levels. Again only just barely audible if listening carefully at high volume on headphones or nice speakers. Aside from the intermittent issue audio was quite clean throughout.

Someone also mentioned they heard phasing in the sound clips - it might have been an early reflection, the presenter's voice bouncing off the lectern surface, mixing with the direct sound. The shotgun mic capsule was placed a few inches above and slight angle up from the lectern surface.

Good to hear the K12's were just set up for the event, what sounded like a rescheduled graduation.

When the system was powered up with the mic open was there any background hum or noise
in the system?

I'm guessing you had the camera XLR inputs set to line level, any chance they could have been set to mic level with the phantom power turned on?
Some equipment will act strange when it's outputs get hit with phantom power.
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Mike Caldwell

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Re: Intermittent signal drop and distortion during live event
« Reply #8 on: July 21, 2020, 12:04:27 am »

Mike - correct, rescheduled graduation, and yes camera audio input was set to line level, as I was sending it line level signal from the aux send.

There was one point when the audio was at its worst that both a hum and hiss were just barely audible, given I had to crank the signal 25db or so in post production to match levels. Again only just barely audible if listening carefully at high volume on headphones or nice speakers. Aside from the intermittent issue audio was quite clean throughout.

Someone also mentioned they heard phasing in the sound clips - it might have been an early reflection, the presenter's voice bouncing off the lectern surface, mixing with the direct sound. The shotgun mic capsule was placed a few inches above and slight angle up from the lectern surface.

I mentioned the phasing sound, it sounded more electronically induced to me or maybe it's from various audio file conversions of the clips you posted.

I did a series of drive in parking lot graduations back at the end of May.

Art Welter

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Re: Intermittent signal drop and distortion during live event
« Reply #9 on: July 21, 2020, 01:15:39 pm »


From what I remember, yes the audio issue occurred everywhere - headphones monitoring off of mixer, the livestream, and in the loudspeakers.

Yes, 1 culprit could be the fact that I was drawing electrical power from 2 different places, probably different circuits.

Someone also mentioned they heard phasing in the sound clips - it might have been an early reflection, the presenter's voice bouncing off the lectern surface, mixing with the direct sound. The shotgun mic capsule was placed a few inches above and slight angle up from the lectern surface.

Chris,

Your description of the problem and the sound of the audio clips do not lead me to an AC electrical or RF problem.

It's possible the mic diaphragm/backplate encountered some sweat (or spit) dropping from the talker, causing a partial short circuit, which went back to normal after it evaporated.

Might still be some evidence if you can detect droplet marks on the mic.

Art
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Chris Grimshaw

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Re: Intermittent signal drop and distortion during live event
« Reply #10 on: July 22, 2020, 08:54:01 am »

+1 on Art's diagnosis - sounds like a mic issue to me. Humidity can cause condensers to drop out completely.

Chris
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Tim McCulloch

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Re: Intermittent signal drop and distortion during live event
« Reply #11 on: July 22, 2020, 09:38:15 am »

Hi Chris-

I've also read the replies you got over at Controlbooth.com and I think the audio cut-out is at the head end.  That you could see/hear it at the mixer means you can eliminate all *hardware* that comes after the mixer as part of that problem.

Your video problems are likely related to the use of UTP for the HDMI converters (I've only used them on the output - display and projector side - and wasn't aware they would work for cameras, we usually convert to SDI for that).

That you could not replicate *any* of these problems back at the shop is frustrating.
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Chris Jolocon

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Re: Intermittent signal drop and distortion during live event
« Reply #12 on: July 23, 2020, 04:52:08 pm »

Hi, I will double check the shotgun mic. It had a windscreen on it the whole time and the presenters were wearing face masks the whole time which I think would rule out people as the condensation source. It was stored in a case the night before, then within a crate with lid, and the crate within a storage building. There was some wind throughout setup and the event, and a little bit of dew in the morning, but no precipitation.  There was more sun and rising temperatures as the event progressed, which does correlate with how the signal cleaned up on its own for the 2nd half of event. I will double check the other equipment at head of signal chain - xlr connections, cable, inputs of mixer.
HDMI to Cat6 boxes I used were Ocean Matrix: https://www.bhphotovideo.com/c/product/1244029-REG/ocean_matrix_omx_hdmi_hdbit_1080p_hdmi_extender_over.html

Any thoughts on whether shielded cat6 cable is necessary, or so long as it's twisted pair it's ok?
I see comprehensive makes a Cat6 cable that is Shielded, twisted pair,
https://www.comprehensiveco.com/store/p/19386-Cat6-Snagless-Solid-Shielded-Black-Patch-Cable-100ft.html

 but also a more expensive one that is "heavy duty", twisted pair but does not mention shielding.
https://www.comprehensiveco.com/store/p/18848-Pro-AV/IT-CAT6-Heavy-Duty-Snagless-Patch-Cable-Black-100ft.html


« Last Edit: July 23, 2020, 05:02:38 pm by Chris Jolocon »
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Re: Intermittent signal drop and distortion during live event
« Reply #12 on: July 23, 2020, 04:52:08 pm »


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