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Author Topic: large scale LED displays, processing latency  (Read 5655 times)

Tim McCulloch

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large scale LED displays, processing latency
« on: June 30, 2012, 12:36:01 pm »

I had a recent outdoor event where the end client wanted a "jumbotron" display for I-mag.  The primary AV contractor brought in a 40' x 22' LED wall mounted inside a semi trailer... the kind with the hydraulic mast that elevates the panel to height.  The audience area was roughly 400' x 400' with a 32 x 24' stage.

Corporate politics came into play when the executive assistants wanted the Imag synced with audio.  Most of the VDO switchers I encounter have a couple of frames of latency, and the speed of sound vs. distance usually makes this a non-issue when the audience is 60' or so from the PA.  But... somewhere in the processing chain we picked up at least another 4 or 5 frames of processing latency, requiring a 180ms delay on every audio source.

Normally this wouldn't be a significant issue, but in the site setup the PA had to be placed along the upstage line.  The PA closest to the stage was easily heard by live presenters - 200ms (24' stage depth) late.  We put audio monitors at the lectern, but they required so much level to overcome the louder PA/delay that the mons were on the edge of feedback.  Despite careful tuning and EQ, a turned head, a hat or cap bill would be sufficient to create that feedback; reducing the level only made the delay more obvious and confusing to the presenters.

My questions:  how much processing occurs "in the truck" to put this 720p signal up on the LED wall?  Is there a mode or other settings that can reduce the latency?  I know we're stuck with whatever switcher latency exists and that some additional processing latency is inevitable, but I'm trying to learn more about this to provide a better service to our customer.

Oh, throw in a 34 mph wind that overloaded the lectern mics for extra fun... anyone have suggestions for more wind-resistant microphones/windscreens?  We had 3 layers of windscreens and a good wind burst would send the inputs into clip (with the amount of gain needed to pick up the presenters).

Ah, outdoor corporate/industrial shows outdoors in Kansas...
« Last Edit: June 30, 2012, 12:39:40 pm by Tim McCulloch »
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Doug Fowler

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Re: large scale LED displays, processing latency
« Reply #1 on: June 30, 2012, 12:39:16 pm »

At least it wasn't hot outside.
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Tim McCulloch

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Re: large scale LED displays, processing latency
« Reply #2 on: June 30, 2012, 12:41:35 pm »

At least it wasn't hot outside.

The hell you say. ;-)

The morning, first shift presentation wasn't too bad, it was over by noon.  The presentation for second shift was from 6-7:30pm, and it was so hot the 2000 folks at that show were lingering in the shadows of buildings on the perimeter of the audience area.  I drank 5 bottles of water in 2 hours and never peed.
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Doug Fowler

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Re: large scale LED displays, processing latency
« Reply #3 on: June 30, 2012, 01:38:58 pm »

The hell you say. ;-)

The morning, first shift presentation wasn't too bad, it was over by noon.  The presentation for second shift was from 6-7:30pm, and it was so hot the 2000 folks at that show were lingering in the shadows of buildings on the perimeter of the audience area.  I drank 5 bottles of water in 2 hours and never peed.

It's been desert hot here, 108 and low humidity, around 22% yesterday. 

Baseball game starts in about 45 minutes, we'll see how many show up for that.
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John Livings

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Re: large scale LED displays, processing latency
« Reply #4 on: July 10, 2012, 11:10:03 pm »

Hi Tim,

This may help;

http://forums.creativecow.net/livestageevents

Regards,  John
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TonyWilliams

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large scale LED displays, processing latency
« Reply #5 on: July 11, 2012, 08:22:46 am »

I'm sure you already thought of this, but is there multiple video processors going on? Such as multiple scan converters, graphic switchers, or extra drop box converters like SDI to fiber type units.


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Tim McCulloch

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Re: large scale LED displays, processing latency
« Reply #6 on: July 13, 2012, 12:13:41 pm »

I'm sure you already thought of this, but is there multiple video processors going on? Such as multiple scan converters, graphic switchers, or extra drop box converters like SDI to fiber type units.


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None that I could see, Tony.  I don't now recall what they ran to the LED truck, but for some reason I recall coax.  Definitely no fiber.  I don't remember any processing at FOH that was inserted after the switcher, but I wasn't involved in setting up VDO world.

Mostly I'm trying to find out if, in a generic sense, there was something I should have known to ask the LED guys.  The corporate in-house VDO guys didn't seem to think there was anything that could be changed and I didn't want to make it look like I was trying to blame another department.

The suck factor on this was the client insistence that audio and video be in sync *everywhere* in the audience area, but particularly up front where the VIPs were.

Thanks for the reply, and thanks to John L for the Creative COW link.  Danny A - yeah, that is pretty much my presumption of the signal chain and assumption as to how the LED wall works.  Next time I do this set up (probably in October) I'll get the client to approve moving the PA downstage so the lectern is completely upstage and minimize the amount of audio the presenter can hear.

Tim Mc
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Joe Kipfer

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Re: large scale LED displays, processing latency
« Reply #7 on: January 16, 2013, 03:45:01 pm »

I came across this blog post in the search engines and thought I could shed some light on the subject.  I work for an LED screen company (www.neoti.com) and we have Trailer unit's like you're describing.  I think I know the company you worked with (can make an educated guess) and we use pretty much the same equipment, especially the image scaler/switcher.  I've never had to deal with video latency on our end (the LED side...just think of it as a giant monitor or TV, for an easy way to think of it)...honestly, with the signal we get, there is zero noticeable latency with our ImagePro Scalers (industry standard)...usually when there's a video latency issue, it's happening at the Tri-caster or whatever multi-input switcher the video crew is using.  Just for what it's worth, I thought i'd let you know...but as to the question of if there was anything the LED guys could have done, a lot of time the LED techs have dealt with this before, and could have combined some knowledge with the camera/video guys to isolate the problem and fix it...going forward I would definitely try to get the LED tech's help/opinion.
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Tim McCulloch

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Re: large scale LED displays, processing latency
« Reply #8 on: January 21, 2013, 01:51:24 pm »

Joe, thanks for your reply.

The nature of this aerospace client is that their people are plenty smart and already have the experience and knowledge.  Any problem must be the fault of the contractors.

I'd never experienced this much delay in a live setting, either, and was taken aback by both the latency and the stubbornness of the personalities involved.  I now (several months later) believe audio was the scapegoat in the pissing contest between the client's creative services department and the AV contractor (who sub-hired the LED trailer).

Postmortem, consulting with another AV contractor, both the client & I were told "we never have that problem."  And I believe them because on the shows I've worked with that firm, they supply EVERYTHING up to the LED trailer... from camera lens to the signal line running to the display.  Nothing supplied by client, and I really think that was where the problem came from... a daisy chain of client and contractor supplied playback, switching, scaling... and the wall of ego that kept the client's technicians from admitting that their plan wasn't delivering the results they had promised to the executive staff.

I also believe the client has told the contractor they don't want me back (my phone hasn't been ringing for work there), but nobody has the integrity to tell me so.  Not losing sleep over it, for sure, but the point of this thread was for me to get a better understanding of what was going on from the VDO standpoint and better prepare me to deal with such things in the future.  Your clarification has helped, and I thank you for your comments.

Tim Mc

I came across this blog post in the search engines and thought I could shed some light on the subject.  I work for an LED screen company (www.neoti.com) and we have Trailer unit's like you're describing.  I think I know the company you worked with (can make an educated guess) and we use pretty much the same equipment, especially the image scaler/switcher.  I've never had to deal with video latency on our end (the LED side...just think of it as a giant monitor or TV, for an easy way to think of it)...honestly, with the signal we get, there is zero noticeable latency with our ImagePro Scalers (industry standard)...usually when there's a video latency issue, it's happening at the Tri-caster or whatever multi-input switcher the video crew is using.  Just for what it's worth, I thought i'd let you know...but as to the question of if there was anything the LED guys could have done, a lot of time the LED techs have dealt with this before, and could have combined some knowledge with the camera/video guys to isolate the problem and fix it...going forward I would definitely try to get the LED tech's help/opinion.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

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Re: large scale LED displays, processing latency
« Reply #8 on: January 21, 2013, 01:51:24 pm »


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