Delayed Video

[Tom]

I was at Hyde Park last night. I don't often go to this sort of big outdoor event type gig so forgive my ignorance.

 

There were a couple of big video screens about half way back from the stage. However, I found them unwatchable as they were completely out of sync with the sound - it felt like almost a second out. The sound was obviously delayed but why not delay the pictures as well? I understand that it will never be perfectly in sync, except for at one distance from the stage, but any form of delay would have helped.

 

Is there a glaringly obvious reason why this is not done?

 

T

[boatman]

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Is there a glaringly obvious reason why this is not done?

 

T

 

Two reasons probably. Firstly the cost and secondly because the organisers don't ever see it and don't really care. These days with video recorders comprising vast arrays of RAID discs it's a fairly simple task to delay video by as much as you need. It just isn't cheap. In BBC sport many years ago we used to use two quadruplex VTR machines with tape buffer between them. Mostly due to the 2" tape handling problems, we couldn't get much more than a few tens of seconds delay, but it did work well enough most of the time

[AHYoung]

The problem generally is the other way round, in that the video processing introduces so much delay that the audio arrives first but in larger venues the video traveling at the speed of light catches up with the audio traveling at the speed of sound by tthe time it reaches the mix positionso it all averages out, for the video to be a second in front of the audio you half to have been about a quarter of mile from the stage, surely the sites not that big? Also even if you do delay the relay screens, you cant delay the ones either side of the stage, or for that matter the actual real onscreen action...

[dwright2104]

I noticed something similar when watching the Diamond Jubilee Concert, on some occasions bbc cut to a view down the street with several big screens and u seen them all change one at a time but not in order,

I'm guessing the video processors were all out slightly with different cable runs ect.

[Pete McCrea]

With the screens on the Mall, there could be differences in the hardware between each screen (processor, modules etc) that could have given the different switching times. realistically the cabling isn't going to make any difference, it'll be the hardware in the units.

[Tom]

Two reasons probably. Firstly the cost and secondly because the organisers don't ever see it and don't really care. These days with video recorders comprising vast arrays of RAID discs it's a fairly simple task to delay video by as much as you need. It just isn't cheap. In BBC sport many years ago we used to use two quadruplex VTR machines with tape buffer between them. Mostly due to the 2" tape handling problems, we couldn't get much more than a few tens of seconds delay, but it did work well enough most of the time

Well I can't believe it's because the organisers don't see them. If you followed that through then you wouldn't bother delaying the sound back there either. Besides, I'd have thought a lot of people working on an event like that do end up watching from the rear of the arena as they need to be able to move about and can't get stuck in the middle of the crowd.

So is it cost? In my naivety I'm surprised it's that expensive to achieve - particularly in the grand scheme of things. Those cameras on wires flying around over the crowd can't come cheap but they didn't seem to be getting much use out of it.

 

The problem generally is the other way round, in that the video processing introduces so much delay that the audio arrives first but in larger venues the video traveling at the speed of light catches up with the audio traveling at the speed of sound by tthe time it reaches the mix positionso it all averages out, for the video to be a second in front of the audio you half to have been about a quarter of mile from the stage, surely the sites not that big? Also even if you do delay the relay screens, you cant delay the ones either side of the stage, or for that matter the actual real onscreen action...

OK - maybe not a second - but the screens were probably a good 150m from the stage so it is more than half a second. The point is the delay made them unwatchable.

I'm aware they can't delay the ones either side of the stage - though any inherent delay would be an advantage and probably not a problem for those right at the front who, having stood there for hours in the rain and endured the squash were presumably planning to be watching the boss in person and not on a screen.

 

So is it only money and indifference?

 

T

[paulears]

I don't think people actually care that much. I produced a video in a venue with in-house cameras and big screens - in some shots it's very obvious these screens suffer from a very noticeable delay - made worse I think by going through two digital processors - so probably a 10-12 frame delay just because that's the internal delay of the mixers and scalers. I doubt any punters have complained - probably just accepting these things are how they are!

[timsabre]

[Those cameras on wires flying around over the crowd can't come cheap but they didn't seem to be getting much use out of it.

 

They would be using that on a separate mix for a show DVD. If the screens are for IMAG (helping the audience to see the act) you try to stick to stage closeups on the screens.

[BlueShift]

I noticed something similar when watching the Diamond Jubilee Concert, on some occasions bbc cut to a view down the street with several big screens and u seen them all change one at a time but not in order,

I'm guessing the video processors were all out slightly with different cable runs ect.

 

I think you'll find those screens were delayed deliberately, hence the staggered switching you saw.

 

It doesn't take a massive venue to experience these kinds of delay issues. Lets take an example room. its 80m long. Stage is at 0m. PA and screen also at 0m

For the punter who is behind the barrier (2m) the audio arrives in sync with the stage. lets assume analog signal path, a millisecond of delay max. A typical mid-range camera switcher chain might impart 3 frames of processing delay. Thats 120ms. Now assuming a very rough audio delay calculation of 1ms per foot (3ms per m) then the video and audio would be in sync at 40m from the stage. By the back of the room, the video is now 120ms ahead of the audio, which is enough to be distracting. Push this out to a more complex video system with more delay and a audio delay tower at 200m, and the difference is becoming massive. What you want to do though is delay the video to the midpoint of the desired audience, to minimise the absolute delay for any one audience member.

 

The issue is cost. essentially, digital delays just work by buffering data. If I have a digital audio console running at 48k, 24bit and want to delay the audio 120ms then I need ~16.8 kilobytes of memory to do it. trivial. Delaying a HD SDI video signal the same amount needs 22.28 megabytes of memory. A primitive example, but essentially shows how much more data there is in a digital video stream, which is one of the reasons its more expensive to do.

Also because when you put the word 'broadcast' into a piece of gear's name, you add a 0 to the price...

[Pete McCrea]

The thing with the screens down the Mall wasn't a delay increasing in length away from the stage, it was almost random, so at a delay point, one screen would switch quicker than the paired unit on the other side of the road, then the front would switch before the final pair at the furthest delay point. This said to me that it wasn't possibly delayed, and that processing in each screen trailer was the influencing factor in the switching.

[Peter F]

The thing with the screens down the Mall wasn't a delay increasing in length away from the stage, it was almost random, so at a delay point, one screen would switch quicker than the paired unit on the other side of the road, then the front would switch before the final pair at the furthest delay point. This said to me that it wasn't possibly delayed, and that processing in each screen trailer was the influencing factor in the switching.

 

You are right, there was something odd about the timings of those screens along the Mall, I knew it wasn't just in my imagination.

 

According to an article in Lighting & Sound International, this is what happened:

"Josh sent Stuart Young from CT, who installed all the screens down the Mall, all the different delay times for the V-DOSC systems deployed along its length. Stuart then meticulously synchronised all the screens to their local PA system: that had a huge effect in the energy from the crowd. Guy Freeman, who directed TV, commented on it."

 

So, were the "local PA systems" delayed very strangely? Maybe the sound was synced to the local screens, not screens to sound as the quote implies? It seems extremely unlikely. Maybe the differences in processing times of the screens, which has been noticed, has now been turned into a "feature" through a creative press release?

 

If anyone is on here and in a position to explain the reason those screens had such unusually differing timings on them I would love to hear it.

 

Cheers,

 

Peter

[AHYoung]

Often the greatest "skill" that one can have is to explain the fantastic benefits offered by a somewhat hastily handled instal...

Personally ive never heard of screens meticuliously being delayed to match the audio as its kind of irrelevant considering you can see more than one set of screens and the stage itself so somethings going to be wrong, at the most you might expect that groups of screens are the same model using the same processing so the delays are consistent.

You can cope with it being out of time and your brain works out whats going on as we are used to the audio / video out of sync as it obviously happens in nature, it gets strange when the audio arrives first as thats not natural, or if there is more than one image at different timings, that hurts your head.

[sleepytom]

The issue is the slow speed of sound traveling through the air.

 

Every 13(ish) meters from the PA is equivalent to 1 frame of (PAL) video.

 

The general perceived wisdom is that the processing delay of the video signal path kind of equals the audio delay so there isn't too much of a problem. Of course perfect sync is only possible in a very small area of the audience.

 

The trend for running cameras upstream of media server processing is making everything worse (or better depending on where your looking from!) as the media servers I/O video delay can be 5 or so frames.

 

LED screens can introduce there own latency + the obligatory Folsom Image Pro that will be put in the chain at each screen will add a frame.

 

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"Josh sent Stuart Young from CT, who installed all the screens down the Mall, all the different delay times for the V-DOSC systems deployed along its length. Stuart then meticulously synchronised all the screens to their local PA system: that had a huge effect in the energy from the crowd."

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That is rubbish - for starters CT don't have the kit to do it, and secondly "synchronised" simply doesn't exist on a gig of that scale where each of the "local PA systems" is covering an area of 50+ meters at the minimum, (see first point re speed of sound).

 

In other news line arrays sound terrible when its windy. Actually large scale sound reinforcement of any kind won't ever be that good in the open air. The industry simply ignores this and gets on with the show as best they can given the inconvenience of the laws of physics. People who go to big gigs / festivals generally don't notice that much as they are having a really good time dancing with 100,000 other people. If you want perfect sync and hifi audio then buy the DVD and watch it at home. If you still want to go to big concerts and you find the lipsync to be too off putting then simply move closer / further away - simply changing position by 20 or so meters could make a dramatic difference to your experience.