compensating for video latency

[timtheenchanteruk]

This is our setup this week PC, 1 fixed camera, 2 operated cameras (all S-Vid) though an Edirol vis mixer out to 2 projectors via composite.

 

there is a noticable lag on the video, whilst, for the purpous of this even isnt really an issue, in future it may be, so how do you minimise the latency? and for pre-recorded video etc, how could you either minimise it or compensate for it in the mix stage?

 

This is a small venue, so adding a delay onto the audio would look silly, as you can easily and clearly see the stage.

 

I realise there will always be some lag, and im guessing this is all processing time, as one cable is alot longer than the other feeding the projectors. is it down to the quality of the equipment being used?

[J Pearce]

Part of it is due to the mixer having to resync the cameras. If you were using posh kit the cameras could all be genlocked and then they wouldn't have to be resynced before mixing.

 

The mixers resync by holding the video frame in a buffer and letting it out at the right time to get it back in sync, so you can lose a frame just in that. Most other operations will take a frame each (it has to process a complete frame), so each mix buss will lose a frame, any colour correction will be another frame, effects maybe 2 or 3 frames, the output buss might lose another frame if its got a resync for external keying.

Then you get to your projector which will have colour processing, keystone processing, scaling so there are possibly another 3 frames gone there.

 

All adds up!

 

As I'm sure you know cable length makes very little difference unless you've got a few thousand km coiled up somewhere. Its like pumping water into an already full pipe. Electrons work on displacement, the first electron doesn't need to get to the end to relay the signal.

 

You can get away with a bit of delay on the sound, some might even consider it advantageous as it should move the perceived acoustic location back to the stage rather than the stacks due to the precedence effect.

 

Although better kit helps with less delay its a problem that doesn't really get properly solved until you get to screens so big that you don't see the stage...

[Pete McCrea]

As Jon says, it's ultimately down to the quality of the kit, and the processing delay that it puts in the chain. We regularly run out Sony EX3 cameras (£4500 each) -> Panasonic AV-HS400 Mixer (£6500) -> Barco Screen PRO II (£10,000+) -> Barco CLM HD8 (£15,000), and with out spending £000,000 this is about as low a latency as you'll get. But there are still little tricks that we use to drive it as low as possible. Genlocking and taking frame stores out of the line, as well as matching resolutions where possible to minimise processing time are all key, but only features that become available on higher end kit.

 

With the likes of the Edirol, it's the trade off for the cost of the equipment and the feature set.