Jump to content

Video / audio sync – Best practice


bthebassman

Recommended Posts

We’re using a new WXGA over CAT6 video sender (LKV383VGA-POE) from stage to our video mixer (Roland V40HD), and routing the audio from the stage via DI and AR2412 to our GLD80. The video is (shockingly) delayed by about 600mS compared to the audio!!! There are several ways to tackle this…

1. Delay the audio to sync up to the video in the GLD

2. Send audio and video from the video mixer only (no issues)

3. Buy in a dedicated audio delay (I have an Alesis Microverb somewhere?)

4. Change the WXGA sender / receiver

 

Option 1:

On the Allen and Heath GLD80, I found a built in input channel delay of 100mS – not nearly enough, so then thought I’d use our FX delays for as clean as possible delay. First I tried routing through a group, but for some reason hit a brick wall when trying to send the group to main mix?? (Maybe I’m just having a dumb day?). Anyway, I know how to route the Group through FX to delay as a stereo pair, and I’ll probably solve it tonight as I nod off,(am I in good company here?).

So, after being defeated, I then thought, use an insert into each input channel – which works well, BUT – although the FX delay is stereo in/out, the FX channel will only allow 1 insert at a time, so I had to use 2 FX channels, one for Lin and 1 for Rin. Working but not ideal.

Option 2 and 3 are practical but not the elegant solution I’m looking for. Option 4 is admitting defeat.

 

So the question is, for around 500-600mS of good, clean, stereo delay, what is best practice?

 

...or...

 

How can I fix the video delay?

 

 

Link to comment
Share on other sites

I would have thought the delay is mostly in the cat6 video sender. The mixer won't be adding more than 1 or 2 frames delay.

 

The Roland V40HD has a variable audio delay 1-12 frames which might sort you out though, if your application can live with everything being delayed.

Edited by timsabre
Link to comment
Share on other sites

3. Buy in a dedicated audio delay (I have an Alesis Microverb somewhere?)

 

Keep an eye on eBay for a 'BEL' audio delay. They go for not much money and are often fitted with oodles of delay time. (oddles = >1 second)

Link to comment
Share on other sites

I would start by determining where in the signal chain all that delay is coming from. It may be possible to change some settings to reduce it at the source. 600ms is 20 frames of video, that is quite a lot. There is probably at least a frame of delay at each conversion point, and possibly several frames in the video mixer, depending on how the mixer is set. Leaving video effects in the mixer enabled when you are not using them will add delay.

 

In a large hall there will also be acoustic delay based on time of flight. I usually consider being plus or minus 1 frame to be acceptable lip sync, so you have an audience depth of around 20m where you can achieve good sync. beyond that you will have to decide which 20m of audience depth is most important to have as that good sync zone.

 

Mac

Link to comment
Share on other sites

Met an interesting effect when using iPlayer's "Restart programme", when the sound is continuous, but the picture jumps forward every now & again, so ends up several seconds ahead of the sound. Only noticed so far on Mr Portillo's railway journeys.
Link to comment
Share on other sites

Met an interesting effect when using iPlayer's "Restart programme", when the sound is continuous, but the picture jumps forward every now & again, so ends up several seconds ahead of the sound. Only noticed so far on Mr Portillo's railway journeys.

 

Trains are never on time though... :P

Link to comment
Share on other sites

Extenders should not be giving you that level of delay - HDBaseT and HDBitT should both be near zero latency. Even AV over IP 1GHz solutions are sub-frame latency. It is relatively easy to check if it is the extender - get a laptop or PC with two outputs. Plug in two computer monitors and duplicate the displays. Run a timer app with milliseconds on it and take a video of the two screens. They should pretty much be in sync. Then put the extenders in line. Take another video. Compare. I would then start to add everything else in the video chain in line as well. If you are getting 600ms of delay from the extenders, return them and demand a full refund, because they are not conforming to the HDBitT spec.
Link to comment
Share on other sites

  • 1 month later...

Sorry to disappear, but I'm back now.

 

I presume the delay is due to the mixer and not the CAT6?

Thanks Dave. It's defo not the mixer or CAT6 - tested negative. It's the Lenkeng TX/RX.

 

 

 

I would start by determining where in the signal chain all that delay is coming from. It may be possible to change some settings to reduce it at the source. 600ms is 20 frames of video, that is quite a lot. There is probably at least a frame of delay at each conversion point, and possibly several frames in the video mixer, depending on how the mixer is set. Leaving video effects in the mixer enabled when you are not using them will add delay.

The Lenkeng TX/RX was the culprit. We have cheaper (nastier) VGA and HDMI extenders - all those are fine!

In a large hall there will also be acoustic delay based on time of flight. I usually consider being plus or minus 1 frame to be acceptable lip sync, so you have an audience depth of around 20m where you can achieve good sync. beyond that you will have to decide which 20m of audience depth is most important to have as that good sync zone.

The hall is 12m deep and 20m wide, so very little acoustic delay to worry about. Thanks for the input mackerr.

 

Extenders should not be giving you that level of delay - HDBaseT and HDBitT should both be near zero latency. Even AV over IP 1GHz solutions are sub-frame latency. It is relatively easy to check if it is the extender - get a laptop or PC with two outputs. Plug in two computer monitors and duplicate the displays. Run a timer app with milliseconds on it and take a video of the two screens. They should pretty much be in sync. Then put the extenders in line. Take another video. Compare. I would then start to add everything else in the video chain in line as well. If you are getting 600ms of delay from the extenders, return them and demand a full refund, because they are not conforming to the HDBitT spec.

 

Hmm. Thanks Mac for that. No surprise that, support@lenkeng.com bounces straight back! Time to contact the cc for refund.

Edited by bthebassman
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.