Delay Compensation Messing Up Video Sync..?

[Jon_Atkinson]

Hey all,

Just started scoring a feature film... I'm in 10.3.6 with CPTK on 10.8.4.

All is very solid, and the workflow with Vienna Ensemble Pro hosting VI's is great.

However, I've just imported the master movie file into my PT setup (and the relevant audio file), and just going through spotting music cue moments... And I noticed that the dialogue wasn't spot on in sync... Which was odd as it looked 'in' when I watched it with the director yesterday.

The movie file is ProRes LT playing out a Blackmagic Intensity Pro card. I have video sync offset at 13 quarter frames, which I believe to be correct for this setup.

So I checked and I'd left Delay Compensation switched to 'Long'... Largest compensated track was in fact the one hosting VEPro (1056 samples so well within the 'Long' setting)

Switched that back to off, and now everything looks spot on in sync.

Does Delay Compensation not deal with video too...? Really..?!

What's it actually doing here? Is it just delaying the audio from the video file, but leaving the video in place? If so, what's the best workaround?

Thanks in advance

Jon

[Chief Technician]

Originally Posted by Jon_Atkinson

Does Delay Compensation not deal with video too...? Really..?!

What's it actually doing here? Is it just delaying the audio from the video file, but leaving the video in place? If so, what's the best workaround?

Audio Delay Compensation (ADC) is completely separate from Video Sync Offset. The purpose of ADC is to time all of your audio tracks and routing so that when each signal gets to the point in the signal chain where it is sent to your interface output, all audio signal paths are time aligned.

Note that if you are using an output to nowhere (aka dummy output) that this will screw up ADC. All audio paths must go somewhere, even if it is to an interface output that you will never use.

Video Sync Offset effectively advances the video by the specified amount so that it is close to in sync with the audio at your interface output.

Note that both audio delay compensation and video sync offset are applied under the hood. Nothing on the timeline is ever shifted to achieve video sync or audio delay compensation.

Now for the truth about QuickTime playback in Pro Tools v10. Even though you can set your video sync offset in quarter frame intervals, it will be rounded to the nearest whole frame when playing out a peripheral! If your video sync offset is 17 quarter frames, Pro Tools treats it as 16 quarter frames.

Why is this? The QuickTime engine is, as far as Pro Tools is concerned, a black box. This black box does not sync itself to video reference, even if you connect a video reference signal to your playback peripheral! Have you ever noticed that once in a while you will press play and things will look egregiously out of sync? If you stop and press play again, they will most likely be in sync. This is related to the +/- 1/2 frame error you can get with QuickTime video playback every time you press play.

Now, if you do not have enough audio delay compensation for your signal paths, your audio may very well be out of sync. The video sync offset is based upon your audio signal path already being synced. If you exceed your audio delay compensation value, then the audio will no longer be synced with the video. I had this happen on a system at the facility I work at when it was upgraded to HDX. I had set the system to use Long delay compensation to maintain compatibility with TDM systems during the HDX rollout. This HDX user had so much native processing going on in his session that he exceeded the Long delay compensation value. Once the system was switched to Maximum delay compensation (4x long), audio and video were back in sync.

[John McDaniel]

HOLY CR@P, John….

I don't think i've seen that much good arcane, technical --and important-- information in one post in a LONG time, if ever.

Dummy outputs mess up ADC? Seriously? By a fixed amount? I've been trying to figure out a problem with a session where I was printing a full mix and 3 stems at once and the DX stem was a AHEAD while another was dead on and another behind --all by very small amounts. WTH? Doesn't one of the AVID Post Production templates have Dummy outputs? I know I've received large sessions from other audio post facilities that used Dummy outputs.

And QT playback in PT 10….. we just installed a new projector and spent a lot to time trying to get the delay right with a Syncheck box…. I guess this accounts for variations in readings from the Syncheck?

UGH…….

[Chief Technician]

Originally Posted by John McDaniel

HOLY CR@P, John….

I don't think i've seen that much good arcane, technical --and important-- information in one post in a LONG time, if ever.

Thank you John!

Originally Posted by John McDaniel

Dummy outputs mess up ADC?

Yes.

Originally Posted by John McDaniel

Seriously?

Yes.

Originally Posted by John McDaniel

By a fixed amount?

No, it is a variable amount, and it depends on what is happening before the dummy output.

For example, lets say you have an audio track that feeds an aux, followed by another aux whose output goes nowhere (dummy output bus). Pro Tools' ADC code does not know what to do for timing the output bus to nowhere, since it's basically an infinity delay (never reaches a destination). What ADC will do is compensate two points back in the signal chain. In this scenario, ADC will compensate up until the signal reaches the first aux input. If the previous points in the signal chain have few plug-ins, then the delay value will be small. If the same audio track in this example has a DSP plug-in (i.e. EQ) followed by a Native plug-in (i.e. noise reduction), then those processes will require much more ADC, hence the variability of the value.

This is one of the dirty little secrets of ADC in Pro Tools v10 (and earlier). I haven't tested this with v11. The facility I work at errs on the side of caution and doesn't use dummy output busses. We make sure all of our audio goes somewhere, even if it is a dummy interface output. A dummy interface output could be output 8 of your 192 I/O (mono signal path). This output would not be connected anywhere else, you wouldn't monitor it, etc. The point is that this is a physical output, which means the audio went somewhere, therefore ADC can calculate the amount of compensation necessary.

Originally Posted by John McDaniel

I've been trying to figure out a problem with a session where I was printing a full mix and 3 stems at once and the DX stem was a AHEAD while another was dead on and another behind --all by very small amounts. WTH? Doesn't one of the AVID Post Production templates have Dummy outputs?

I haven't checked the AVID Post Production templates. I prefer to build my templates from a blank session.

Originally Posted by John McDaniel

I know I've received large sessions from other audio post facilities that used Dummy outputs.

Maybe the amount that those sessions were off by was too small to notice?

Originally Posted by John McDaniel

And QT playback in PT 10….. we just installed a new projector and spent a lot to time trying to get the delay right with a Syncheck box…. I guess this accounts for variations in readings from the Syncheck?

UGH…….

Yes, the +/- 1/2 frame of slop that you can get any time you press play is most likely the reason for the variations. When I am measuring for video sync offset, I expect to get a few outliers in terms of quarter frame values. I will do several start/stop operations with my Syncheck session and observe which value keeps coming up more often than the others. The value that comes up most often is what I settle on for the video sync offset.

If you have a Mojo or Mojo SDI, you do not get this +/- 1/2 frame slop when using Avid video. You would get this +/- 1/2 frame slop with a Mojo or Mojo SDI if you were playing QuickTime video.

[Jon_Atkinson]

Hi Jonathan,

Thanks so much for the detailed and incredibly helpful answer on Delay Compensation...!

It pretty much confirmed to me that Delay Comp behaves exactly as I thought it should... Which make me think that I have something a bit odd going on in my setup... (perhaps I have something routed incorrectly...?)

The info on dummy outputs is something I wasn't aware of. What specifically would be an example of a dummy output, just so I'm 100% clear..?

To confirm then, if I have one video track, and it's associated audio file (single stereo file imported from the video), and delay compensation is enabled and is set to greater than any inherent delay in the system (in this case the AUX which has VEPro instantiated), then in theory the audio file *should* play back in time with it's video with delay comp enabled?

In my setup it's not, so I should probably do some digging and find out where I'm going wrong..!

Thanks for the insight!

Jon

[Chief Technician]

Originally Posted by Jon_Atkinson

Hi Jonathan,

Thanks so much for the detailed and incredibly helpful answer on Delay Compensation…!

You are welcome.

Originally Posted by Jon_Atkinson

It pretty much confirmed to me that Delay Comp behaves exactly as I thought it should... Which make me think that I have something a bit odd going on in my setup... (perhaps I have something routed incorrectly...?)

The info on dummy outputs is something I wasn't aware of. What specifically would be an example of a dummy output, just so I'm 100% clear..?

A dummy output would be a Pro Tools internal routing bus (i.e. Bus 1) that is assigned to any track's output while never being assigned to the input of any other track in your session.

Originally Posted by Jon_Atkinson

To confirm then, if I have one video track, and it's associated audio file (single stereo file imported from the video), and delay compensation is enabled and is set to greater than any inherent delay in the system (in this case the AUX which has VEPro instantiated), then in theory the audio file *should* play back in time with it's video with delay comp enabled?

In theory, yes, though your example assumes a video sync offset of zero. Even a computer display has a video sync offset.

Originally Posted by Jon_Atkinson

In my setup it's not, so I should probably do some digging and find out where I'm going wrong..!

If you have a Syncheck, test your setup with a session that only has the Syncheck movie file and its associated beep track. Once you do this you will know what your video sync offset is. If you go back to your problematic session and you still have video sync issues, then considering session routing, video codec, video and audio on different drives, etc.

[Jon_Atkinson]

Originally Posted by Chief Technician

A dummy output would be a Pro Tools internal routing bus (i.e. Bus 1) that is assigned to any track's output while never being assigned to the input of any other track in your session.

Ok, thanks... that's what I thought you meant, but just wanted to confirm!

Originally Posted by Chief Technician

If you have a Syncheck, test your setup with a session that only has the Syncheck movie file and its associated beep track. Once you do this you will know what your video sync offset is. If you go back to your problematic session and you still have video sync issues, then considering session routing, video codec, video and audio on different drives, etc.

I don't, but have borrowed one in the past to set up this current rig. I always use ProRes LT, and nothing else has changed (so I'm probably there or thereabouts... Flame suit at the ready, but I'm a composer: a frame here or there is ok...!) I believe my offset to be 13 quarter frames, and as I say, currently if I turn off Delay Comp it looks bang on in sync.
If I turn it on, then it's wildly out (I'm talking maybe 5-8 frames)

I'm going to do some testing now and see if I can pin this down... Thinking that there's something a bit weird going on with VEPro...

Will report back.

[Jon_Atkinson]

Set all tracks to output Analog 1-2. Solved the problem.

Not sure what was wrong routing-wise, but that's cleared it up.

Thanks

[Chief Technician]

Glad to read that you found a solution.

[Jazzbass56]

Not to get off topic ,has anyone tried, or have any experience using these iPhone apps like"Catch n Sync" or "HD AV sync app" to measure and adjust your video/audio off set to various monitors?