Delay Compensation Messing Up Video Sync..?

[David Sandwisch]

I just found this topic via Google and have been pondering the question of video tracks needing to be delayed to be in sync with sessions that have a lot of ADC. Chief, your responses are excellent and very helpful but I still am a bit confused.

I feel I understand ADC as it relates to audio tracks and the need to enable / disable it accordingly to keep things like recorded stem tracks in sync.

I am still not clear about whether the video needs a value entered for video sync offset other than the inherent A/V system delay always present in the room.

For example let's say a track has 16,000 samples of ADC which would be = to 8 frames at 24FPS. With delay comp enabled all the audio tracks will sound correct together but will they be in sync with the video track assuming no video offset of 8 frames?

One of your responses makes me think the video will not be in sync unless I enter an 8 frame video off set. However a later response to the original poster seems to indicate that simply having ADC turned on will fix the issue, no offset needed.

"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."

My reading of Jon's scenario is that the system is delayed more than the inherent system delay because of a processing time hog plugin. His audio should still be out of sync with the video in the room because PT is comping more than the time it takes the video signal to exit the computer and reach his eyes. It would only be in perfect sync had PT randomly needed to delay the audio the exact amount of the inherent video delay.

Based on this discussion it seems one would have to change the video sync offset occasionally as more and more ADC is added to the system. For example large film mixing sessions in which a lot of new time hogging plugins are added.

[ggegan]

Originally Posted by Chief Technician

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!

Wow, this is news to me. I have a BM Decklink Extreme and feed trilevel video reference to it and my sync HD. If I set the Sync HD to generate timecode, my SyncCheck seems to give me very stable sync readings and when I am working I don't perceive any wandering sync, However if I disconnect the reference I get noticeable varying sync offsets every time I stop and start the system. Are you saying this is a placebo effect?

[Chief Technician]

Originally Posted by ggegan

Are you saying this is a placebo effect?

Not entirely. First, the statement I made is only applicable to Pro Tools v10.x. Secondly, I have found that sync is repeatable more than 2/3rds of the times I press play. Once in a while, less than 1/3rd of the times I press play, something will look out of sync. A quick stop and start and things are, 99% of the time, back in sync as expected. This occasional observation that playback is out of sync is attributable to the QuickTime engine not being genlocked.

The problem described above does not exist in Pro Tools v11 and later.

[Chief Technician]

Originally Posted by David Sandwisch

One of your responses makes me think the video will not be in sync unless I enter an 8 frame video off set. However a later response to the original poster seems to indicate that simply having ADC turned on will fix the issue, no offset needed.

ADC compensation keeps the audio in sync with the video. If you have entered a video sync offset value, the ADC will keep the audio aligned to that video sync offset value.

Here is another way to think about this. If you use a Syncheck to set your video sync offset to 8 quarter frames (example only!), then any ADC applied to your session will keep all of your audio in sync with that 8 quarter frame offset. If you change the video sync offset to 4 quarter frames, the audio will be in sync with the 4 quarter frame offset.

Succinctly, any ADC that does not exceed the maximum value of compensation available will be time-aligned with whatever your video sync offset is.

[ggegan]

Originally Posted by Chief Technician

Not entirely. First, the statement I made is only applicable to Pro Tools v10.x. Secondly, I have found that sync is repeatable more than 2/3rds of the times I press play. Once in a while, less than 1/3rd of the times I press play, something will look out of sync. A quick stop and start and things are, 99% of the time, back in sync as expected. This occasional observation that playback is out of sync is attributable to the QuickTime engine not being genlocked.

The problem described above does not exist in Pro Tools v11 and later.

Come to think of it, when I use SyncCheck to determine my video offset while locked to a video reference I'm guessing that maybe 20%-25% of the times that I hit play the reading is off. (Without a reference it is different every time I hit play.) If I stop and hit play again it is correct, but this may suggest that the problem has something to do with an intermittant inability to lock to the reference. Using a video reference is definitely a lot better than not doing so, but it seems that it is not 100% reliable at attaining perfect sync. I'll definitely take 75% of the time over almost never, but it should be perfect every time.

[David Sandwisch]

OK so ADC will keep the audio in sync with the video or more precisely the video sync offset. So given that, is PT 11 holding back (i.e. delaying) the video stream equal to ADC so that they all leave the software in sync? In other words, is the video track having the appropriate amount of delay added to it to let the audio catch up?

If the above is true, then that would make sense to me and goes along with having to keep ADC on for tracks that are recording stems otherwise with ADC off the record tracks aren't in sync.

I think it boils down to is PT reading / processing audio ahead of the play head so once it's processed it reaches the play head in sync with everything or is the play head what is being read and what you hear is slightly behind the play head = to the ADC value of the session? From having worked on and observed some large film sessions at a big studio, it appears that what you are hearing is behind where the play head is on screen when there is large ADC happening. Then the video should have delay added to it to keep in sync with the ADC of the audio tracks and hence it still looks in sync in the room. All under the hood and the timeline stays in total sync regardless of ADC.

[ggegan]

When dealing with very large ADC values I have found that while the audio is in sync with itself and the picture (providing it has the correct video offset), hardware controllers, such as a D-Command and D-Control faders and knobs may record automation late when very high latency plugins are present in the signal path.

This would suggest that in order to accomplish ADC the playback of audio, video and the visual representation of the timeline are all delayed relative to the actual input timeline. That means that your response to visual or audio cues will be late. This isn't an issue with very small amounts of ADC, but when you get to really large values that result in as much as 33msec or more of delay (approx 1 video frame) it can really throw off your timing.

I first noticed this issue in PT9, but have always avoided using super high latency plugins in the signal path since that time, so I can't confirm that it continues to be true in PT10, 11 and 12. I would expect that it is, although PT11 and 12 may be able to process audio faster, thus avoiding extreme latency which would keep the delay within tolerable limits.

[David Sandwisch]

This makes great sense to me. I don't know if pt 11 or higher accounts for adc when recording automation from a control surface but it should if it doesn't already. This is a great thread and I thank everyone for your input!

[antonis]

From all the afore-mentioned do we conclude that:

ADC:

The system, under the hood, calculates the plugin induced delay, and the system prioritises the track playback by playing the track with the highest plugin induced latency first, and the rest follow after the mathematical computation?

Video Sync Offset:

As there is less delay to play back audio (as opposed to the video signal been sent and converted by the monitor), the whole of the audio is been offset (delayed) by the quarter frames set by the menu option, irrespectively of or if ADC is been applied (i.e. all the audio outs will be offset regardless if the plugin induced delay is been compensated or not)?

Deviating slightly from the subject, I have to mention the significant improvement on PT11 and the ability to offset using milliseconds instead of quarters frames. But from what has been noted above, this brings up the question: when PT11 is using the QT engine, is PT still using video as a 'black box' and only "Avid Video Engine" follow video reference? Again, apologies for shifting the subject a bit?

[Chief Technician]

Originally Posted by antonis

Deviating slightly from the subject, I have to mention the significant improvement on PT11 and the ability to offset using milliseconds instead of quarters frames.

While it does not say this in the v11 Reference Guide, I do believe that milliseconds is only applicable if you are not using a genlocked video playback device. If your video playback device is genlocked, I do believe your offset value is rounded to the nearest whole frame.

Originally Posted by antonis

when PT11 is using the QT engine, is PT still using video as a 'black box' and only "Avid Video Engine" follow video reference? Again, apologies for shifting the subject a bit?

From the v11 Reference Guide: Pro Tools uses the Avid Video Engine (AVE) for video playback. AVE lets you simultaneously view desktop video (native video) and video monitored using a supported Avid or third-party video peripheral. AVE provides support for a wide range of QuickTime and Avid HD and SD MXF video formats on Pro Tools video tracks without transcoding (including Avid DNxHD®).