Can sync depend on file size?

[Pythagor]

Hi
10.6.8 PT 8.1 Decklink Intensity Pro Mac 3,1
Syncheck shows 16 qtf video offset for Qt ProRes HQ 1920x1080 using Syncheck supplied files.
Yesterday i received 45 min video QT ProRes HQ 1920x1080 with sound where i offset looks OK with 7 qtf

the question is:
can offset depend on file size?
any help is much appreciated
many thanks

[Chief Technician]

Originally Posted by Pythagor

can offset depend on file size?

Unlikely. Is the offset constant? Does it change based upon how far into the movie you are?

[Postman]

Frame rate, yes. Codec, maybe. Movie length or file size, should not happen. Can you paste a Syncheck movie onto the video track in the middle of your session? Route beeps like all other session audio. Start playback well ahead of the point and solo the beeps track when you reach it for measurement. If sync has changed unexpectedly ahead of that point, you'll know.

In the bad old days and earlier versions of Pro Tools, QuickTime movies could slip sync and play remaining video out of sync until stopped and started again. I have even seen the same thing happen in Media Composer Video Satellite when playing certain files linked through AMA (which I believe is similar to how PT11's video engine works now). It shouldn't happen but it does.

[Postman]

10.6.8 PT 8.1 Decklink Intensity Pro Mac 3,1
Syncheck shows 16 qtf video offset for Qt ProRes HQ 1920x1080 using Syncheck supplied files.
Yesterday i received 45 min video QT ProRes HQ 1920x1080 with sound where i offset looks OK with 7 qtf

I have more thoughts and question for you. First, cut in a sync check measurement movie into your working session.

1. Check offset with automation on and automation suspended. Do you get the same measurement both ways?

2. Are you using a Sync or SyncHD with common reference to it and Decklink? Without both, you'll never achieve a consistent offset.

3. Do you know that 1//4 frame adjustments do not result in 1/4 frame changes to the AV offset?

[Pythagor]

I've t inserted syncheck files at the beginning and end of 66 gb movie. I found the difference of sync mesured at the beginning and the end is about 1/3 frames and that is not what i am looking for.
Postman, could you please advise why 1//4 frame adjustments do not result in 1/4 frame changes to the AV offset?

[Postman]

I am unable to use fewer words to express what happens. Please forgive my long expanation.

Video is a stream of individual video frames, one after another in a very precise way. The video card hardware is designed to output that, using data you send to it. It does not go any faster or slower, always one frame followed by the next. QuickTime and Pro Tools also outputs a stream, one frame at a time, but not necessarily at the same time as the card. The card will receive that data stream. As each new frame is received it is placed into a buffer where it is held until the card's hardware is ready to begin output of a new frame. In this way, the video card always "quantizes" the video stream from Pro Tools/QuickTime so that it fits into the card's hardware timing. If the card is ready to output the next frame but it has not completely received from Pro Tools a new frame in time to begin outputting it, it has no choice but to repeat the same frame it already is showing and wait for the hardware to start the one. This means the audio could be playing for a frame that has not yet begun to be output. Audio and video are out of sync. If Pro Tools/QuickTime has gotten ahead of the card and sends a new frame before one already in the buffer has been output, one of the other of those undisplayed frames must be dropped, and again there is a sync problem.

As you make 1/4 frame adjustements, Pro Tools may in fact send video to the card 1/4 frame sooner or later (relative to the audio), but the card still must adhere to its own hardware timing. Ideally, you want Pro Tools to send each frame of video just in time for the card to be ready to output it. If Pro Tools is too early, audio is ahead of video. If it is too late, a frame will be dropped and the last one repeated, meaning video stutter and bad sync. Best video and audio playback is like perfect dance partners! As long as both the card and Pro Tools do their parts always on time you will get repeatable AV offset. If one or the other changes in any way, you will see AV offset changes.

A perfectly timed "dance" between Pro Tools and the video card is only possible when both are operating at the same video frame rate.

BTW, this exact same sequence of events happens when video is being displayed on your desktop, without a video card. Why is a video card better? It is not better except for one very, very important difference. A regular desktop video output is displayed at a frame rate which is NOT related to the video from Pro Tools. There is no way for the precise dance to happen. A video card can lock its hardware to YOUR reference signal. As long as Pro Tools can also lock its hardware to YOUR reference, using a SyncIO (SyncHD), the dance is possible.

Once you realize this, you can begin to understand why it is critical to use a common reference signal for both audio (SyncHD) and video (the card). Unless both are timed in repeatable, precise ways, the "dance" fails.

[Chief Technician]

Some elaboration on Postman's description.

QuickTime video playback in Pro Tools v10 and earlier can be thought of as a black box. QuickTime is the black box. All Pro Tools can do is send video into QuickTime (black box). QuickTime decides when to output the video that came into it. While it can do this in time with a sync input, QuickTime was really designed as a FIFO (First In First Out) process, where as soon as it receives a frame, it sends it to the output destination and then immediately looks for another frame.

Let's revisit the idea that changing your offset by a single 1/4 frame does not make a difference. When you are using a genlocked video card for output, its only concept of time is a whole frame interval (4 quarter frames). While Pro Tools lets you insert single quarter frames (such as 17) for an offset, the video card doesn't understand that 17th quarter frame's position within a sync pulse (whole frame). The end result? Your 17 quarter frame offset is rounded down to 16 quarter frames! If you have a 19 quarter frame offset, it's rounded up to 20 quarter frames! Halfway values, such as 18 quarter frames, usually round up to the nearest whole frame.

These issues are supposedly gone with v11 and the Avid Video Engine (AVE). Instead of QuickTime's black box, Avid has more control over the whole process from the moment you press play until the moment the signal is at the card's physical output.

[simonchase]

Originally Posted by Chief Technician

These issues are supposedly gone with v11 and the Avid Video Engine (AVE)

Sadly my tests with sync check seem to show that it's still one frame at a time in v11. Although it drifts back and forth way less.

[Postman]

AVE in PT11 can be MUCH better than a QuickTime window in PT10! You will still get a 1/2 or 1 frame drift. It is unavoidable unless PT, video hardware, and display monitor are all locked together at the exact same frame rate. But yeah, AVE is way better.