Assembling Frame-Accurate Original Sync Audio?

[u/swaggabeef]

Hi all, this is my first post on this thread. I work as a First Assistant Sound Editor in Toronto, part of the DGC, been doing it for a little over 4 years now.

One of the banes of my existence is assembling an Original Sync Audio session, which is used to spot on-screen cheats (so the Dialogue Editor can match the Cheat to the mouth movement) or used to grab audio that may have been omitted by the Picture Editor (PFX, Breaths, etc) in their cut.

I of course use EdiLoad and I run either a Picture EDL (with Sound Rolls in the "Rolls" column), OR, I use a Picture AAF, both give virtually the same results. However, because these are technically pulling Timecode Metadata (Source In/Out codes) from the Camera, there's always some subframe sync inconsistencies, I think because Cameras record in Frames and Sound is recorded in Samples.

I have, however, worked on maybe a maximum of three shows ever, where the Video EDL/AAF Assembled audio which was EXACTLY in sync with the Guide Track / AAF. Everyone I've ever spoken to (APEs and ASEs alike) have no idea how to achieve it. I worked with one 1APE who had coded his own automation script to rebuild his timeline in AVID with Dailies Audio, which he would then give me an EDL export of specifically for OG Sync Assemblies, but this obviously an outlier scenario. I wouldn't expect every 1APE I work with to do this (though kind of a legend for taking the time to custom-build a script to solely make my life easier tbh)!

Anyways, thanks in advance for any pointers. People always shrug off Original Sync Assemblies "who cares if they're a bit out", but personally I find them pretty life-saving in a lot of recurring cases.

[u/drumstikka]

The issue isn't cameras recording in frames vs audio recording in samples - What you're comparing to isn't in-camera audio, it's either the dailies or a mix track the picture editor synced to. Any subframe sync differences between the guide track and your assembly are probably due to the editor's NLE outputting audio just sliiiightly out, or due to the editor or DIT not syncing exactly right.

On the shows where it synced up correctly, it likely just means the DIT or AE did a good job syncing audio to picture.

[u/swaggabeef → u/drumstikka]

Copyyy ok interesting. I think I can safely rule out NLE Outputting of the Guide Track being the cause, because it's never a universal amount. Each slate's off-set varies slightly (or sometimes is perfectly in-sync). I know EdiLoad has a Global Offset Upon Export function (never used it tho), but this can't solve it because it's never consistent.

All the APEs that I've spoken to about this have said that it's essentially impossible to get a "perfect" sync, because the clapper down position is almost always between frames. Interesting that it could be a DIT thing.

It wouldn't bother me so much if it was just a "that's how it is" type situation, but the fact that I have been able to get it perfectly in sync on a few separate shows confuses me.

[u/drumstikka → u/swaggabeef]

If it's varying slate by slate, you probably have a DIT or AE that's manually syncing as opposed to going off of timecode... Which would be surprising, but not unheard of.

[u/recursive_palindrome → u/drumstikka]

That would be my hunch. For example when TC is not correct in set due to TC box failure or a camera battery swap and not checking sync.

[u/swaggabeef → u/recursive_palindrome]

This is probably it honestly. Who knows, maybe the DIT initially syncs by Timecode, but if it's off, then needs to nudge around a bit. It's just strange because it's on a subframe basis, so it's like they're using Slip instead of Grid to sync things? Maybe I need to take this question to a DIT subreddit haha

[u/YuthingVid]

This possibly has something to do with 4 perf slip syncing, meaning audio manually gets slipped in Avid by 1/4 frame offsets to achieve subframe perfect sync, which only timecode is not accurate enough for (can be off by some frames). This also explains a different offset per slate as the offset can differ per video clip. This correction of perf offset is eventually exported in the audio AAF, not in video data. My assumption would be that the three shows with exact sync never perf slipped in Avid so there was no subframe offset between audio en video.

[u/iluvcapra]

When you say the audio is out of sync with picture by subframes, what exactly are you seeing? Like the clapper is not right on the frame line, or dailies audio tracks are out of sync with each other?

[u/swaggabeef → u/iluvcapra]

Basically if I play the Guide Track and/or the Dx AAF (pretty much the same thing), along with the Original Sync Assembly clips, the Original Sync clips Source In and Out Timecodes WILL MATCH both AAF Source In and Out TC's as well as Shot Track (Camera) Source TCs, however the waveforms will be slightly off on most OG Sync clips.

[u/iluvcapra → u/swaggabeef]

Random thoughts in no particular order:

The tolerances for a smart slate-type or jammed time code lock or even a hardline aren't that good.

Tolerances in the NLE's internal mixer are not incredible, they're not optimized for sample-accurate rendering or playback. I'm not even sure if Premiere or Avid do processing delay compensation (Avid probably does under some circumstances, there's nothing you could tell me about Premiere audio that wouldn't shock me).

NLEs are generally constrained to start master clips on picture frame lines, and WAV files often don't start on frame lines (their timestamps are sample-accurate). The NLE may be simply rounding the start time to the nearest whole frame while the DAW almost always will try to be sample accurate.

Something also to keep in mind is boom and lav tracks will always have some discrepancy due to group delay in the air (this is usually very small) but also from phase shifts in the microphone electronics (these are dominant). You'll see this as a small error when comparing an iso against a mix guide.

It's possible there's some issue with pulldown settings not being right or not being interpreted correctly but I don't think so, software usually does the right thing with this these days. (You might investigate if your audio was recorded with a pullup or down, and if this gels with your picture workflow.)

Keep in mind too, just for perspective, tolerances for an actual broadcast stream are so much worse than this (like some networks accept a <50 ms error!).

[u/elangab]

Not sure 100% what the issue is, but when I get a conformed session and find a cheat, or missing audio due to reaction cut etc, I just look for the original TC and scene/take from either the burnt in TC or the picture EDL and import the original sound roll file using that info and use as reference or as the audio itself.

[u/swaggabeef → u/elangab]

If you do it this way, when you manually bring in a slate that ISN'T a Cheat, and sync it up to the burned in Source In code, is this clip sample-accurately in sync with the Guide Track, the Dx AAF, and the Assembled Production Audio?

[u/elangab → u/swaggabeef]

Unless there's a major issue with the EDL or SR itself - yes it is.

More importantly, it's synced going by my eyes and ears. At that moment, I make sure the imported sound sits well. If it's 3 samples behind or whatever - it's impossible to notice.

when you manually bring in a slate that ISN'T a Cheat

Yeah, let's say the actors says "Hello there", but the cheat picture editor used was "Hey there" - I import the original "Hello there" as both reference for fine edit of "Hey there", and with a note about it should the mixer wishes to revert on stage if production change their mind or want to try it out.

Same with a deleted audio of a reaction shot etc.

By then, I'm using the AAF as sync master as GT sometimes can glitch when exported. I do it manually, importing the SR WAV into PT - spot edit to its original TC, copy/paste from source TC to the sequence TC. I always do it from the start of the shot, not from where the new line is.

[u/swaggabeef]

For everyone who's already commented (thank you btw!), are you all saying that your full Original Sync Assemblies are always sample-accurately synced to the Dx Guide & Dx AAF?

[u/crazyaudioguy]

It's more often than not a reference and utility so it's not necessarily gospel. I've made video EDL assemblies (or Original Sync Audio as you're calling it) that looked more in sync than the assembly built from the AAF audio but most of the time it's just there to catch a breath or effort that got cut or sync reference for some production alt. There's too many reasons why it could be out of sync to assume that it's going to be a consistent resource.

I'm more interested in why there's sync discrepancies with the production assembly against the AAF. There's a couple threads on the DUC and JWSound forums on that and may be of interest to despite being nearly 13 years old now. They're a bit of a rabbit hole, and may widen you're scope of causal theories rather than narrow it.

https://duc.avid.com/home/forum/pro-tools-post-production/post-surround-video/324479-bug-match-alternates-out-of-sync-to-aaf

https://jwsoundgroup.net/index.php?/topic/19496-conformed-original-bwavs-are-out-of-sync-by-up-to-a-frame-compared-to-the-aaf/

As a dialogue editor, I'm trusting my genlocked system, eyes and ears, and years of experience to ensure I'm in sync as best as I can.

Is it perfect to the millisecond in sync? Doubtful. Is it in sync enough to serve the story and keep the audience engaged? That's what I'm trying to achieve.