Sub-Frame drift when exporting multi mono for 5.1

[Rhetoric122]

Hey all,

First post here on the forums!

I recently sent a 5.1 mix to a video editor and got a comment when he laid the eight channels on top of each other when marrying picture and audio in Premiere Pro. There was some kind of sub frame drift happening between the export of the multi-mono 5.1 and the multi mono LtRt causing there to be slight phase issues during playback. I've exported using the exact same method and when laying the eight multi-mono tracks on top each other for a different project, there is no phase present. Issues like this cause paranoia, so if anyone has an explanation why this happening, that'd be great.

Thinking down the road when these mixes are played, some device is ether folding them down to either 5.1 or stereo, making it passable the phase was present during playback of all eight channels at once since the drift was so slight.

I'd love to hear some comments about why this is happening, and what to do in the future to prevent it from happening when exporting a 5.1 mix.

Specs located in my signature.

Thanks!

[reichman]

Based on your post, it's hard to tell exactly what the video editor was listening to, but playing a discrete 5.1 mix along with the LtRt version of the same mix is definitely going to sound wonky.

[Rhetoric122]

Hey reichman, thanks for your input. I asked a few colleagues about this, some said as you did and that laying a 5.1 and LtRt mixes over each other would sound weird. However, opening it up in Pro Tools, the stereo mix that I printed within PT had drifted slightly. So that is where my cause for concern is coming from. We fold down using SurCode for Dolby Pro Logic II. So my first thought is using that plug-in, that is where the delay is coming from.

[reichman]

Not to be nit-picky, but let's look at terminology for a moment: "drift" usually means a change in the time that is variable over a defined length. Very rarely seen in these digital days. "Offset" or "latency" refer to a change in the time that is consistent over a defined length. Assuming you have a latency problem, you might find that routing your 2-pop through the encoding plug-in solves the issue. i.e. everything is delayed by the same amount. There are plug-ins out there that don't report the correct delay compensation to Pro Tools. Do you have delay compensation on? And, how much is the delay? If it's less than a tenth of a frame, a SynCheck would pick up the problem, but most viewers and clients would never notice.

[BScout]

If you are printing back into your session using a plugin that might create delay, you should turn off auto-low latency on that track so delay comp is working. Otherwise, delay comp for the track that is bussed into and recording from the track with a plugin isn't being compensated. (You do this by right clicking on the delay comp area of the recording track in the mixer window and selecting "disable auto-low latency".) All the delay comp numbers will turn blue.

Do that, try sync pops, and you'll see everything line up.
Minnetonka SurCode is a particularly bad plugin because it puts latency in the chain in order to do some of its functions (transport linked to metering calculations) that most people don't use (I've had the discussion with them to be able to disable that but they seem really lax.) More or less it has a look ahead buffer.

[Sonny Keyes]

I just recently found that my entire LtRt print was being delayed by one frame because of the Neyrinck encoder on that track. I was tearing my hair out trying to figure out why that was the only track that was delayed until I realized it was the only one with the encoder on it.

[Rhetoric122]

Thanks BScout, I think you described exactly the situation I'm experiencing. I'm going to take a pass using the method you described when I get a chance and report back on the findings. Thanks for taking the time to respond.

Originally Posted by BScout

If you are printing back into your session using a plugin that might create delay, you should turn off auto-low latency on that track so delay comp is working. Otherwise, delay comp for the track that is bussed into and recording from the track with a plugin isn't being compensated. (You do this by right clicking on the delay comp area of the recording track in the mixer window and selecting "disable auto-low latency".) All the delay comp numbers will turn blue.

Do that, try sync pops, and you'll see everything line up.
Minnetonka SurCode is a particularly bad plugin because it puts latency in the chain in order to do some of its functions (transport linked to metering calculations) that most people don't use (I've had the discussion with them to be able to disable that but they seem really lax.) More or less it has a look ahead buffer.