Timing dialogue - audio buffer delay?

RoyBU wrote on 9/22/2008, 5:26 AM
A project I'm working on involves quite a bit of dubbed dialogue, which has made me more sensitive to the timing of dialogue. In reviewing the final version in Vegas on my computer it seemed that all the dialogue was off by just a bit. I compared the dubbed dialogue to the original and it lined up almost perfectly, but even the original dialogue seemed to come just a bit early for the video. Then it occurred to me that there is a playback buffering setting under preferences/audio device that defaults to a tenth of a second. This, for a 24P project, is just over 2 frames, which is about the delay I needed to introduce. So am I correct in thinking that really my audio is fine and what seems to be a mismatch is an artifact of that buffering? (Obviously I will check this by burning a DVD, but I like to understand what is going on.)


musicvid10 wrote on 9/22/2008, 7:30 AM
Playback buffering is the amount of data that is stored in memory when preview playback begins. It is not a "delay" setting, and has no meaningful impact on audio / video synchronization as long as you do not hear gapping or stuttering when playing the preview. It also has no impact whatsoever on the final (rendered) product.

That being said, you didn't tell us what source material or sound interface you are using, how many audio tracks, plugins, envelopes, etc. you have on the timeline. All of these can affect preview playback, and you should burn a dvd, or whatever your final output will be, to check. Even so, different dvd drives and players handle audio differently, so check it by playing on several machines.

I don't recommend changing the buffer properties unless you are having a problem. There is a little ? in the corner of the properties box that you drag to a specific item to see detailed information for that setting. For best playback and lowest hardware recording latency you should use the ASIO audio drivers if available.
RoyBU wrote on 9/22/2008, 7:58 AM
Thanks for the reply. The source material is Cineform 24P and wave files. When I am trying to time the dialogue I use the selective pre-render option, and then play it back on my computer monitor and through my sound card (Audiophile 2496) which is connected to my computer speakers. What mystifies me is that even the original dialogue, which is presumably correctly lined up, seems ahead a couple of frames.
musicvid10 wrote on 9/22/2008, 8:06 AM
I have experienced this, and in one case had to move the audio track a couple of frames to get the rendered output right.
I assume your video and audio project properties are set to match your sources exactly?
RoyBU wrote on 9/22/2008, 2:45 PM
Yes, they match. I wonder if it has anything to do with the fact that this material was recorded on a Canon HV20, which puts the 24P in a 60i wrapper, which I then used Cineform to redo as a 24P avi? And it doesn't seem entirely consistent; that is, some clips seem to sync properly.
nolonemo wrote on 9/22/2008, 3:04 PM
I have had the experience a couple of times working in SD on an older computer when I was tweaking video going by the preview, sliding the audio so that it would match up to the speaker's lip movements, though the sync looked perfect in the preview window (and the window indicated that my playback was at the full 29.9 fps) when I went to render it was a little out of sync. I figured that the processing load for the video was high enough that the audio was de-syncing. Note that in this case I was syncing up separately recorded audio and video, and I couldn't rely on the slating because the camera was some distance from the speaker. Remember that with in-camera audio you will get about 1 frame of de-sync for every 30 feet the camera is from the speaker because light and sound travel at different speeds.
johnmeyer wrote on 9/22/2008, 5:18 PM
1. Use the internal monitor (the display within Vegas), not an external monitor via Firewire, when checking lip sync.

2. Make absolutely certain that you are getting full frame-rate playback before proceeding with sync. This is shown in the Vegas display. It should be rock solid at 24.00 for your video. If it is anything lower, then decrease the preview quality until you get stable 24. If that still doesn't work, then do a RAM preview of the segment you are syncing.

3. You probably already know this, but words which start with "B" and "P" are great words to sync on.

I just synced dozens of short clips and the other thing I did was to turn off Quantize to Frame, so I could slip the audio to exactly match the lip movement. I always move the audio, not the video, so I can get precise control. This is even more important when doing 24 fps, because there is even a bigger time difference between frames than with 60i.
Robert W wrote on 9/23/2008, 11:27 AM
Does the HV20 shoot at a dead 24fps or is it 23.976 or something like that? If the true speed of the camera was a little slower than what the camera returns in it's header it would drop a frame or two every few minutes against an accurately clocked sound sample. I.e. If your project is set to 24 frames and the media imports as 24 frames when it was really shot at 23.976 this could cause a sync issue.

What happens if you change the media and project to 23.976? Or have I got the issue round backwards?
Grazie wrote on 9/23/2008, 11:40 AM
"What happens if you change the media and project to 23.976? Or have I got the issue round backwards?"

Excellent question Robert!!