Syncing woes

ajcrm125 wrote on 9/27/2010, 7:55 PM

I'm creating a video for our church. We captured some audio/video using an digital camcorder. The audio was faint since the camera was so far away I decided to use the audio recorded from the speaker's lapel mic.

I layed down the video from the camera and the audio from the mic and synced them up perfectly. Looks and sounds great on the timeline.

When I render the audio and video are lined up for the first few minutes and them progressively gets worse.

Any ideas?

Comments

musicvid10 wrote on 9/27/2010, 9:46 PM

It's clock drift, and it occurs whenever two audio recordings are not synced by a "wire."

There are two approaches, the first may be simpler for you, and I prefer the latter.

1) Locate a distinct short waveform near the beginning and the end of the camera audio. Normalize the event audio so you can see it better. Now line up the matching waveforms at the beginning, and stretch or squeeze the new audio event (Ctrl+Drag) until the end matches. Then go back to the beginning, line them up again, and repeat until the head and tail are lined up perfectly. The drawback is that time stretch always introduces some quantization noise, or echo / flange if it is a lot, and intermediate drift can occur if it is a long program.

2) I prefer to chop the new audio into ~10 minute chunks, being careful to make my cuts in quiet areas exactly at zero crossings, then lining up a waveform near the middle of each chunk with the original audio. I then mix the tracks to give a pleasing balance and cover any microgaps that may occur as a result of moving them around. Since I have Vegas Pro, this is very easy using Pluraleyes. However, doing it by hand in VMS could become very tedious, esp. on a long program.

There are lots of refinements to these techniques, but this should get you started.

TOG62 wrote on 9/27/2010, 11:36 PM

It seems to me that if the audio syncs before rendering, but not afterwards, it might be worth rendering to a high quality intermediate format and then re-syncing that in VMS. This is a stab in the dark as I have no idea why the problem is occurring.

ajcrm125 wrote on 9/28/2010, 4:30 AM

Thanks for the advice.. I'll try those techniques.

Just out of curiosity... why does this happen? I mean, if it's perfect in the timeline why doesn't the media render correctly? Are there some characteristics of the source media that are different from the desired render settings that may be different that I'm not aware of?

I guess in my ignorance I thinking this should "just work" out of the shoot.

ajcrm125 wrote on 9/28/2010, 4:33 AM

By the way, this 'clock drift'... should it be noticable in the timeline? Like I said earlier, this think looks/sounds perfect in the timeline, it's only after rendering that things looks off.

Thanks.

richard-amirault wrote on 9/28/2010, 4:42 AM

When I render the audio and video are lined up for the first few minutes and them progressively gets worse.

As others have said ... clock drift ... but you haven't really said .. did you check further down the timeline to see if the audio stayed in sync *before* you rendered?

That's how clock drift works. You can sync up perfectly at the start of a clip, but it will (if the two clocks are not running the same) gradually drift as the clip progresses. For short takes it may not be noticeable but for longer clips it can be quite significant.

ajcrm125 wrote on 9/28/2010, 5:26 AM

Yep.. I checked further in the timeline (the end actually) juuuust to make sure things looked/sounded ok and they did.

richard-amirault wrote on 9/28/2010, 2:33 PM

Ok, then it does not sound like clock drift, but I have no idea what it may be.

What file format is the new audio? Was it converted from it's original format to something else?

OzSpaceAce wrote on 10/5/2010, 5:51 AM

Im not sure why it happening in your case, but something ive noticed in VMS, is that what you see in the preview window is not nessasarily what you'll get in the render. By this i mean, unless you have a very fast "you beaut" PC, the preview window will frameskip, depending on, timeline zoom, active effects windows, preview settings [eg - draft, preview, good etc], amount of video FX running on the tracks, among other factors. I have a E2500 dual intel, FSB800, 2gb ram, basic NVidia graphics card, and hardly any background tasks running, and i still get frameskipping quite often.

Also, audio sync has been a problem for me too, check the audio device setting in the preferences screen, theres an option for track buffering and adjust for latency, experiment with these settings, track buffers will cause latency, so set as low as possible. I use ASIO on both my soundcards, and im wondering if this is causing my problems, as ASIO bypasses window audion maybe vegas doesnt like this, im not sure, still nutting it out. BTW, Is VMS9, ASIO compatable?

ajcrm125 wrote on 10/5/2010, 7:14 AM

The audio format is an MP#, captured from the guy's lapel mic.
I've heard of people having sync issues when using video and an underlying MP3. They've suggested converting the MP3 to a WAV first. I'll try that and then adjusting the audio settings as suggested. Hopefully we'll get this thing solved cuz I have a bunch of videos to render. :)