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Old September 4th, 2004, 11:46 PM   #1
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Audio before video?

I just finshed a recent edit of a 10 minute shoot, with my GL2 and a shotgun, from about 10' away from a podium, and I found that the audio was 4-5 frames ahead of the video. It did not drift, it was the same sync start to finish.

I ran the original tape direct to a TV to see if it was a capture problem, but the sync was out there too...

Obvoiusly easy to correct in post, but is it a common problem?
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Old September 5th, 2004, 12:54 AM   #2
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Sound travels much slower than light. so if anything, the audio should be 3-4 frames behind the video. Ahead doesn't 'sound' right at all.

Let's play with some numbers. Assume sound travels at 700 mph. Using 'close' numbers for quicker calculations:

700 x 5000 feet (mile) = 3,500,000 feet per hour. Divide by 60 to get feet per minute = 58,333 fpm. Divide by 60 to get feet per second = 972 fps Divide by 30 to get feet per frame = 32 feet.

So, using these rough numbers, for every 32 feet away from the event you set your camera, an on-camera microphone will record the audio one frame later that it actually occured. If you use a wired or wireless microphone located at the event, there is no delay of course.

I think that you need to look for another answer if your audio is leading your video
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Old September 5th, 2004, 07:15 AM   #3
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How about trying a head cleaning? Could be a small build up is your problem.

Just a thought.
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Old September 5th, 2004, 09:40 AM   #4
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<<<-- Originally posted by Robert J. Wolff : How about trying a head cleaning? Could be a small build up is your problem.

Just a thought. -->>>

Dirty heads would cause drop outs , not a timing problem. This is a real time issue and I can't see how the camera could be at fault.
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Old September 5th, 2004, 09:57 AM   #5
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Bryan,

As I said it was just a thought. My thinking is that the video portion of the head, could be clogged enough to "lift" the tape as the video is laid down, possibly forcing audio to lay down earlier.

It was just a slim chance that cleaning the head might correct the problem.

If tried, and it does not correct the problem, Chris may have to take it to the camera doctor. The only other item that comes to my mind, is time-base correction has changed some how.

I would be interested in hearing from Chris as to what it takes to solve the problem.
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Old September 5th, 2004, 10:50 AM   #6
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It is very strange indeed. I have had issues with the audio behind in the past, but that was when I was quite far from the source, and I realized that that is just how it works (Thanks for the ft/frame calculations Mike!)..

I have not used the camera again since, so I will do a few test tonight and see if it was some type of fluke, or a problem that is still there. I guess if it exists, I will have to send my baby away :(
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Old September 5th, 2004, 12:33 PM   #7
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Canon, at least in their earlier cameras, didn't follow the rules with regard to handling the video/audio interleave with the result that special consideration had to be incorporated into NLEs to accomodate the issue.

I've not heard of those cameras inserting the audio before the video, though. It was always a few frames after the video and in a burst so the audio could catch up.

Might be interesting to post a question about audio embedding in the GL2 forum to see if anyone recognizes the problem.

Just out of curiosity, you are running 16 bit audio into channels 1 & 2 and haven't somehow switched the camera to 12 bit, insert audio into channels 3 & 4?
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Old September 5th, 2004, 02:57 PM   #8
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Re: Audio before video?

<<<-- Originally posted by Chris Thomas : I just finshed a recent edit of a 10 minute shoot, with my GL2 and a shotgun, from about 10' away from a podium, and I found that the audio was 4-5 frames ahead of the video. It did not drift, it was the same sync start to finish.

I ran the original tape direct to a TV to see if it was a capture problem, but the sync was out there too...

Obvoiusly easy to correct in post, but is it a common problem? -->>>

What deck or camera are you using for playback (same one to record?)?
What speakers?
What monitor?

If you used a shotgun, how did you get the signal into the camera?

Did you record at 16bit/48Khz or 12b/32K?

I don't own a GL2, but the GL series is known for mis aligned heads.

Have you had this issue before, or just on this shoot?
Have you ever used the shotgun on a different gig and had the same issue?
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Old September 5th, 2004, 10:34 PM   #9
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Quote:
What deck or camera are you using for playback (same one to record?)?
What speakers?
What monitor?
I used the same GL2 for playback of the original tape on my TV. I only noticed the problem after the edit, for which I use a Sony DCR-HC20 as a deck, and to connect to an external monitor. After seeing the problem in the edit, I checked the raw footage and saw it there too.
Quote:
If you used a shotgun, how did you get the signal into the camera?
I use an XLR to Mini 1' cable
Quote:
Did you record at 16bit/48Khz or 12b/32K
16bit record, capture, and output, no audio changes on the camera during recording or capturing. All 16bit.
Quote:
Have you had this issue before, or just on this shoot?
It is the first time I have noticed the problem, and I did a few tests tonight and it is still there. I am going to look back at some footage I have shot and not used yet over the past few weeks and see if it is there too. 4 frames is hard for me to spot on a small monitor unless there is a good closeup to verify lip sync. I'll see what happens
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Old September 6th, 2004, 12:20 PM   #10
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Are you using LCD or CRT monitor for viewing?

Try connecting the camera direct to a TV (with CRT) monitor and see if you still have a problem with the raw footage.
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Old September 6th, 2004, 02:40 PM   #11
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I have seen problems with audio and video sync when viewing on
LCD monitors, but this sounds like the camera has a problem.

After confirming that eveything shot on it has this issue I would
send it back to Canon ;(
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Old May 12th, 2005, 06:33 PM   #12
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I seemed to be having a similar problem. While capturing the footage shot with XM2 through Sony DSR-25P, I could see on TV that the audio was in sync. But playing back the captured footage (both TV and LCD), the audio was visually ahead by 5 frames.

I then checked the playback frame offset (FCP HD 4.5 in system setting- not sure what this does, I couldn't find a reference in help) and the value was '4'. I set it to '0' and during the playback on the LCD screen (G4 1.5MHz) the audio visually looked in sync. However, on TV it was still ahead visually. Scratching revealed that the audio was 1 frame ahead on both TV and LCD.

I'm confused. It seems you can't test the sync by using scratch. I might end up shifting the audio at the fine cut stage so that it visually looks OK.
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Old May 12th, 2005, 09:29 PM   #13
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Chris, are you saying the original raw footage DV tape played back in the shooting camera , output composite or s cable to a NTSC monitor the audio leads the video?
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Old May 13th, 2005, 02:19 AM   #14
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If your question was for me then the original footage was OK and did not have any problems.
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