Videomaker – Learn video production and editing, camera reviews › Forums › Technique › Sound › Audio Sync
- March 1, 2007 at 2:45 PM #41225
After reading the review this month, I purchased the Zoom H4 Digital recorder. I enjoy recording our school band concerts, plays etc, and though that moving the recorder closer to the action would subtantially improve my video quality.
The H4 works great – very much improved quality from Camera. 😛
I had assumed that because the recorder is digital, as is my camera (older Digital 8 system), that the two files should sync nicely. Unfortunately their is about a 6 second bust after 30 minutes. 😡 I had to "stretch" my video, in order to match it with the audio. Is this normal? I plan to do a test with a watch over 15-20 minutes to see if it is the camera, or H4, or both that are out of sync.
I can see where a "clapper" would be very handy at the start (and apparently also at the end) of a sceen. Unfortunately this is not always possible…..
I would start by testing results between your recording format. My feeling is that if you’re using mp3 as a recording format you will experience such problems. You might get better results in WAV format, however, there’s no guarantee. I think the root issue is that the device does not have the same time-base format as video. That means the device will interpret time differently. In your case the H4 compresses the time. It’s only noticeable when you’re recording for longer durations.
As for compensating for that time compression, you’re technique of slowing down the video fractionally makes perfect sense. Although, I think that could be damaging to the quality of the video so I’ll suggest another idea. If you can find a few places (say every 8-10 minutes) in your video where there’s a lull or a similar audio environment (such as an applause or silence), cut the audio in those places and then slide each section and align them to fit naturally with the video. Then fill in the gaps with some selections from that audio file the imitate the audio environment of that particular scene. Make sure to mix it in using you volume/gain controls so that it sounds natural.
Another technique is to stretch the audio and adjust the pitch to compensate for the slow down. This in many ways can be easier to pull off than my first suggestion. But, you’ll need the right software/audio filter effects to do it. What do you have at your disposal?
Thanks Hank and Mark for the suggestions.
I made a short 12 min movie last night as a trial, but ran out of time before I had a chance to load it on the computer and put the video and wave files side by side. I used a stop watch, and clapped my hands at various time intervals. I should be able to see which component is different.
Mark – I think you may be on to something with the recording format. I used the MP3 format on the H4 Digital recorder. Today before work, I loaded the mp3 file into an audio program, and saved it as a wav file. This file matched the video much more closely, althouth not perfectly. I have a feeling that if I had used the wav format on the original recording, it would have worked better. I will do another test and let you know. I hope to have some time this weekend.
I hadn’t thought about the reduction in video quality by changing slowing it down or speeding it up slightly, but it makes sense. I do have some software that will let me do it with the audio – so that may work better. I suspect that with the very minimal change (2-3 seconds over 30 minutes – 0.2%) it should not change the pitch noticably – particularly when there is no music in this particular project. The idea of chopping out little bits of audio is also interesting. I’ll give it a try and let you know how I make out.
My trials continue to show a slight time sync difference between the digital recorder and my digital 8 camcorder.
The camera appears to be very accurate – at least camparing the timestamp and a stopwatch.
In each case my Audio is a bit longer than the video.
I took your advise can cut small sections of the audio out. This was some trouble but worked very well.
I removed two frames every minute. I was able to target relatively silent spots in the audio – so it is not noticable.
The effect seems to be consistent with each new recording. Still not sure if it is my software, the recorder or the cam…
At least I have a workaround…
Have you tried recording with the H4 set at different sample rates as well? The difference between 44.1Khz and 48Khz recordings may not always be immediately (audibly) apparent but if you import a 44.1Khz file into a 48Khz video project there will be a noticeable difference in the duration of the files.
I have an H4 here but haven’t had a chance to try it on a video project yet, I’ve just been sound-collecting so far. I would be interested in knowing more details regarding your tests.
I just got my H4 for I am working on my first project – which was a 3 act school play. Each act was about 30 minutes long.
I had assumed that the audio sync would be a no-brainer. I was supprised when I found that by the end of each act (30 minutes) the audio and video were out of sync. I originally recorded in mp3 format – and the audio was out by about 3 seconds at the end of 30 minutes. Based on the advice above, I converted the audio to wav. It was then much closer, but still about 1/2 second out after 30 minutes – very noticable.
As above, I solved the issue by cutting out 2 frame segments of the audio every minute or so of the video. Some hassle, but it saved the project and worked very well.
I did a test by setting up my video camera in the kitchen and running it along side the H4 and a stopwatch for 15 minutes. I tried both mp3 format and wav as recording format. I did not try different sample rates, but maybe should.
I then loaded the audio and video files into my ULEAD 10 editing program.
The timecode on the camera matched the stopwatch exactly (as close as I could tell). I made a clapping sound at the start and end of the test. In each test the sound file did not match up with the video. In each case it was about 1/2 second longer than the corresponding video. When I get a chance, I will muc with the sample rates to see if that makes a difference – both on the H4 and the Editing software..
It would be much simpler if the audio and video would sync without all of the extra work…