How do recorders like the Zoom H4N sync up audio to different framerates?

H

Hutchman

Active member
Hey guys,
Sorry if this is a basic question, but I want to learn:

If I get a zoom h4n and record audio, and shoot on my 7D at 24fps, then shoot other footage at 25 or 30fps, is there a setting in the recorder that you switch framerates with?

Or am I just not totally understanding the process....?

PLEASE HELP

Thanks
 
Zoom (as well as other) recorders don't care about framerate. A second is a second is a second. The problem you will have is how to get various frame rate clips onto a single timeline.

Let me explain it another way. If you shoot 10 minutes of footage at 25 fps and record that same 10 minutes of audio onto the zoom, and then drap both clips onto a 25 fps timeline, audio and video will sync just fine. The same holds true if the video is 24 or 30. The problem surfaces when you drag a 24 fps clip onto a 30 fps timeline. The timeline is expecting the 30 frames to occupy 1 second of timeline, so the 24 fps clip will fall 6 frames short each second, which will very quickly throw off sync of audio already on the timeline.

So the problem is never what framerate the clip was shot at, the problem is ensuring that the clip is stretched or shrunk to conform to the timeline.
 
Very well explained! Thank you very much.

Does stretching or shrinking clips work well or does it sound a bit 'off'? Any tips? Thanks.
 
Hutchman said:
Very well explained! Thank you very much.

Does stretching or shrinking clips work well or does it sound a bit 'off'? Any tips? Thanks.
Click to expand...

For taking 24 fps to produce something that runs correctly at 29.97 fps, the procedure is called adding a 3:2 pulldown, or 'telecine'. Since this has been a problem for NTSC TV since the beginning, it is a well used procedure.

In the case of 25 fps, if the material is only to be presented in PAL land, the video rate is 25, so problem's solved... 24 fps material can be slightly sped up, as a 'cheap' method of conversion.

The standard time base for 'audio' is 48k samples/s. This is independent of the frame rate of the video. If the camera records 48K concurrently with whatever frame rate for the video, everything is 'sync'd' up. However if one is using a Zoom or any other external recorder, the clapper on the slate is used to set a sync point of the audio that can be matched to the video frame where the two clapper blades meet.

Depending on the quality of the timing circuit there may be drift over long periods of time in the unsyncronized sound recorder, such as the Zoom. So, the advice is to make short recordings, and synchronize with the clapper regularly.

For long recordings, drift may be accounted if it is stable by 'stretching' the audio time without any perceptible, at least by humans, change.
 
May be a bit more would be helpful.

What was said above boils down to as long as you don't change the speed of your footage sound will stay in sync.

The 3/2 pull down does in fact change the speed of your footage so you need to do some compensation if you go from film speed to video speed.

For the OP though, he is not actually shooting 24. He would be shooting "24p" which is really 23.98 and running at video speed. So as long as he doesn't do a filmout he will always be working at the same speed so no sync issues. Well at least not because of frame rate issues.

The above assumes your in NTSC land. If your in PAL land you should shoot at 25a and again you won't have any frame rate issues unless you do a filmout.

It is very unlikely that anybody is going to shoot video and do a filmout these days so it's kind of a moot point.
 
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